Enterprise ServerApplication · GitHub
CVE-2026-5921
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedA 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThis is an SSRF vulnerability in GitHub Enterprise Server's notebook rendering service that allows unauthenticated attackers to reach internal services when private mode is disabled. The vulnerability chains an open redirect with the notebook viewer's failure to revalidate redirect destinations, combined with a timing side-channel attack against internal API regex filters to infer secret environment variables character by character.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Enterprise ServerApplication
< 3.14.26>= 3.15.0, < 3.15.21>= 3.16.0, < 3.16.17>= 3.17.0, < 3.17.14>= 3.18.0, < 3.18.8>= 3.19.0, < 3.19.5= 3.20.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- High
- Privileges
- None
- User interaction
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- High
- Availability
- Low
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify GitHub Enterprise Server versionAccess the GitHub Enterprise Server management console or admin dashboard. The version is displayed on the main admin page or can be retrieved via the /api/v3/version endpoint if unauthenticated access is allowed.Affected if The installed version falls into any of these ranges: < 3.14.26, >= 3.15.0 < 3.15.21, >= 3.16.0 < 3.16.17, >= 3.17.0 < 3.17.14, >= 3.18.0 < 3.18.8, >= 3.19.0 < 3.19.5, or equals exactly 3.20.0.
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Verify private mode configurationNavigate to the GitHub Enterprise Server management console, go to Settings > Authentication, and check the 'Private mode' setting. This setting controls whether the instance requires authentication for viewing repositories.Affected if Private mode is disabled, allowing unauthenticated access to the notebook viewer and enabling the redirect-following behavior.
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Confirm notebook rendering service is accessibleAttempt to access a notebook file (e.g., .ipynb) on the GHES instance without authentication, or verify the notebook rendering endpoint responds to unauthenticated requests.Affected if The notebook viewer endpoint is accessible to unauthenticated users, which is required for the SSRF exploitation chain.
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Check for internal network exposureReview whether the GHES instance can make outbound HTTP requests to internal services. This is typically a default capability but can be restricted via firewall rules.Affected if The GHES server has the ability to follow redirects to internal network addresses, enabling the SSRF attack vector.
You are affected if your GitHub Enterprise Server version is within any of the vulnerable ranges AND private mode is disabled, allowing unauthenticated attackers to exploit the notebook redirect following behavior for SSRF and timing-based secret extraction.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped3.14.263.15.213.16.17
Upgrade GitHub Enterprise Server to a fixed version (3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, 3.19.5, or 3.20.1 or later) and ensure private mode is enabled to prevent exploitation.
3.20.1 (or the latest 3.x release in your upgrade lineage: 3.14.26 for 3.14.x, 3.15.21 for 3.15.x, 3.16.17 for 3.16.x, 3.17.14 for 3.17.x)
- 1. Identify the current GitHub Enterprise Server version by navigating to the management console or running: ghe-version
- 2. Based on the current version, determine the appropriate target upgrade path from the fixed versions: 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, 3.19.5, or 3.20.1
- 3. Back up the GitHub Enterprise Server instance including all data and configuration before proceeding with the upgrade
- 4. Download the appropriate upgrade package from the GitHub Enterprise downloads page matching your target version
- 5. Follow the standard GitHub Enterprise Server upgrade procedure: upload the upgrade package via the management console or run the installation command
- 6. After upgrade completes, verify the instance is functioning correctly
- 7. Confirm the fix is applied by checking that the notebook viewer now properly validates redirect destinations and no longer allows SSRF to internal services when private mode is disabled
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Enterprise Server
Scoped from the published advisory
- Consultation4.0 h
- Implementation16.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,960.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-5921 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-5921 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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