CVE-2017-12880 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 • 11/07/2019
The vulnerability described in CVE-2017-12880 represents a critical security flaw in the PyJWT library version 1.5.0 and earlier, specifically within the HMACAlgorithm.prepare_key method. This issue stems from an insufficient validation mechanism that fails to properly identify and reject certain PEM encoded public keys, creating a pathway for attackers to exploit the cryptographic system. The flaw particularly affects PKCS1 formatted public keys that begin with the identifier '-----BEGIN RSA PUBLIC KEY--- --' which are not properly accounted for in the invalid_strings check, allowing them to be incorrectly processed as symmetric keys.
The technical nature of this vulnerability can be categorized under CWE-327, which addresses the use of weak cryptographic algorithms and improper implementation of cryptographic functions. The flaw manifests in the cryptographic library's inability to distinguish between symmetric and asymmetric keys during the key preparation phase, enabling a form of key confusion attack. When a PKCS1 PEM encoded public key is processed through the HMACAlgorithm.prepare_key function, the validation logic fails to recognize that this particular key format should be treated as an asymmetric key rather than a symmetric one. This misclassification occurs because the specific string pattern used to identify PKCS1 keys is not included in the list of invalid strings that would normally prevent such keys from being processed in a symmetric context.
The operational impact of this vulnerability is severe and directly enables attackers to perform symmetric/asymmetric key confusion attacks against systems utilizing the affected PyJWT library. An attacker who can influence or control the key material used for JWT signature verification can exploit this flaw to forge valid JWT tokens without possessing the actual secret key. This occurs because the system incorrectly treats PKCS1 formatted public keys as if they were symmetric keys, allowing the attacker to craft signatures that will be accepted by the verification process. The attack vector specifically targets applications that use the library to validate JWT tokens, where the attacker can substitute a maliciously crafted key that appears to be a valid public key but is actually processed as a symmetric key, thereby bypassing the intended security controls.
Mitigation strategies for this vulnerability require immediate library updates to versions that address the insufficient validation logic in the HMACAlgorithm.prepare_key method. Organizations should prioritize upgrading to PyJWT version 1.6.0 or later, where the invalid_strings check has been properly enhanced to include all relevant PEM key formats. Additionally, security teams should implement comprehensive key validation procedures that explicitly check for proper key format identification and ensure that public keys are correctly processed according to their cryptographic type. The remediation process should also include thorough code reviews to identify any instances where JWT libraries are used without proper validation, and security testing should be performed to verify that key confusion attacks are properly mitigated. From an ATT&CK perspective, this vulnerability aligns with techniques involving credential access and privilege escalation through cryptographic weakness, making it particularly dangerous in environments where JWT tokens are used for authentication and authorization purposes.