SSRF timing side-channel secret exfiltration in GitHub Enterprise Server notebook viewer
CVE-2026-5921 is a server-side request forgery vulnerability in GitHub Enterprise Server (GHES) affecting versions prior to 3.21. In affected deployments, the notebook rendering/viewer service would follow HTTP redirects without revalidating the destination host when private mode was disabled. This allowed an unauthenticated attacker to abuse the notebook viewer as an SSRF primitive to reach internal services. The issue could then be chained with regex-based filter queries against an internal API and a timing side channel to infer sensitive environment variable values character by character. The attack relied on measuring response-time differences to determine whether guessed characters matched secret data. The vulnerability was fixed in GHES versions 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.8, 3.19.5, and 3.20.1.
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Impact, mitigation & remediation
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Impact
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Mitigation
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Exploits
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Affected products & vendors
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Recent activity
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Every observed campaign linking this CVE to a named adversary.
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Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.