CVE-2016-10555 in jwt-simple
Summary
by MITRE
Since "algorithm" isn't enforced in jwt.decode()in jwt-simple 0.3.0 and earlier, a malicious user could choose what algorithm is sent sent to the server. If the server is expecting RSA but is sent HMAC-SHA with RSA's public key, the server will think the public key is actually an HMAC private key. This could be used to forge any data an attacker wants.
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Analysis
by VulDB Data Team • 03/19/2023
The vulnerability described in CVE-2016-10555 represents a critical security flaw in the jwt-simple library version 0.3.0 and earlier, specifically within the jwt.decode() function implementation. This issue stems from the absence of algorithm enforcement during JSON Web Token validation, creating a fundamental weakness that allows attackers to manipulate the cryptographic verification process. The flaw occurs when the server expects tokens to be signed using RSA algorithms but receives tokens signed with HMAC-SHA algorithms, where the RSA public key is mistakenly treated as an HMAC secret key. This misinterpretation of cryptographic algorithms creates a dangerous scenario where the server's trust model becomes compromised, as it validates tokens using incorrect cryptographic parameters.
The technical implementation of this vulnerability exploits the trust relationship between the JWT library and the server's cryptographic expectations. When a malicious user sends a token with an HMAC-SHA signature but claims it was signed with RSA, the jwt-simple library fails to verify that the algorithm specified in the token header matches the algorithm expected by the server. This mismatch allows an attacker to effectively bypass the intended cryptographic security measures by using the public key as a secret key for HMAC generation. The vulnerability directly maps to CWE-347, which addresses the improper verification of cryptographic signatures, and demonstrates how weak cryptographic implementation can lead to complete bypass of authentication mechanisms. The flaw operates at the intersection of authentication and cryptographic integrity, where the server's expectation of RSA signatures is undermined by the attacker's ability to manipulate the signature algorithm.
The operational impact of this vulnerability is severe and far-reaching, as it enables complete token forgery capabilities for attackers. An attacker can forge any data they wish to include in the token payload by leveraging the server's incorrect handling of cryptographic algorithms. This means that malicious users can impersonate legitimate users, gain unauthorized access to protected resources, and manipulate the system according to their malicious intent. The vulnerability affects any system relying on jwt-simple for JWT validation, particularly those implementing server-side token verification logic that doesn't explicitly enforce algorithm matching. The attack vector is particularly dangerous because it can be executed without requiring knowledge of the actual private keys, as the system incorrectly validates tokens using the public key as a secret. This vulnerability directly aligns with ATT&CK technique T1552.001, which covers "Unsecured Credentials" and demonstrates how improper cryptographic implementation can lead to credential compromise.
Mitigation strategies for this vulnerability require immediate implementation of explicit algorithm enforcement in JWT validation processes. Organizations should upgrade to jwt-simple version 0.4.0 or later, where the algorithm enforcement has been properly implemented. The recommended approach involves explicitly specifying the expected algorithm during token decoding and ensuring that the algorithm in the token header matches the expected algorithm. Server-side implementations must validate that the algorithm specified in the JWT header matches the algorithm used for verification, preventing the scenario where an attacker can manipulate the cryptographic parameters. Additionally, system administrators should implement proper JWT validation practices that include explicit algorithm checking, which aligns with security best practices outlined in NIST SP 800-57 for cryptographic key management. The fix should also include comprehensive testing of JWT validation logic to ensure that all cryptographic parameters are properly verified before accepting any token as valid, thereby preventing the exploitation of this algorithm enforcement bypass vulnerability.