What is CVE-2026-5921?
A 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.
The record classifies CVE-2026-5921 under weakness class CWE-918. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 167 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 8.9 — HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L - CVSS v4.0 base score
- 8.9
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:H/SI:H/SA:L/E:P - EPSS (FIRST)
- 0.0% probability of exploitation in the next 30 days, higher than 23.3% of all scored CVEs
- CISA KEV
- Not listed in the CISA Known Exploited Vulnerabilities catalog
- Threadlinqs priority
- 5.4/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- Published
- 2026-04-21, last modified 2026-04-22
Is CVE-2026-5921 being exploited?
It currently carries a trending score of 11 in the Threadlinqs vulnerability feed.
Affected products and versions
- Github: Enterprise Server
How to fix CVE-2026-5921
Vendor advisory: https://docs.github.com/en/enterprise-server@3.14/admin/release-notes#3.14.26. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2026-5921
1 tracked threat in the Threadlinqs corpus references CVE-2026-5921, either in the campaign’s CVE list or as an indicator on the campaign record.
Sources
Seeded from cveorg and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.