What is CVE-2026-77987?

CVE-2026-77987 is GitHub Enterprise Server notebook viewer vulnerable to Server-side request forgery. The supplied enrichment data classifies it as SSRF mapped to CWE-918 Server-Side Request Forgery (SSRF); CWE-208 CWE-208.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-77987. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-77987 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-77987. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS, cloud metadata access logs, egress proxy logs where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-77987 using CVSS 9.3, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

criticalSSRF

CVE-2026-77987

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

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.

Published Sep 22, 2026Updated Sep 22, 2026Sources: NVD, GitHub (CNA), GitHub, GitHub Security Advisory

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

Not scored

FIRST has not published a score

Affects

GitHub

6 products listed

CVSS base

9.3

critical

Remediation

The vendor's own words where we have them.

Upgrade Enterprise Server to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
GitHub	Enterprise Server	3.17.0 to < 3.17.*	Vulnerable
GitHub	Enterprise Server	3.18.0 to < 3.18.*	Vulnerable
GitHub	Enterprise Server	3.19.0 to < 3.19.*	Vulnerable
GitHub	Enterprise Server	3.20.0 to < 3.20.*	Vulnerable
GitHub	Enterprise Server	3.21.0 to < 3.21.*	Vulnerable
GitHub	Enterprise Server	3.22.0 to < 3.22.*	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
9.3	CVSS 4.0	critical	—	—	GitHub (CNA)

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

Weakness & attack patterns

CWE-918Server-Side Request Forgery (SSRF)
CWE-208

Inferred from the weakness class — not observed against this CVE.

CAPEC-664 · Server Side Request Forgery

T1592.002Gather Victim Host Information: Software
T1082System Information Discovery

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable Enterprise Server.

web/proxy/WAF logsapplication logsnetwork IDS/IPScloud metadata access logsegress proxy logs

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 9.3 (AV:N)
Sep 22, 2026 · NVD
Initial · CVE published
Sep 22, 2026 · NVD
Added · GHSA-5p5v-w43x-q8v7 published (critical)
Sep 22, 2026 · GitHub Security Advisory

References

8 on the record

docs.github.com/en/[email protected]/admin/release-notes#3.17.21
Exploit, Third Party Advisory
docs.github.com/en/[email protected]/admin/release-notes#3.18.15
Exploit, Third Party Advisory
docs.github.com/en/[email protected]/admin/release-notes#3.19.12
Exploit, Third Party Advisory
docs.github.com/en/[email protected]/admin/release-notes#3.20.8
Exploit, Third Party Advisory
docs.github.com/en/[email protected]/admin/release-notes#3.21.6
Exploit, Third Party Advisory
docs.github.com/en/[email protected]/admin/release-notes#3.22.1
Exploit, Third Party Advisory
nvd.nist.gov/vuln/detail/CVE-2026-77987
NVD
github.com/advisories/GHSA-5p5v-w43x-q8v7
Exploit, Third Party Advisory

Elsewhere on this site

GitHubevery CVE for this vendor
SSRFsame class
CWE-918other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-77987 is GitHub Enterprise Server notebook viewer vulnerable to Server-side request forgery, a critical vulnerability affecting Enterprise Server from GitHub. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-77987. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-77987. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 9.3 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.