PHP JWT Framework Algorithm Confusion Vulnerability (TOCTOU)

The web-token/jwt-framework and web-token/jwt-library PHP packages are affected by a Time-of-Check/Time-of-Use (TOCTOU) vulnerability that allows attackers to perform algorithm confusion attacks. Specifically, in JWSVerifier::getAlgorithm() and JWEDecrypter, header merging logic (... spread operator or array_merge()) incorrectly prioritizes the unprotected alg (algorithm) parameter over the integrity-protected one when duplicate keys exist. This means that while the protected header's alg might be validated (e.g., RS256), the actual signature verification or decryption might proceed with an attacker-specified alg from the unprotected header (e.g., HS256 or none). This bypasses cryptographic integrity checks, enabling authentication bypass, unauthorized access, or information disclosure, making it critical for applications relying on these libraries for secure JWT handling.

Attack Chain

1. Initial Access / Reconnaissance: An attacker identifies a web application utilizing JSON Web Tokens (JWTs) for authentication or authorization.
2. Malicious JWT Creation: The attacker crafts a JWT containing a protected header with a strong, integrity-protected algorithm (e.g., alg: RS256) and an unprotected header specifying a weaker or symmetric algorithm (e.g., alg: HS256 or alg: none), intending for the unprotected alg to override the protected one.
3. Token Submission: The attacker sends this crafted, malicious JWT to the vulnerable web application, typically within an HTTP Authorization header or as a cookie.
4. Header Merging (TOCTOU): Upon receiving the JWT, the application's JWSVerifier or JWEDecrypter component merges the protected and unprotected headers. Due to the vulnerability, the alg parameter from the unprotected header overwrites the alg from the protected header in the internal merged array.
5. Algorithm Validation (Time-of-Check): An initial check (e.g., by HeaderCheckerManager) might validate the alg from the protected header (e.g., RS256), which passes, creating a false sense of security.
6. Signature/Decryption (Time-of-Use): The JWSVerifier or JWEDecrypter proceeds to verify the JWT signature (or decrypt the payload) using the alg parameter that was overridden by the unprotected header (e.g., HS256 or none).
7. Authentication Bypass / Data Compromise: If the attacker chose an alg like none or could forge a valid signature for a symmetric key (HS256), the system may successfully validate the JWT.
8. Impact: This leads to unauthorized access, impersonation of legitimate users, or decryption of sensitive data, allowing the attacker to bypass authentication mechanisms.

Impact

If exploited, this vulnerability leads to a severe authentication bypass, allowing attackers to forge valid JSON Web Tokens (JWTs) and gain unauthorized access to web applications. This could result in full account takeover, privilege escalation, and access to sensitive data or functionality that should be restricted. The impact is significant for applications that rely on web-token/jwt-framework or web-token/jwt-library for secure session management, API authentication, or inter-service communication. Organizations across all sectors using PHP applications with these specific JWT libraries are at risk, as the integrity of their authentication and authorization mechanisms is compromised.

Recommendation

• Immediately update composer/web-token/jwt-framework to a patched version (e.g., newer than 4.2.99) to address the algorithm confusion vulnerability.
• Immediately update composer/web-token/jwt-library to a patched version (e.g., >= 3.4.10, >= 4.0.7, >= 4.1.7) to address the algorithm confusion vulnerability.
• Review application logs for entries indicating JWT verification failures or unexpected algorithm usage for authentication (refer to the Detect JWT Algorithm Verification Errors rule).
• Ensure verbose application logging is enabled for JWT processing and verification steps to aid in detection of anomalous alg parameter usage (refer to the Detect JWT 'none' Algorithm Usage rule).
• Deploy the Sigma rules in this brief to your SIEM and tune for your environment, specifically for webserver logs that might contain application-level JWT processing details.