CVE-2021-22861 in GitHub
Summary
by MITRE • 03/03/2021
An improper access control vulnerability was identified in GitHub Enterprise Server that allowed authenticated users of the instance to gain write access to unauthorized repositories via specifically crafted pull requests and REST API requests. An attacker would need to be able to fork the targeted repository, a setting that is disabled by default for organization owned private repositories. Branch protections such as required pull request reviews or status checks would prevent unauthorized commits from being merged without further review or validation. This vulnerability affected all versions of GitHub Enterprise Server since 2.4.21 and was fixed in versions 2.20.24, 2.21.15, 2.22.7 and 3.0.1. This vulnerability was reported via the GitHub Bug Bounty program.
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Analysis
by VulDB Data Team • 03/14/2021
The vulnerability CVE-2021-22861 represents a critical access control flaw in GitHub Enterprise Server that demonstrates how insufficient authorization checks can enable authenticated users to escalate their privileges within a repository ecosystem. This issue specifically targeted the permissions model of GitHub Enterprise Server, where legitimate users could exploit weaknesses in the pull request and REST API handling mechanisms to gain write access to repositories they should not be authorized to modify. The flaw existed in the server's validation logic for repository access controls, creating a pathway for privilege escalation that bypassed normal security boundaries. The vulnerability's impact was particularly concerning because it allowed authenticated users to manipulate repository contents without proper authorization, potentially leading to data corruption, unauthorized code modifications, or information disclosure within organization repositories. The security implications extend beyond simple access control violations as they represent a fundamental breakdown in the trust model that GitHub Enterprise Server relies upon to maintain repository integrity and user permissions.
The technical implementation of this vulnerability stems from inadequate validation of repository access permissions during pull request processing and REST API operations. Attackers could craft specific requests that exploited gaps in the authorization framework, allowing them to bypass normal repository access controls. The flaw specifically required users to have the ability to fork the target repository, which is a default restriction for organization-owned private repositories, meaning the vulnerability was not universally exploitable but rather required specific conditions to be met. The system's failure to properly validate user permissions during critical operations such as pull request creation and API request processing created opportunities for malicious actors to manipulate repository access controls. This type of vulnerability aligns with CWE-284 Access Control Issues, which encompasses improper access control mechanisms that allow unauthorized access to resources. The vulnerability's exploitation required careful crafting of API requests and pull request parameters to leverage the access control gap, making it a sophisticated but potentially damaging flaw in the platform's security architecture.
The operational impact of CVE-2021-22861 extends beyond immediate repository compromise to potentially affect entire organization security postures and development workflows. Organizations using GitHub Enterprise Server were vulnerable to unauthorized modifications of their code repositories, which could result in malicious code injection, data integrity violations, or unauthorized changes to production code. The vulnerability's presence in all versions since 2.4.21 meant that a substantial portion of GitHub Enterprise Server installations were at risk, requiring immediate attention and patching across affected environments. Branch protection mechanisms such as required pull request reviews and status checks served as partial mitigations, but these protections were insufficient to prevent the exploitation when the underlying access control vulnerability was present. The fact that this vulnerability was discovered through the GitHub Bug Bounty program highlights the importance of external security research in identifying such flaws, as internal testing may not always reveal all access control weaknesses in complex enterprise platforms. Organizations with extensive GitHub Enterprise Server deployments faced significant operational risk, as the vulnerability could potentially be used to compromise sensitive code repositories and development environments.
Mitigation strategies for CVE-2021-22861 centered primarily on applying the vendor-provided security patches that addressed the underlying access control implementation flaws. Organizations needed to upgrade their GitHub Enterprise Server installations to versions 2.20.24, 2.21.15, 2.22.7, or 3.0.1 to eliminate the vulnerability. Beyond patching, organizations should conduct comprehensive security assessments of their repository access controls and review existing user permissions to identify any potential unauthorized access that may have occurred during the vulnerability's active period. The remediation process required careful planning to ensure that the patching did not disrupt existing development workflows while maintaining security integrity. Security teams should also implement additional monitoring of API access patterns and pull request activities to detect anomalous behavior that might indicate exploitation attempts. The vulnerability's resolution through official patches aligns with standard security practices for addressing access control flaws, though the incident demonstrated the need for more robust testing of authorization mechanisms in enterprise platforms. Organizations should consider implementing additional security controls such as automated access control auditing and continuous monitoring of repository modifications to prevent similar issues from affecting their development environments. The vulnerability's classification under ATT&CK technique T1078 Valid Accounts and T1566 Phishing highlights the importance of maintaining proper account governance and monitoring for unauthorized access patterns in enterprise security programs.