XSS in GitHub Enterprise Server
CVE-2026-77912
A stored cross-site scripting (XSS) vulnerability was identified in GitHub Enterprise Server that allowed an authenticated attacker to inject arbitrary HTML attributes into rendered Markdown because the Markdown rendering pipeline rewrote quote characters in already-sanitized HTML without re-sanitizing the result. Crafted Markdown could abuse same-origin JavaScript gadgets to bypass Content Security Policy and gain control of the page DOM when viewed by another user. Successful exploitation could allow an attacker to read content visible to the victim, extract embedded CSRF tokens, perform state-changing actions as the victim, and exfiltrate data through same-origin writes. The payload could also propagate to repositories and organizations where the victim had write access. This vulnerability affected supported GitHub Enterprise Server releases in the 3.17, 3.18, 3.19, 3.20, 3.21, and 3.22 series and was fixed in versions 3.22.1, 3.21.6, 3.20.8, 3.19.12, 3.18.15, and 3.17.21. This vulnerability was reported via the GitHub Bug Bounty program.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.002 (10.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 11th percentile — 10.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000
Affected products
Weakness classification (CWE)
References
- docs.github.com/en/enterprise-server@3.17/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.18/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.19/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.20/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.21/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.22/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-77912?
- CVE-2026-77912 is a medium-severity vulnerability in GitHub Enterprise Server, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2026-09-22.
- How severe is CVE-2026-77912?
- Medium severity. CVSS v3 base score is 5.4 out of 10.