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Vulnerability · Published Sep 22, 2026 · Assigned by GitHub_P

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

Critical 9.3

In plain English

A critical-severity server-side request forgery in Enterprise Server from GitHub. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.9% (higher than 59% of all known vulnerabilities).

Severity (CVSS 4.0)9.3Critical
Exploit odds (30 days)0.9%top 40.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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 (as reported)

VendorProductVersions
GitHub Enterprise Server 3.17.0 – before 3.17.*3.18.0 – before 3.18.*3.19.0 – before 3.19.*3.20.0 – before 3.20.*3.21.0 – before 3.21.*3.22.0 – before 3.22.*

Weakness: CWE-918, CWE-208 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

References