Security readout for executives and security teams
Plain-English summary
GitHub Enterprise Server had an authorization flaw that could let a logged-in attacker merge their own pull request without push access. The issue required a fork-based pull request, clean checks, and no branch protection on the target branch. For affected self-hosted GitHub environments, this is a source-code integrity risk.
Executive priority
Treat as a high-priority platform integrity issue if your organization runs GitHub Enterprise Server. Prioritize upgrades for instances hosting production code, regulated software, or release pipelines. The scenario is constrained, but successful abuse could bypass intended code ownership and review controls.
Technical view
CVE-2026-1999 is CWE-863 incorrect authorization in GitHub Enterprise Server’s enable_auto_merge pull request mutation. It affects versions before 3.17.11, 3.18.5, and 3.19.2. The CVSS 4.0 score is 7.1. Exploitation required low privileges and specific repository conditions involving forks, clean pull request status, and unprotected branches.
Likely exposure
Exposure is limited to GitHub Enterprise Server deployments running affected 3.17.x, 3.18.x, or 3.19.x versions before the fixed releases. Risk applies where repositories allow forking and branches lack branch protection rules. GitHub.com is not identified as affected in the provided sources.
Exploitation context
The sources do not report active exploitation, and the CVE is not marked in KEV. Practical exploitation requires an authenticated attacker, an attacker-controlled fork, a clean pull request status, and a target branch without branch protection. The business impact is unauthorized code changes entering protected workflows or release paths.
Researcher notes
Key conditions are authorization bypass in enable_auto_merge, fork-origin pull request, clean status, and no branch protection. The provided record does not include proof-of-concept details or evidence of exploitation in the wild. Validation should focus on affected versions and repository governance controls, not speculative exploit testing.
Mitigation direction
- Upgrade GitHub Enterprise Server to 3.17.11, 3.18.5, 3.19.2, or later supported releases.
- Review GitHub release notes for any environment-specific upgrade requirements.
- Enable branch protection rules on important branches where appropriate.
- Review repository fork policies for sensitive codebases.
Validation and detection
- Inventory all GitHub Enterprise Server appliance versions.
- Confirm no instance runs below 3.17.11, 3.18.5, or 3.19.2.
- Identify repositories that allow forks and have unprotected target branches.
- Review recent pull request merges from forked repositories for unexpected approvals or automation.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-863: Authorization and privilege behavior lookup
Authorization weaknesses can support privilege escalation and valid-account review, depending on exploit path. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupPrivilege behavior lookup
The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-1999 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.1 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:L/SC:L/SI:H/SA:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:L/SC:L/SI:H/SA:N——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
7.1HighVector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:L/SC:L/SI:H/SA:N
Source materials
- CVE List V5 sourceCVE List V5
- https://docs.github.com/en/enterprise-server@3.17/admin/release-notes#3.17.11CVE reference · release-notes
- https://docs.github.com/en/enterprise-server@3.18/admin/release-notes#3.18.5CVE reference · release-notes
- https://docs.github.com/en/enterprise-server@3.19/admin/release-notes#3.19.2CVE reference · release-notes
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Incorrect Authorization
Incorrect Authorization represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.