CVE-2015-9235 in Jsonwebtoken Module
Summary
by MITRE
In jsonwebtoken node module before 4.2.2 it is possible for an attacker to bypass verification when a token digitally signed with an asymetric key (RS/ES family) of algorithms but instead the attacker send a token digitally signed with a symmetric algorithm (HS* family)
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Analysis
by VulDB Data Team • 03/17/2023
The vulnerability identified as CVE-2015-9235 represents a critical security flaw in the jsonwebtoken node module that affected versions prior to 4.2.2. This issue stems from improper implementation of algorithm validation during JWT token verification processes, creating a significant bypass opportunity for malicious actors seeking to compromise authentication systems. The vulnerability specifically impacts applications that utilize asymmetric key algorithms such as RS256, RS384, RS512, ES256, ES384, and ES512 for signing tokens, yet fail to properly enforce algorithm consistency during verification. The flaw allows attackers to exploit a mismatch between the expected asymmetric signing algorithm and the actual symmetric signing algorithm used in the token payload.
The technical implementation of this vulnerability occurs due to inadequate validation of the algorithm field within the JWT token header. When an application expects a token signed with an asymmetric algorithm such as RS256, it should verify that the token was indeed signed using an asymmetric key. However, the vulnerable implementation fails to enforce this constraint, permitting attackers to substitute a token signed with a symmetric algorithm like HS256, HS384, or HS512. This substitution attack exploits the fact that the jsonwebtoken module does not strictly validate that the algorithm specified in the token header matches the expected verification algorithm. The vulnerability can be categorized under CWE-347, which addresses improper certificate validation and authentication bypass issues in cryptographic implementations.
The operational impact of CVE-2015-9235 extends beyond simple authentication bypass, potentially enabling attackers to escalate privileges, access unauthorized resources, and compromise entire authentication systems. Applications relying on JWT tokens for session management, API access control, and user authentication become vulnerable to this attack vector, particularly those using asymmetric key pairs for security-sensitive operations. The vulnerability allows attackers to forge tokens that would normally require access to private keys, effectively rendering asymmetric encryption mechanisms useless for authentication purposes. This type of vulnerability maps directly to ATT&CK technique T1550.002, which describes the use of valid credentials to gain access to systems, as attackers can effectively impersonate legitimate users by bypassing the expected cryptographic verification process. The attack requires minimal technical expertise and can be automated, making it particularly dangerous for widespread exploitation.
Mitigation strategies for CVE-2015-9235 involve immediate upgrading of the jsonwebtoken module to version 4.2.2 or later, where proper algorithm validation has been implemented. Organizations should also implement explicit algorithm whitelisting in their token verification processes, ensuring that tokens are validated against the expected signature algorithm. Security configurations should enforce strict algorithm matching between the expected signing method and the actual token signature, preventing the substitution attack pattern. Additionally, developers should implement comprehensive logging and monitoring of authentication events to detect potential exploitation attempts. The vulnerability highlights the importance of adhering to cryptographic best practices and proper input validation, as outlined in NIST SP 800-57 and other security standards that emphasize the need for robust algorithm validation in cryptographic implementations. Organizations should also consider implementing additional security layers such as token binding and challenge-response mechanisms to provide defense-in-depth against similar vulnerabilities.