Auth bypass in Github Enterprise Server

CVE-2026-1999

An incorrect authorization vulnerability was identified in GitHub Enterprise Server that allowed an attacker to merge their own pull request into a repository without having push access by exploiting an authorization bypass in the enable_auto_merge mutation for pull requests. This issue only affected repositories that allow forking as the attack relies on opening a pull request from an attacker-controlled fork into the target repository. Exploitation was only possible in specific scenarios. It required a clean pull request status and only applied to branches without branch protection rules enabled. This vulnerability affected GitHub Enterprise Server versions prior to 3.19.2, 3.18.5, and 3.17.11, and was fixed in versions 3.19.2, 3.18.5, and 3.17.11. This vulnerability was reported via the GitHub Bug Bounty program.

Vulnerability class: Broken Access Control

EPSS: 0.003 (21.6th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N.

Affected products

Weakness classification (CWE)

Public proof-of-concept exploits

References

Frequently asked questions

What is CVE-2026-1999?
CVE-2026-1999 is a medium-severity vulnerability in Github Enterprise Server, classified under Incorrect Authorization. CVSS score: 6.5/10. Published 2026-02-18.
How severe is CVE-2026-1999?
Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2026-1999 known to be exploited?
2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.