CVE-2026-77987: GitHub Enterprise Server notebook viewer vulnerable to Server-side request forgery

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-22 · updated 2026-10-02
Weakness: CWE-208

Description

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.

Affected products

GitHub Enterprise Server: from 3.17.0 before 3.17.21; from 3.18.0 before 3.18.15; from 3.19.0 before 3.19.12; from 3.20.0 before 3.20.8; from 3.21.0 before 3.21.6; from 3.22.0 before 3.22.1 — fixed in 3.17.21, 3.18.15, 3.19.12, 3.20.8, 3.21.6, 3.22.1

How to fix

Upgrade to a fixed version: GitHub Enterprise Server 3.17.21, 3.18.15, 3.19.12, 3.20.8. Check the vendor advisory for mitigations.

References

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