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

• Severity: high (CVSS 8.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-21 · updated 2026-10-26
• Weakness: CWE-918

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.

Affected products

• GitHub Enterprise Server: before 3.14.26; from 3.15.0 before 3.15.21; from 3.16.0 before 3.16.17; from 3.17.0 before 3.17.14; from 3.18.0 before 3.18.8; from 3.19.0 before 3.19.5 — fixed in 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, 3.19.5

How to fix

Upgrade to a fixed version: GitHub Enterprise Server 3.14.26, 3.15.21, 3.16.17, 3.17.14. Check the vendor advisory for mitigations.

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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