SSRF in GitHub Enterprise Server
CVE-2026-5921
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.
Vulnerability class: SSRF (Server-Side Request Forgery)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.9 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L.
EPSS exploit prediction
EPSS: 0.006 (49.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 49.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.006 · change: +0.002
Affected products
- GitHub Enterprise Server — versions 3.14.0, 3.15.0, 3.16.0
- GitHub Enterprise Server — versions 3.20.0
Weakness classification (CWE)
References
- docs.github.com/en/enterprise-server@3.14/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.15/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- docs.github.com/en/enterprise-server@3.16/admin/release-notes (release-notes, Release Notes, Vendor Advisory)
- 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)
Frequently asked questions
- What is CVE-2026-5921?
- CVE-2026-5921 is a high-severity vulnerability in GitHub Enterprise Server, classified under Server-Side Request Forgery (SSRF). CVSS score: 8.9/10. Published 2026-04-21.
- How severe is CVE-2026-5921?
- High severity. CVSS v3 base score is 8.9 out of 10.