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A conversation that I had with Claude 3 (Sonnet) over lunch

The following is a conversation that I had with Claude 3 (Sonnet) over lunch. I was trying to clarify some questions I had about OAuth, how to decipher the flows, and about related security concerns when using it with server-hosted and server-less applications.

I find this type of conversation to be very useful, although I find that I still need to have some awareness of the subject and approach the responses with some critical thought and push-back appropriately. You may notice that when I asked it to list the OAuth flows, it left out ‘hybrid’. I somewhat understand why, since it’s not as straightforward, but I found the omission to be interesting anyway.

Also, it helps me to rephrase what it tells me and to ask it to clarify. Sometimes it catches things that I miss, and other times, it provides validation that I’ve stated the ideas correctly. And stating back what I understood helps my memory…

I’m leaving this here partly due to posterity, as I’m sure that the capabilities of AI will change over time. I also offer it as an example of a way to use a LLM AI tool that recognizes both its value and current limits.

I need some help understanding oauth flows. I’m looking at a URL that appears to kick off the oauth login. What can I tell from this URL to help me understand what part of the OAUTH flow is happening. I’m specifically interested in how the OAUTH works, and also understanding which type of flow this represents.

what can you tell me about this URL, with regard to OAUTH

BC
https://testsso.hostname.com/OAUTH.aspx?response\\_type=code&client\\_id=abcd1234&redirect\\_uri=http%3A%2F%2Flocalhost%3A5002%2Fauthorize&scope=openid+profile+email+phone&state=xyzpdqM&nonce=roflmao

The URL you provided contains several parameters that provide insights into the OAuth flow being initiated. Let’s break it down:

  1. response_type=code: This parameter indicates that the OAuth flow being used is the Authorization Code Flow, which is one of the most common and recommended flows for server-side web applications. In this flow, the client (your application) will receive an authorization code from the authorization server (in this case, testsso.hostname.com) after the user grants permission. This code will then be exchanged for an access token and potentially a refresh token.
  2. client_id=abcd1234: The client_id is a unique identifier assigned to your application by the authorization server during the client registration process. It helps the authorization server identify which client is making the authorization request.
  3. redirect_uri=http%3A%2F%2Flocalhost%3A5002%2Fauthorize: The redirect_uri is the URL where the authorization server will redirect the user’s browser after the user grants or denies permission to your application. In this case, it’s set to http://localhost:5002/authorize, which likely represents a local development environment.
  4. scope=openid+profile+email+phone: The scope parameter specifies the permissions or access rights your application is requesting from the user. In this case, the scopes requested are openid (OpenID Connect authentication), profile (access to the user’s profile information), email (access to the user’s email address), and phone (access to the user’s phone number).
  5. state=xyzpdq: The state parameter is an opaque value used to maintain state between the request and the redirect from the authorization server. It’s typically used to mitigate Cross-Site Request Forgery (CSRF) attacks and to provide additional security by verifying that the response from the authorization server matches the original request.
  6. nonce=roflmao: The nonce parameter is a unique, one-time value that is included in the authentication request to help prevent replay attacks. It’s often used in combination with OpenID Connect to ensure that the ID token issued by the authorization server is fresh and not a replay of a previous authentication.

Based on the parameters in the URL, it appears that this request is initiating the Authorization Code Flow of the OAuth 2.0 protocol, specifically using OpenID Connect (OIDC) for authentication. The client application is requesting access to the user’s profile, email, and phone number, and it expects the authorization server to redirect the user’s browser back to http://localhost:5002/authorize after the user grants or denies permission.

It’s important to note that this is just the initial step in the Authorization Code Flow. After the user grants permission, the client application will receive an authorization code, which it will then exchange for an access token and potentially a refresh token, allowing it to access the requested resources on behalf of the user.

Claude does not have internet access. Links provided may not be accurate or up to date.

I know there other types of OAUTH flows. Can you give me a modified URL that could indicate the different parameters for each of the other OAUTH Flows?

BC

Certainly! Here are examples of modified URLs that could indicate different OAuth flows, along with explanations of the parameters:

  1. Implicit Flow:
https://testsso.hostname.com/OAUTH.aspx?response_type=token&client_id=abcd1234&redirect_uri=http%3A%2F%2Flocalhost%3A5002%2Fcallback&scope=openid+profile+email+phone&state=xyzpdq&nonce=roflmao

In the Implicit Flow, the response_type parameter is set to token. This flow is typically used for client-side applications (e.g., single-page applications or mobile apps) where the client cannot securely store a client secret. Instead of an authorization code, the access token is directly returned as a fragment in the redirect URI.

  1. Resource Owner Password Credentials Flow:
https://testsso.hostname.com/OAUTH.aspx?grant_type=password&client_id=abcd1234&[email protected]&password=userpassword&scope=openid+profile+email+phone

In the Resource Owner Password Credentials Flow, the grant_type parameter is set to password. This flow is suitable for trusted first-party clients where the user directly provides their credentials to the client application. The client then exchanges these credentials for an access token from the authorization server.

  1. Client Credentials Flow:
https://testsso.hostname.com/OAUTH.aspx?grant_type=client_credentials&client_id=abcd1234&client_secret=xyz123&scope=read_data

In the Client Credentials Flow, the grant_type parameter is set to client_credentials. This flow is used for machine-to-machine authentication, where the client application itself (not a user) is authorized to access resources. The client exchanges its client ID and client secret for an access token from the authorization server. Note that this flow does not involve any user interaction or consent.

  1. Refresh Token Flow:
https://testsso.hostname.com/OAUTH.aspx?grant_type=refresh_token&client_id=abcd1234&client_secret=xyz123&refresh_token=abc456

The Refresh Token Flow is not a separate flow but rather a way to obtain a new access token using a previously obtained refresh token. The grant_type parameter is set to refresh_token, and the client sends the refresh token along with its client ID and client secret (if applicable) to the authorization server to obtain a new access token.

It’s important to note that the specific parameters and their values may vary depending on the authorization server implementation and configuration. Additionally, some flows may require additional parameters or use different parameter names. Always refer to the documentation provided by the authorization server for the correct parameter names and values.

Claude does not have the ability to run the code it generates yet.

Is it reliable to say that looking at the URL sent to the OAUTH identity provider, I can derive the flow from these parameters in the URL?

response_type=code indicates ‘Authorization Code Flow’

response_type=token indicates ‘Implicit Flow’

grant_type=password indicates ‘Resource Owner Password Credentials Flow’

grant_type=client_credentials indicates ‘Client Credentials Flow’

grant_type=refresh_token indicates ‘Refresh Token Flow’

BC

Yes, it is generally reliable to derive the OAuth flow from the parameters present in the URL sent to the OAuth identity provider (authorization server). The specific parameters you mentioned are commonly used to indicate the different OAuth flows:

  • response_type=code indicates the Authorization Code Flow
  • response_type=token indicates the Implicit Flow
  • grant_type=password indicates the Resource Owner Password Credentials Flow
  • grant_type=client_credentials indicates the Client Credentials Flow
  • grant_type=refresh_token indicates the Refresh Token Flow

These parameters are defined in the OAuth 2.0 specification (RFC 6749) and are typically implemented by OAuth identity providers (IdPs) and authorization servers in a consistent manner.

However, it’s important to note a few caveats:

  1. Implementation variations: While the core parameters are standardized, some IdPs or authorization servers may use slightly different parameter names or additional parameters due to their specific implementation or custom extensions.
  2. Hybrid flows: Some IdPs or authorization servers may support hybrid flows that combine aspects of different OAuth flows. In such cases, the URL parameters may include a combination of parameters representing different flows.
  3. Additional security parameters: The URL may include additional security parameters, such as state, nonce, or code_challenge, depending on the specific requirements and security measures implemented by the IdP or authorization server.
  4. URL encoding: The URL parameters are typically URL-encoded, which can make them appear different from their raw form.

Despite these potential variations, the parameters you listed are generally reliable indicators of the respective OAuth flows. It’s always a good practice to cross-reference the parameters with the documentation provided by the specific OAuth identity provider or authorization server to ensure a proper understanding of their implementation and any deviations from the standard.

Additionally, it’s important to note that the OAuth flow should be chosen based on the specific requirements and security considerations of your application, as each flow has its own advantages and limitations.

Are there other oauth flows besides these? I’ve seen one called ‘hybrid’

BC

Yes, there is another OAuth flow called the Hybrid Flow, which you mentioned. The Hybrid Flow is a combination of the Authorization Code Flow and the Implicit Flow, designed to address the limitations of each individual flow.

In the Hybrid Flow, the authorization request includes both the response_type=code and response_type=token parameters, separated by a space. For example:

https://testsso.hostname.com/OAUTH.aspx?response_type=code%20token&client_id=abcd1234&redirect_uri=http%3A%2F%2Flocalhost%3A5002%2Fcallback&scope=openid+profile+email+phone&state=xyzpdq&nonce=roflmao

In this flow, the authorization server returns both an authorization code (like in the Authorization Code Flow) and an access token (like in the Implicit Flow) in the redirect response.

The Hybrid Flow is beneficial in scenarios where you have a client-side application (e.g., a single-page application or a mobile app) that needs to interact with a backend server. The client-side application can use the access token for immediate access to the user’s resources, while the backend server can exchange the authorization code for an access token and a refresh token, providing a more secure and long-lived access to the user’s resources.

It’s important to note that the Hybrid Flow inherits the security characteristics of both the Authorization Code Flow and the Implicit Flow. It provides the security benefits of the Authorization Code Flow for the backend server, while also allowing the client-side application to receive an access token directly, as in the Implicit Flow.

However, the Hybrid Flow is not widely adopted or supported by all OAuth identity providers or authorization servers. It’s considered a non-standard flow and may require additional configuration or implementation effort on both the client and server sides.

To summarize, the Hybrid Flow is an additional OAuth flow that combines aspects of the Authorization Code Flow and the Implicit Flow, aiming to provide a more flexible and secure solution for client-side applications that interact with backend servers. However, it’s not part of the core OAuth 2.0 specification and may not be universally supported.

Claude does not have the ability to run the code it generates yet.

If I see a refresh_token flow, will there usually be another flow type that happens prior to it?

BC

Yes, that’s correct. The Refresh Token Flow is typically preceded by another OAuth flow that initially obtains the refresh token.

The Refresh Token Flow is not a standalone flow but rather a way to obtain a new access token using a previously obtained refresh token. Refresh tokens are usually provided alongside the initial access token response from the authorization server.

Here’s how the Refresh Token Flow relates to the other OAuth flows:

  1. Authorization Code Flow: In the Authorization Code Flow, the client first obtains an authorization code from the authorization server. The client then exchanges this code for an access token and a refresh token. The refresh token can then be used in the Refresh Token Flow to obtain new access tokens when the original access token expires.
  2. Implicit Flow: The Implicit Flow does not provide a refresh token mechanism by default. However, some implementations may extend the Implicit Flow to also return a refresh token along with the access token.
  3. Resource Owner Password Credentials Flow: In this flow, the client directly exchanges the user’s credentials for an access token. Some implementations may also return a refresh token along with the access token.
  4. Client Credentials Flow: The Client Credentials Flow is used for machine-to-machine authentication and does not involve user credentials or consent. Refresh tokens are generally not used in this flow since the access tokens are typically long-lived and bound to the client application itself.

In most cases, the Refresh Token Flow is preceded by the Authorization Code Flow, as this is the recommended approach for server-side web applications and provides the most secure way to obtain refresh tokens.

When you see a Refresh Token Flow in action, it typically means that the client application has already gone through one of the other OAuth flows (usually the Authorization Code Flow) and obtained a refresh token. The client can then use this refresh token to request new access tokens from the authorization server when the previous access token expires, without requiring the user to re-authenticate or provide their credentials again.

It’s important to note that refresh tokens have an expiration time as well, and the authorization server may revoke or invalidate them for security reasons. In such cases, the client would need to initiate the initial OAuth flow again to obtain a new refresh token.

In the case where authorization code flow is seen along with refresh token flow, that the authorization code will be seen with the user’s browser session, and the refresh token flow will happen between the application server and the identity provider?

BC

Yes, you’re correct. In the case where the Authorization Code Flow is used in conjunction with the Refresh Token Flow, the flows typically involve different components and occur at different stages.

  1. Authorization Code Flow:
    • This flow involves the user’s browser session and the authorization server (identity provider).
    • The user’s browser is redirected to the authorization server’s login page, where the user authenticates and grants permission to the client application.
    • After the user grants permission, the authorization server redirects the user’s browser back to the client application’s redirect URI, along with an authorization code.
  2. Refresh Token Flow:
    • This flow happens between the client application’s server and the authorization server, without the user’s direct involvement.
    • Using the authorization code obtained from the Authorization Code Flow, the client application’s server exchanges the code for an access token and a refresh token from the authorization server.
    • When the access token expires, the client application’s server can use the refresh token to obtain a new access token from the authorization server, without requiring the user to re-authenticate or provide their credentials again.

So, in summary:

  • Authorization Code Flow: Involves the user’s browser session, where the user authenticates and grants permission to the client application, resulting in an authorization code being returned to the client application’s redirect URI.
  • Refresh Token Flow: Happens between the client application’s server and the authorization server, using the previously obtained refresh token to request new access tokens without user interaction.

This separation of concerns provides better security and a more seamless user experience. The user only needs to authenticate and grant permission once during the Authorization Code Flow. After that, the client application’s server can use the refresh token to obtain new access tokens behind the scenes, without requiring the user to re-authenticate or be redirected back and forth between the client application and the authorization server.

It’s important to note that while the Refresh Token Flow happens between the client application’s server and the authorization server, the initial Authorization Code Flow may involve additional backend communication between the client application’s server and the authorization server, depending on the specific implementation and configuration.

It looks like the authorization code flow requires the ‘client id’ only, and the refresh token flow requires the ‘client id’ and also ‘client secret’.

Is this always the case?

Also, are there other flows that require both the ‘client id’ and also the ‘client secret’?

For the flows that only include ‘client id’, how is the ‘client secret’ used, if at all?

BC

You’re correct in your observations about the use of client_id and client_secret in different OAuth flows. Here are the details:

  1. Authorization Code Flow:
    • The initial authorization request typically includes only the client_id.
    • The client_secret is used when the client exchanges the authorization code for an access token and refresh token.
  2. Refresh Token Flow:
    • The request includes both the client_id and client_secret.
    • This is because the refresh token flow is considered a “credential” grant type, and the client_secret is used to authenticate the client application.
  3. Other flows that require client_id and client_secret:
    • Resource Owner Password Credentials Flow: The request includes both client_id and client_secret.
    • Client Credentials Flow: The request includes both client_id and client_secret.

In the flows that only include the client_id in the initial authorization request (e.g., Authorization Code Flow), the client_secret is still used, but at a later stage:

  • During the client registration process, the authorization server issues the client_id and client_secret to the client application.
  • The client_id is included in the initial authorization request to identify the client application.
  • When the client exchanges the authorization code for an access token and refresh token, it must authenticate itself by providing both the client_id and client_secret.

The client_secret is essentially a credential that proves the identity of the client application to the authorization server. It’s kept confidential and should never be exposed to the user’s browser or any other untrusted environment.

In summary:

  • The Authorization Code Flow typically only includes the client_id in the initial authorization request, but the client_secret is still required when exchanging the authorization code for tokens.
  • The Refresh Token Flow, Resource Owner Password Credentials Flow, and Client Credentials Flow require both client_id and client_secret in the request, as they are considered “credential” grant types.
  • The client_secret is a sensitive credential that should be kept secure and used only between the client application’s server and the authorization server.

It’s important to note that some authorization server implementations or configurations may vary in their exact requirements for client_id and client_secret in different flows. It’s always a good practice to refer to the specific documentation provided by the authorization server you’re working with to ensure you’re meeting their requirements and following best practices for security and authentication.

I’ve heard of cases where there is a serverless application which is using OAUTH to authenticate the user. What flow(s) would be involved, and how would the authentication be enforced. I’m wondering how an application which runs only in the browser could safely store and use the ‘client secret’ without making it available to the user by looking at the information within the browser session

BC

In the case of a serverless application running entirely in the browser and using OAuth for authentication, the recommended flow is the Implicit Flow or the Authorization Code Flow with PKCE (Proof Key for Code Exchange).

Implicit Flow: The Implicit Flow is designed for client-side applications, such as single-page applications (SPAs) or mobile apps, where the client cannot securely store a client secret. In this flow:

  1. The application redirects the user’s browser to the authorization server’s login page, including the client_id, redirect_uri, and requested scope.
  2. After the user authenticates and grants permission, the authorization server redirects the user’s browser back to the redirect_uri, including the access token as a hash fragment in the URL.
  3. The application can then extract the access token from the hash fragment and use it to make API requests on behalf of the user.

However, the Implicit Flow has some security concerns, as the access token is exposed in the browser’s URL and may be accessible to third-party scripts or shared across multiple browser tabs/windows.

Authorization Code Flow with PKCE: The Authorization Code Flow with PKCE (Proof Key for Code Exchange) is generally considered more secure for client-side applications. In this flow:

  1. The application generates a random code_verifier and derives a code_challenge from it.
  2. The application redirects the user’s browser to the authorization server’s login page, including the client_id, redirect_uri, code_challenge, and requested scope.
  3. After the user authenticates and grants permission, the authorization server redirects the user’s browser back to the redirect_uri, including an authorization code.
  4. The application sends the authorization code and the original code_verifier to the authorization server, along with the client_id.
  5. The authorization server verifies the code_verifier against the code_challenge and, if valid, returns an access token and optionally a refresh token.

In both flows, the client application running in the browser never needs to store or use a client_secret. The client_id is a public identifier and can be included in the client-side code.

However, it’s important to note that in a serverless application, there is no secure server-side component to exchange the authorization code for an access token and refresh token. Therefore, the application must use the Implicit Flow or the Authorization Code Flow with PKCE, which allows the entire authentication flow to occur within the browser.

Additionally, since the browser environment is inherently less secure than a server-side environment, it’s crucial to follow best practices for secure token storage and handling, such as:

  1. Storing tokens securely: Use browser storage mechanisms like localStorage or sessionStorage to store tokens, but be aware of their limitations and potential risks.
  2. Enforcing secure communication: Always use HTTPS to prevent token exposure during transmission.
  3. Implementing token expiration and renewal: Implement mechanisms to handle token expiration and renewal (if using refresh tokens) to minimize the risk of token misuse.
  4. Protecting against cross-site scripting (XSS) and cross-site request forgery (CSRF): Implement appropriate security measures to mitigate these types of attacks, which could potentially expose or misuse tokens.

It’s worth noting that serverless applications may have additional security considerations and limitations compared to traditional server-side applications. In some cases, it might be better to have a minimal server-side component to handle the more sensitive parts of the OAuth flow, such as exchanging the authorization code for tokens and securely storing and handling refresh tokens.

In the example you just gave for Implicit flow, you stated, “The application can then extract the access token from the hash fragment and use it to make API requests on behalf of the user.” This implies that there is a server that is running the API, so in that sense, there must still be a server involved, and that server API is requiring the oauth token. Is that correct?

BC

You make a good point. My previous statement about the Implicit Flow implied the existence of a server-side API that requires the access token for authentication and authorization.

In the context of a truly serverless application running entirely in the browser, there would indeed be no server-side component involved. The application would interact directly with third-party APIs or services that support OAuth authentication and authorize requests using the access token obtained from the Implicit Flow or the Authorization Code Flow with PKCE.

To clarify, in a serverless OAuth scenario:

  1. The client-side application (e.g., a single-page app) initiates the OAuth flow with the authorization server, following either the Implicit Flow or the Authorization Code Flow with PKCE.
  2. The application obtains an access token (and optionally a refresh token) from the authorization server.
  3. The application can then use this access token to make authorized API requests directly to third-party APIs or services that support OAuth authentication and accept the access token for authorization.

There is no server-side component within the application itself. The application running in the browser interacts directly with the authorization server and the third-party APIs or services that accept the OAuth access token.

You’re correct that my previous statement implied the existence of a server-side API within the application, which is not necessarily the case in a truly serverless architecture. Thank you for catching that and helping me clarify the scenario more accurately.

Please verify if this is an accurate statement based on the serverless implicit flow: “The application itself runs in a browser or application that does not use the oauth token, but it will aid the user in obtaining the token to present and authenticate/authorize while accessing resources used by, but not part of, the application itself”

BC

Yes, your statement accurately describes the serverless Implicit Flow scenario:

“The application itself runs in a browser or application that does not use the OAuth token, but it will aid the user in obtaining the token to present and authenticate/authorize while accessing resources used by, but not part of, the application itself.”

In this scenario, the application running in the browser or client-side environment does not directly consume or use the OAuth access token itself. Instead, its role is to facilitate the user in obtaining the access token from the authorization server through the Implicit Flow.

Once the user has obtained the access token, they can then use it to authenticate and authorize themselves when accessing third-party resources or APIs that are external to the application itself. These external resources or APIs are the ones that actually require and validate the OAuth access token for authentication and authorization purposes.

The application acts as an intermediary, guiding the user through the Implicit Flow to acquire the access token, but it does not directly use or consume the token within its own codebase. The token is intended for the user to present to external services or resources that support OAuth authentication and authorization.

This separation of concerns is a key aspect of the Implicit Flow in a serverless context, where the client-side application itself does not have a server component to securely handle and use the access token. Instead, it facilitates the user in obtaining the token, which the user can then use to access and interact with external OAuth-protected resources or APIs as needed.

Your statement accurately captures this dynamic, where the application aids in token acquisition but does not directly use the token itself, as the token is meant for authenticating and authorizing the user with external resources separate from the application.

Let’s go back to ‘Authorization Code Flow with PKCE’. I understand that this will use the authorization code, so the URL sent to the authorization server will include ‘response_type=code’ among the parameters. What is an example of a URL that uses this flow along with PKCE, and what must be present in the URL for the authorization server to recognize this as using PKCE and also respond appropriately?

BC

In the Authorization Code Flow with PKCE (Proof Key for Code Exchange), the initial authorization request URL sent to the authorization server includes additional parameters related to PKCE. Here’s an example URL with the necessary parameters:

https://auth.example.com/authorize?response_type=code&client_id=CLIENT_ID&redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb&scope=openid%20profile&state=STATE_VALUE&code_challenge=CODE_CHALLENGE&code_challenge_method=S256

Let’s break down the relevant parameters:

  1. response_type=code: This indicates that the request is using the Authorization Code Flow.
  2. client_id=CLIENT_ID: The public identifier for the client application, issued by the authorization server during registration.
  3. redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb: The URL-encoded redirect URI where the authorization server will send the authorization code after the user grants permission.
  4. scope=openid%20profile: The requested scopes or permissions that the client application is seeking from the user.
  5. state=STATE_VALUE: An opaque value used to maintain state between the request and the callback, typically used for CSRF protection.
  6. code_challenge=CODE_CHALLENGE: The code challenge derived from the code_verifier using a specified transformation method (in this case, S256).
  7. code_challenge_method=S256: The method used to derive the code_challenge from the code_verifier. In this example, it’s the S256 method, which applies SHA-256 hashing.

The code_challenge and code_challenge_method parameters are specific to the PKCE extension of the Authorization Code Flow. These parameters are used to prevent code injection attacks and ensure that the authorization code is issued to the legitimate client application.

When the authorization server receives this request with the code_challenge and code_challenge_method parameters, it recognizes that the client is using PKCE. The authorization server then associates the code_challenge with the authorization code it issues.

Later, when the client application exchanges the authorization code for an access token, it must also provide the original code_verifier value. The authorization server then verifies that the code_verifier matches the previously associated code_challenge before issuing the access token.

This process ensures that only the legitimate client application that initiated the authorization request can exchange the authorization code for an access token, as it is the only entity that knows the code_verifier value.

It’s important to note that the specific parameter names and values may vary depending on the authorization server implementation. Always refer to the documentation provided by the authorization server for the correct parameter names and values required for PKCE support.

Claude does not have the ability to run the code it generates yet.

In this case, it looks like there are values such as code_verifier that would need to be protected. How can a serverless application protect those values while also facilitating the authentication flow?

BC

In a serverless application scenario, where there is no server-side component to securely store and handle sensitive values like the code_verifier, you need to take additional precautions to protect these values. Here are some approaches you can consider:

  1. Ephemeral Storage: Store the code_verifier value in the browser’s memory or ephemeral storage mechanisms like sessionStorage. This way, the value is only available during the current browser session and is not persisted across sessions or page reloads. However, be aware that this approach is vulnerable to Cross-Site Scripting (XSS) attacks and may not be suitable for all scenarios.
  2. Client-side Encryption: Before storing the code_verifier in the browser’s storage (e.g., localStorage), you can encrypt it using a cryptographic library designed for client-side use, such as the Web Crypto API or a third-party library like Stanford JavaScript Crypto Library (SJCL). The encryption key can be derived from user input (e.g., a password or a biometric factor) or generated securely on the client-side. This adds an extra layer of protection, but remember that client-side encryption is still susceptible to potential vulnerabilities like XSS or code injection attacks.
  3. Third-party Storage Services: Instead of storing the code_verifier in the browser’s storage, you can use a secure third-party storage service, such as a secure cloud-based storage solution or a secure key-value store. This approach removes the sensitive data from the client-side application, but it introduces additional dependencies and potential attack vectors (e.g., man-in-the-middle attacks during data transmission).
  4. Hybrid Approach: Combine client-side encryption with a third-party storage service. Encrypt the code_verifier on the client-side using a secure key, and then store the encrypted value in a third-party storage service. This approach provides an additional layer of security by separating the encryption key from the encrypted data.
  5. Short-lived Tokens: Instead of storing the code_verifier for an extended period, consider generating a new code_verifier for each authentication flow and discarding it after the flow is complete. This approach minimizes the window of exposure for the sensitive value but may not be practical for all use cases.

Regardless of the approach you choose, it’s essential to follow best practices for secure communication (e.g., using HTTPS), implement measures to mitigate common web application vulnerabilities (e.g., XSS, CSRF), and consider the trade-offs between security, usability, and complexity.

It’s also worth noting that serverless applications often have inherent security limitations compared to traditional server-side applications. In some cases, it might be beneficial to have a minimal server-side component to handle the more sensitive parts of the authentication flow, such as securely generating and verifying the code_verifier and code_challenge values.

For a ‘serverless’ application, can you please elaborate on the specific risks that ‘authorization code flow with PKCE’ addresses that are not addressed with ‘implicit flow’? I’m interested in the specific exploits that would be mitigated by using one of these vs the other flow.

BC

The Authorization Code Flow with PKCE (Proof Key for Code Exchange) addresses several security risks and potential exploits that are not mitigated by the Implicit Flow, especially in the context of a serverless application. Here are the specific risks and exploits that PKCE helps to mitigate:

  1. Code Injection Attacks: In the Implicit Flow, the access token is directly exposed in the URL fragment, making it susceptible to code injection attacks, such as an attacker injecting malicious code into the client application’s context. PKCE mitigates this risk by using the code_challenge and code_verifier mechanism, which ensures that only the legitimate client application can exchange the authorization code for an access token.
  2. Interception of Access Token: Since the access token is exposed in the URL fragment in the Implicit Flow, it can be potentially intercepted or leaked, for example, through referer headers, browser history, or server logs. PKCE avoids this risk by not exposing the access token directly in the URL.
  3. Cross-Site Scripting (XSS) Attacks: In the Implicit Flow, if an attacker can inject malicious code into the client application’s context through an XSS vulnerability, they may be able to steal the access token from the URL fragment. PKCE mitigates this risk by not exposing the access token directly in the URL.
  4. Lack of Refresh Tokens: The Implicit Flow does not provide a standardized mechanism for obtaining refresh tokens, which are used to acquire new access tokens without requiring the user to re-authenticate. PKCE, combined with the Authorization Code Flow, allows for the issuance of refresh tokens, providing a more robust and long-lived authentication mechanism.
  5. Mismatched Redirect URI: In the Implicit Flow, if an attacker can redirect the user’s browser to a malicious URI, they may be able to intercept the access token. PKCE mitigates this risk by ensuring that the authorization code can only be exchanged for an access token by the legitimate client application with the correct redirect_uri.
  6. Cross-Site Request Forgery (CSRF) Attacks: Both flows are susceptible to CSRF attacks, but PKCE provides an additional layer of protection by using the state parameter, which can be used to mitigate CSRF attacks.

While the Implicit Flow is designed for client-side applications and avoids the need for a secure server-side component, it comes with inherent security risks due to the exposure of the access token in the URL. PKCE, combined with the Authorization Code Flow, provides a more secure alternative by introducing an additional layer of protection through the code_challenge and code_verifier mechanism, mitigating the risks of code injection, token interception, XSS attacks, and mismatched redirect URIs.

However, it’s important to note that PKCE does not address all security risks in a serverless application. Other security measures, such as secure storage of sensitive data, secure communication (HTTPS), and protection against other web application vulnerabilities like XSS and CSRF, must still be implemented to ensure the overall security of the application.

Is it correct to state that the PKCE method primarily mitigates risks against unfriendly 3rd party attacks, but would be much less helpful against a user trying to modify the serverless application themselves, such as by modifying the serverless code running in their browser? Is this generalization correct, and if now, how not?

BC

Your generalization is mostly correct, but there are a few nuances to consider:

  1. Unfriendly Third-Party Attacks: Yes, the PKCE method primarily mitigates risks against unfriendly third-party attacks, such as code injection, token interception, and redirect URI tampering, where an attacker tries to exploit vulnerabilities in the client application or the communication channel between the client and the authorization server.
  2. User Modifying Serverless Application Code: PKCE does provide some protection against a user trying to modify the serverless application code running in their browser, but this protection is limited and depends on the specific scenario.
    • If the user tries to modify the client-side code to bypass the PKCE mechanism and directly exchange the authorization code for an access token, PKCE can help mitigate this attack. The authorization server will reject the request because the code_verifier provided by the modified client-side code will not match the expected code_challenge.
    • However, if the user modifies the client-side code to access and manipulate the code_verifier value itself, PKCE may not provide effective protection. Since the code_verifier is generated and stored on the client-side, a malicious user with access to the code can potentially obtain and misuse this value.
  3. Additional Security Measures: While PKCE provides an additional layer of security, it does not replace the need for other security measures, such as secure communication (HTTPS), protection against Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) attacks, and secure storage of sensitive data. These measures are still necessary to mitigate risks from both third-party attacks and malicious user modifications.
  4. Serverless Application Limitations: In a serverless application scenario, the client-side code is inherently less secure than a server-side component, as it is exposed to the user’s environment. While PKCE helps mitigate certain risks, it does not provide the same level of security as a server-side component that can securely handle sensitive data and authentication flows.

In summary, your generalization is generally correct: PKCE primarily mitigates risks against unfriendly third-party attacks, but it provides more limited protection against a user trying to modify the serverless application code running in their browser. However, it’s essential to implement additional security measures and consider the inherent limitations of serverless applications when it comes to secure authentication and authorization.

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