CVE-2021-39914 in GitLab
Summary
by MITRE • 11/05/2021
A regular expression denial of service issue in GitLab versions 8.13 to 14.2.5, 14.3.0 to 14.3.3 and 14.4.0 could cause excessive usage of resources when a specially crafted username was used when provisioning a new user
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Analysis
by VulDB Data Team • 11/09/2021
The vulnerability CVE-2021-39914 represents a critical regular expression denial of service weakness that affected GitLab installations across multiple version ranges including 8.13 through 14.2.5, 14.3.0 through 14.3.3, and 14.4.0. This issue stems from insufficient input validation during user provisioning processes where the system fails to properly sanitize username inputs before processing them through regular expression patterns. The flaw exists within GitLab's user management functionality and specifically targets the username validation mechanism that employs regular expressions to validate user identifiers during account creation. When an attacker crafts a maliciously formatted username string that triggers catastrophic backtracking in the regular expression engine, the system becomes vulnerable to resource exhaustion attacks. This vulnerability operates under the common weakness enumeration CWE-400 which categorizes it as a vulnerability involving uncontrolled resource consumption through regular expression denial of service attacks.
The technical execution of this vulnerability occurs when an attacker submits a specially crafted username during user provisioning that contains patterns designed to cause the regular expression engine to perform exponential time complexity operations. The regular expression patterns used for username validation contain constructs that are susceptible to catastrophic backtracking when processing malformed input strings. This creates a scenario where the system's CPU resources become consumed rapidly as the regular expression engine attempts to match the malicious input against the validation pattern. The operational impact extends beyond simple resource exhaustion as this vulnerability can be exploited to cause system instability, service degradation, and potentially complete system unavailability depending on the computational resources available. The vulnerability affects the core user provisioning functionality of GitLab and can be triggered by any user creation operation that involves username validation, making it particularly dangerous in environments where automated user provisioning occurs frequently.
The attack surface for this vulnerability is significant within GitLab environments as it can be exploited through various user provisioning pathways including manual user creation, automated provisioning scripts, and integration with external identity management systems. The exploitation requires minimal privileges as any user with access to the user creation functionality can potentially trigger the denial of service condition. This vulnerability directly maps to the attack technique T1499.004 from the MITRE ATT&CK framework which describes network denial of service attacks and specifically targets resource exhaustion techniques. Organizations using GitLab versions within the affected ranges face potential operational disruptions that can impact development workflows, CI/CD pipelines, and collaborative software development processes that rely on proper user management functionality. The vulnerability's impact is particularly severe in high-traffic environments where user provisioning occurs frequently, as the resource exhaustion can cascade into broader system performance degradation.
Mitigation strategies for CVE-2021-39914 require immediate patching of GitLab installations to versions that contain the regular expression validation fixes. Organizations should implement input sanitization measures that validate username length and character sets before regular expression processing occurs, effectively preventing malicious patterns from reaching the vulnerable regex engine. Network-level rate limiting and resource monitoring should be implemented to detect and prevent exploitation attempts by monitoring CPU usage patterns during user provisioning operations. Security teams should also consider implementing additional validation layers that can identify and block suspicious username patterns that exhibit characteristics of known malicious inputs. The patching process should include thorough testing to ensure that legitimate user provisioning operations continue to function properly while eliminating the vulnerability. Organizations should also review their user provisioning workflows and implement automated monitoring to detect anomalous resource consumption patterns that might indicate exploitation attempts, particularly focusing on the specific user management operations that trigger the vulnerable code paths.