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-2026-77912. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-77912 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-2026-77912. 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 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-2026-77912 using CVSS 7.4, 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.

highCross-site Scripting

CVE-2026-77912

CVE-2026-77912 — Stored cross-site scripting vulnerability in GitHub Enterprise Server allowed HTML attribute injection via the Markdown rendering pipeline

Q: What is CVE-2026-77912?

CVE-2026-77912 is Stored cross-site scripting vulnerability in GitHub Enterprise Server allowed HTML attribute injection via the Markdown rendering pipeline. The supplied enrichment data classifies it as Cross-site Scripting mapped to CWE-79 Cross-site Scripting.

A stored cross-site scripting (XSS) vulnerability was identified in GitHub Enterprise Server that allowed an authenticated attacker to inject arbitrary HTML attributes into rendered Markdown because the Markdown rendering pipeline rewrote quote characters in already-sanitized HTML without re-sanitizing the result. Crafted Markdown could abuse same-origin JavaScript gadgets to bypass Content Security Policy and gain control of the page DOM when viewed by another user. Successful exploitation could allow an attacker to read content visible to the victim, extract embedded CSRF tokens, perform state-changing actions as the victim, and exfiltrate data through same-origin writes. The payload could also propagate to repositories and organizations where the victim had write access. This vulnerability affected supported GitHub Enterprise Server releases in the 3.17, 3.18, 3.19, 3.20, 3.21, and 3.22 series 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 via 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

7.4

high

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

High

Attack Vector

Network

Privileges Req

Low

User Interaction

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
7.4	CVSS 4.0	high	—	—	GitHub (CNA)

CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:L/SI:L/SA:N/AU:Y

Weakness & attack patterns

CWE-79Cross-site Scripting

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

CAPEC-63 · Cross-Site Scripting (XSS)CAPEC-591 · Reflected XSS

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/IPS

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 7.4 (AV:N)
Sep 22, 2026 · NVD
Initial · CVE published
Sep 22, 2026 · NVD
Added · GHSA-6293-4rx2-2p57 published (high)
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-2026-77912
NVD
github.com/advisories/GHSA-6293-4rx2-2p57
Exploit, Third Party Advisory

Elsewhere on this site

GitHubevery CVE for this vendor
Cross-site Scriptingsame class
CWE-79other 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-2026-77912 is Stored cross-site scripting vulnerability in GitHub Enterprise Server allowed HTML attribute injection via the Markdown rendering pipeline, a high 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-2026-77912. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-77912. 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 7.4 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.