CVE-2026-1999: 
GitHub Enterprise Server vulnerability analysis and mitigation

CVE-2026-1999 is an incorrect authorization vulnerability in GitHub Enterprise Server (GHES) that allows an authenticated attacker to merge their own pull request into a target repository without having push access. The flaw exists in the enable_auto_merge GraphQL mutation for pull requests and was disclosed on February 18, 2026. It affects GHES versions prior to 3.17.11, 3.18.5, and 3.19.2, and was fixed in those respective releases. The vulnerability carries a CVSS v3.1 base score of 6.5 (Medium) and a CVSS v4.0 base score of 7.1 (High) (Feedly, GitHub Docs 3.19).

The root cause is an authorization bypass (CWE-863: Incorrect Authorization) in the enable_auto_merge GraphQL mutation. When an attacker opens a pull request from an attacker-controlled fork into a target repository, the mutation fails to properly verify that the caller has push access to the target branch before enabling auto-merge, allowing the PR to be merged without the required permissions. Exploitation requires the target repository to permit forking, the pull request to have a clean (passing) status, and the target branch to lack branch protection rules. No public proof-of-concept code has been identified at this time (Feedly).

Successful exploitation allows an attacker to inject unauthorized code changes into a target repository's branch by merging a pull request they control, directly impacting code integrity. This could be leveraged to introduce malicious code into software supply chains, backdoor applications, or tamper with configuration files. Confidentiality and availability are not directly impacted, but the integrity impact is rated High given the potential for unauthorized code to propagate through CI/CD pipelines (Feedly).

No public exploit code or in-the-wild exploitation has been reported as of the time of this report. The vulnerability was discovered and responsibly disclosed through the GitHub Bug Bounty program. The EPSS score is approximately 0.036%, indicating a low probability of near-term exploitation. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation is constrained by specific preconditions: the repository must allow forking, the PR must have a clean status, and the target branch must lack branch protection rules (Feedly).

1. Reconnaissance: Identify a target GitHub Enterprise Server instance running a vulnerable version (prior to 3.17.11, 3.18.5, or 3.19.2) that hosts repositories with forking enabled and branches without branch protection rules.
2. Fork the target repository: Using a valid (low-privileged) account, fork the target repository to create an attacker-controlled copy.
3. Create a pull request: Open a pull request from the attacker's fork targeting a branch in the upstream repository that has no branch protection rules and a clean (passing) CI/CD status.
4. Invoke the vulnerable mutation: Send a crafted GraphQL request invoking the enable_auto_merge mutation on the pull request, bypassing the push-access authorization check.
5. Achieve unauthorized merge: The authorization bypass causes the server to enable auto-merge and subsequently merge the attacker's pull request into the target branch without verifying push permissions, injecting the attacker's code changes into the repository (Feedly).

• Logs: Audit log entries showing enable_auto_merge GraphQL mutations initiated by users who do not have push access to the target repository; unexpected pull_request.merged events from fork-originated PRs on branches without protection rules.
• Repository Activity: Pull requests from external forks merged into branches without branch protection rules by users lacking push permissions; unexpected commits appearing in protected or sensitive branches originating from fork accounts.
• Network: GraphQL API requests to the GHES instance's /api/graphql endpoint containing enableAutoMerge or enable_auto_merge mutation payloads from low-privileged user tokens.

GitHub has released patched versions addressing this vulnerability: 3.17.11, 3.18.5, and 3.19.2. Administrators should upgrade to one of these versions as the primary remediation. As a temporary workaround, administrators can disable forking on sensitive repositories or enable branch protection rules on all critical branches to eliminate the preconditions required for exploitation. Restricting repository access to trusted contributors also reduces exposure (GitHub Docs 3.17, GitHub Docs 3.18, GitHub Docs 3.19).

The vulnerability was reported through the GitHub Bug Bounty program and disclosed by GitHub as part of their standard security advisory process. No notable independent researcher commentary or significant media coverage beyond standard vulnerability aggregator reporting has been identified for this CVE (Feedly).