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9.3
CVE-2026-77987: GitHub Enterprise Server notebook viewer can leak secrets
CVE-2026-77987 · published 4 days ago
Summary
The notebook viewer in GitHub Enterprise Server versions 3.17 to 3.22 lets an attacker send crafted URLs that reach internal services on the same machine. By measuring response times, the attacker can gradually discover passwords or keys, which could later be used to run code on the server. Update to the latest patched release as soon as possible or apply the vendor’s recommended mitigations.
What to do
- Update github enterprise server to version 3.17.21.
Affected software
| Vendor | Product | Affected versions |
|---|---|---|
| github | enterprise server |
< 3.17.* Fix: upgrade to 3.17.21
|
Original advisory text
GitHub Enterprise Server notebook viewer vulnerable to Server-side request forgery
A server-side request forgery (SSRF) vulnerability was identified in the notebook viewer of GitHub Enterprise Server. The notebook viewer validated the scheme and host of a user-supplied URL but did not validate the port, allowing requests to be directed to internal services listening on other ports of the same appliance. Response bodies were not returned to the requester, but response timing acted as an oracle that allowed instance secrets to be extracted character by character. An extracted secret could then be used in a separate interaction with an internal service to obtain remote code execution on the appliance. Exploitation required network access to the instance and was unauthenticated when private mode was disabled, or required any authenticated user when private mode was enabled. This vulnerability affected GitHub Enterprise Server versions 3.17 through 3.22 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 through the GitHub Bug Bounty program.
References
- https://docs.github.com/en/[email protected]/admin/release-notes#3.17.21
- https://docs.github.com/en/[email protected]/admin/release-notes#3.18.15
- https://docs.github.com/en/[email protected]/admin/release-notes#3.19.12
- https://docs.github.com/en/[email protected]/admin/release-notes#3.20.8
- https://docs.github.com/en/[email protected]/admin/release-notes#3.21.6
- https://docs.github.com/en/[email protected]/admin/release-notes#3.22.1
Severity
9.3
Critical
Exploitation
EPSS <1%
Type
CWE-918Server-Side Request Forgery (SSRF)
CWE-208Observable Timing Discrepancy
Timeline
Published22 Sep 2026
Updated27 Sep 2026
First seen22 Sep 2026
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