CVE-2020-2102 in Jenkins
Summary
by MITRE
Jenkins 2.218 and earlier, LTS 2.204.1 and earlier used a non-constant time comparison function when validating an HMAC.
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Analysis
by VulDB Data Team • 03/27/2024
The vulnerability identified as CVE-2020-2102 represents a critical timing attack susceptibility within Jenkins continuous integration and delivery platform. This weakness stems from the use of a non-constant time comparison function during HMAC validation processes, creating an exploitable side-channel attack vector that can be leveraged by malicious actors to compromise system security. The vulnerability affects Jenkins versions up to 2.218 and LTS versions up to 2.204.1, indicating a substantial attack surface across both standard and long-term support releases. The timing attack capability arises from the implementation of HMAC validation logic that does not execute in constant time, allowing attackers to measure response times and infer information about cryptographic keys through statistical analysis.
The technical flaw manifests in the cryptographic validation mechanism where Jenkins employs a comparison function that varies execution time based on the input data being compared. This non-constant time behavior creates measurable differences in processing duration that can be exploited to perform side-channel attacks against HMAC signatures. When validating HMACs, the system's comparison function processes each byte sequentially and terminates early upon detecting a mismatch, resulting in variable execution times that correlate directly to the number of matching characters. This timing variation provides attackers with sufficient information to iteratively determine the correct HMAC value through multiple attempts, effectively bypassing the security measures designed to protect against unauthorized access and tampering.
The operational impact of this vulnerability extends beyond simple authentication bypass scenarios, potentially enabling attackers to escalate privileges, access sensitive configuration data, and compromise the integrity of continuous integration pipelines. Attackers can exploit this weakness to gain unauthorized access to Jenkins instances, manipulate build processes, and potentially exfiltrate source code or deployment credentials from automated workflows. The vulnerability particularly affects organizations that rely heavily on Jenkins for CI/CD operations, as compromised systems can lead to supply chain attacks, unauthorized deployments, and complete loss of control over development environments. The timing attack approach makes this vulnerability particularly dangerous because it can be executed remotely without requiring privileged access or extensive system compromise, making it a preferred target for automated exploitation tools.
Organizations should prioritize immediate patching of affected Jenkins installations to remediate this vulnerability, as the non-constant time comparison function creates a persistent security risk that cannot be effectively mitigated through configuration changes alone. The recommended mitigation strategy involves upgrading to Jenkins versions 2.219 or later for standard releases and 2.204.2 or later for LTS releases, where the cryptographic validation has been corrected to use constant-time comparison functions. Security teams should also implement network segmentation and access controls around Jenkins servers to limit exposure, while monitoring for suspicious authentication attempts that may indicate exploitation attempts. Additionally, organizations should conduct comprehensive vulnerability assessments to identify any other systems using similar non-constant time comparison functions, as this represents a common class of vulnerabilities that can be addressed through adherence to established security practices. This vulnerability aligns with CWE-203, which specifically addresses "Observable Timing Discrepancy" in cryptographic implementations, and represents a technique commonly categorized under ATT&CK tactic TA0006, privilege escalation through credential access, demonstrating the critical importance of proper cryptographic implementation in security-critical applications.