Rfc 7636 (CloudMonk.io)
RFC 7636
RFC 7636 defines the Proof Key for Code Exchange (PKCE) protocol, which is an enhancement to the OAuth 2.0 authorization code flow. It addresses a specific vulnerability in the traditional authorization code flow, where authorization codes can be intercepted and used by malicious actors to gain unauthorized access to resources. This vulnerability is especially problematic in public clients, such as mobile and single-page applications, where client secrets cannot be securely stored. PKCE solves this problem by adding a layer of security through the use of a dynamically generated code verifier and code challenge.
The PKCE mechanism was introduced because in public client scenarios, authorization codes could be intercepted during transmission. Attackers could then exchange the stolen authorization codes for access tokens. RFC 7636 mitigates this risk by ensuring that the client must prove possession of the original code verifier used in the authorization request to successfully obtain an access token. This method is particularly effective for securing mobile and web applications, where the client cannot be trusted to store confidential information securely.
PKCE introduces two new parameters in the authorization request process: the `code_challenge` and the `code_challenge_method`. The `code_challenge` is a derived value, usually a SHA-256 hash of a randomly generated code verifier, which is sent to the authorization server during the initial authorization request. The authorization server retains this value and compares it to the `code_verifier`, which is sent by the client during the token exchange. If the values do not match, the authorization server rejects the request, preventing the attacker from exchanging a stolen code.
During the authorization process, the client generates a `code_verifier`, which is a random string with sufficient entropy to ensure security. This verifier is hashed using the SHA-256 algorithm to create the `code_challenge`, which is then sent to the authorization server. The server does not see the verifier at this stage, only the challenge. After the user authenticates and the client receives the authorization code, it must send the original `code_verifier` back to the server when exchanging the authorization code for an access token. The server then hashes the verifier and checks if it matches the stored challenge.
The introduction of PKCE in RFC 7636 marked a significant improvement in the security of the OAuth 2.0 authorization code flow. Without this mechanism, public clients could be easily exploited, as authorization codes could be intercepted and exchanged for access tokens by unauthorized entities. With PKCE, even if the authorization code is intercepted, the attacker cannot complete the token exchange without the original code verifier, which is not transmitted over the network.
Originally, PKCE was intended for use in public clients, but it has since become a recommended best practice for all clients implementing the authorization code flow. The reason for this is that PKCE adds a layer of protection that can defend against various attacks, including authorization code interception, regardless of whether the client is public or confidential. This broad adoption of PKCE across different client types is an important step in standardizing secure OAuth 2.0 implementations.
PKCE is particularly valuable in mobile app environments, where users often connect over untrusted networks like public Wi-Fi. These environments are vulnerable to man-in-the-middle attacks, where attackers could intercept authorization codes. However, with PKCE in place, even if an attacker captures the authorization code, they will be unable to obtain an access token because they do not have access to the original code verifier generated by the legitimate client.
The process of using PKCE is simple for developers to implement and does not require significant changes to existing OAuth 2.0 workflows. It only requires the client to generate the code verifier and challenge, and send the appropriate values during the authorization and token exchange phases. By using PKCE, developers can enhance the security of their OAuth 2.0 applications with minimal overhead, while greatly reducing the risk of code interception attacks.
Platforms such as Google, Facebook, and Microsoft have widely adopted PKCE as a mandatory or recommended practice in their OAuth 2.0 implementations. This helps ensure that third-party applications using these platforms are protected against authorization code interception attacks, further promoting the secure adoption of OAuth 2.0 in mobile and web applications. This shift towards enforcing PKCE has led to its becoming a standard practice in secure API authorization workflows.
The use of PKCE also aligns with other security practices in modern authorization protocols, such as the use of TLS for encrypted communications and token expiration strategies to limit the impact of leaked tokens. By combining these techniques, developers can build secure applications that protect sensitive user data and maintain the integrity of their authorization flows, even in environments that are more prone to security threats.
Conclusion
RFC 7636 introduced PKCE as a critical enhancement to the OAuth 2.0 authorization code flow, providing additional security against code interception attacks, particularly for public clients. By requiring clients to prove possession of the code verifier during the token exchange, PKCE ensures that only the legitimate client can successfully obtain an access token. Its simplicity, effectiveness, and wide adoption make it a crucial component of secure OAuth 2.0 implementations, providing much-needed protection for mobile and web applications in environments where storing client secrets securely is not feasible.
Network Security: Important Security-Related RFCs, Awesome Network Security (navbar_network_security - see also navbar_security, navbar_networking, navbar_rfc)
Request for Comments (RFC): List of RFCs, GitHub RFCs, Awesome RFCs, (navbar_rfc - see also navbar_network_security, navbar_security, navbar_networking)
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