CVE-2017-11424 in PyJWT
Summary
by MITRE
In PyJWT 1.5.0 and below the `invalid_strings` check in `HMACAlgorithm.prepare_key` does not account for all PEM encoded public keys. Specifically, the PKCS1 PEM encoded format would be allowed because it is prefaced with the string `-----BEGIN RSA PUBLIC KEY-----` which is not accounted for. This enables symmetric/asymmetric key confusion attacks against users using the PKCS1 PEM encoded public keys, which would allow an attacker to craft JWTs from scratch.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability CVE-2017-11424 resides in the PyJWT library version 1.5.0 and earlier, representing a critical security flaw that undermines the integrity of JSON Web Token authentication mechanisms. This issue stems from an insufficient validation process within the HMACAlgorithm.prepare_key method, where the library fails to properly identify and reject certain PEM encoded public keys. The flaw specifically targets PKCS1 formatted public keys that begin with the identifier `-----BEGIN RSA PUBLIC KEY-----`, which are incorrectly accepted as valid keys despite being asymmetric in nature. This oversight creates a fundamental misalignment between the cryptographic operations and the key type validation, enabling attackers to exploit the system through a sophisticated key confusion attack vector.
The technical implementation of this vulnerability exploits a gap in the input validation logic that should distinguish between symmetric and asymmetric cryptographic keys. When PyJWT processes a JWT token, it performs a check to validate the key type against the expected algorithm, but the `invalid_strings` validation in HMACAlgorithm.prepare_key does not comprehensively cover all valid PEM formats. This particular PKCS1 format, while legitimate for asymmetric encryption, bypasses the validation because the specific string pattern `-----BEGIN RSA PUBLIC KEY-----` is not included in the list of forbidden identifiers. The consequence of this oversight allows attackers to substitute a symmetric key with a PKCS1 formatted public key, effectively creating a scenario where the system treats an asymmetric key as if it were a symmetric key. This confusion enables attackers to forge JWT tokens by crafting signatures using their own private keys, which the system incorrectly validates as legitimate due to the malformed key type checking.
The operational impact of CVE-2017-11424 extends beyond simple authentication bypasses, creating a significant risk for systems relying on JWT tokens for authorization and identity management. Organizations using affected PyJWT versions face potential unauthorized access to protected resources, as attackers can generate valid JWT tokens without possessing the actual secret key. This vulnerability directly maps to attack techniques described in the MITRE ATT&CK framework under credential access and privilege escalation categories, where adversaries exploit implementation flaws to manipulate authentication mechanisms. The attack surface includes any application that utilizes PyJWT for token validation, particularly web applications, API gateways, and microservices architectures that depend on JWT-based authentication. The vulnerability is particularly dangerous because it can remain undetected for extended periods, allowing attackers to maintain persistent access to systems while the legitimate key management processes continue to operate normally, creating a stealthy attack vector that can be difficult to trace and remediate.
The remediation strategy for CVE-2017-11424 requires immediate upgrading to PyJWT version 1.5.1 or later, where the fix addresses the incomplete validation by properly including all PEM format identifiers in the `invalid_strings` check. System administrators should also implement comprehensive key management practices, including regular audits of cryptographic key usage and validation of all token generation processes. Organizations should consider implementing additional security controls such as token introspection services, enhanced monitoring of authentication patterns, and regular penetration testing to identify similar implementation flaws. The vulnerability aligns with CWE-295, which addresses improper certificate validation, and demonstrates the importance of comprehensive input validation in cryptographic libraries. Security teams should also consider implementing automated dependency scanning tools to identify vulnerable library versions and establish processes for rapid patch deployment across all affected systems. The fix implemented in newer versions ensures that all PEM encoded public keys, including PKCS1 format, are properly rejected when used with HMAC algorithms, thereby preventing the symmetric/asymmetric key confusion that enabled the attack vector.