---
id: CVE-2026-77987
title: >-
  A server-side request forgery (SSRF) vulnerability was identified in the
  notebook viewer of GitHub Enterprise Server
summary: >-
  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
  reques…
severity: critical
cvss: 9.3
cvssVector: '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'
cwe:
  - CWE-208
  - CWE-918
vendor: GitHub
product: Enterprise Server
affected:
  - enterprise_server >= 3.17.0 < 3.17.*
  - enterprise_server >= 3.18.0 < 3.18.*
  - enterprise_server >= 3.19.0 < 3.19.*
  - enterprise_server >= 3.20.0 < 3.20.*
  - enterprise_server >= 3.21.0 < 3.21.*
  - enterprise_server >= 3.22.0 < 3.22.*
published: '2026-09-22'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T21:16:28.120'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77987'
references:
  - url: >-
      https://docs.github.com/en/enterprise-server@3.17/admin/release-notes#3.17.21
    label: product-cna@github.com
  - url: >-
      https://docs.github.com/en/enterprise-server@3.18/admin/release-notes#3.18.15
    label: product-cna@github.com
  - url: >-
      https://docs.github.com/en/enterprise-server@3.19/admin/release-notes#3.19.12
    label: product-cna@github.com
  - url: >-
      https://docs.github.com/en/enterprise-server@3.20/admin/release-notes#3.20.8
    label: product-cna@github.com
  - url: >-
      https://docs.github.com/en/enterprise-server@3.21/admin/release-notes#3.21.6
    label: product-cna@github.com
  - url: >-
      https://docs.github.com/en/enterprise-server@3.22/admin/release-notes#3.22.1
    label: product-cna@github.com
tags:
  - nvd
  - cve.org
epss: 0.00891
epssPercentile: 0.57632
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-09-23T15:26:50.899891Z'
cvssSource: cna
ingestedAt: '2026-09-22T21:11:40.324Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.