CVE-2021-39940 in Community Edition
Summary
by MITRE • 12/13/2021
An issue has been discovered in GitLab CE/EE affecting all versions starting from 13.2 before 14.3.6, all versions starting from 14.4 before 14.4.4, all versions starting from 14.5 before 14.5.2. GitLab Maven Package registry is vulnerable to a regular expression denial of service when a specifically crafted string is sent.
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Analysis
by VulDB Data Team • 12/16/2021
The vulnerability CVE-2021-39940 represents a critical security flaw in GitLab's Maven package registry functionality that affects multiple version ranges including 13.2 through 14.3.5, 14.4 through 14.4.3, and 14.5 through 14.5.1. This issue manifests as a regular expression denial of service vulnerability that specifically targets the package registry's handling of malformed input strings. The vulnerability exists within GitLab's community and enterprise editions, making it a widespread concern for organizations utilizing these platforms for software package management. The flaw exploits the regular expression engine's handling of specially crafted strings that can cause excessive backtracking during pattern matching operations.
The technical implementation of this vulnerability stems from insufficient input validation within GitLab's Maven registry processing logic. When a malicious user submits a specially constructed string to the package registry, the system's regular expression matching algorithms become susceptible to catastrophic backtracking scenarios. This occurs because the regular expressions used to parse and validate package metadata contain patterns that can be exploited through input manipulation to cause exponential execution time growth. The vulnerability is classified under CWE-400 as a Regular Expression Denial of Service (ReDoS) which is a well-documented class of vulnerabilities affecting systems that rely heavily on regular expression processing for input validation and parsing.
The operational impact of CVE-2021-39940 extends beyond simple service disruption as it can be leveraged for broader attack vectors within GitLab environments. An attacker could potentially exploit this vulnerability to consume excessive system resources, causing denial of service conditions that affect legitimate users attempting to access package repositories. The vulnerability affects the core package registry functionality, which means that organizations relying on GitLab for software dependency management could experience complete service unavailability during exploitation attempts. This makes the vulnerability particularly dangerous in CI/CD environments where package repositories are frequently accessed and where such disruptions can cascade into larger development workflow failures.
Organizations should prioritize immediate remediation by upgrading to GitLab versions 14.3.6, 14.4.4, or 14.5.2 which contain patches addressing the regular expression denial of service vulnerability. System administrators should also implement network-level mitigations including rate limiting and input validation at proxy layers to reduce the impact of potential exploitation attempts. The vulnerability aligns with ATT&CK technique T1499.004 which covers network denial of service attacks and represents a classic example of how seemingly benign input validation can become a critical security weakness. Additionally, implementing proper input sanitization and limiting the complexity of regular expressions used in package metadata parsing would provide additional defense in depth measures against similar future vulnerabilities. Security monitoring should be enhanced to detect unusual patterns in package registry access that might indicate exploitation attempts, particularly focusing on unusual string patterns that could trigger catastrophic backtracking behaviors in the system's regular expression engines.