Instituto Nacional de ciberseguridad. Sección Incibe
Instituto Nacional de Ciberseguridad. Sección INCIBE-CERT

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CVE-2026-77912

Severity CVSS v4.0:
HIGH
Type:
CWE-79 Cross-Site Scripting (XSS)
Publication date:
22/09/2026
Last modified:
02/10/2026

Description

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.

Impact

Vector 4.0
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 CVSS v4.0 Severity and Metrics:

Base Score: 7.40 HIGH
Vector: 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

Attack Vector (AV): Network
Attack Complexity (AC): High
Attack Requirements (AT): None
Privileges Required (PR): Low
User Interaction (UI): Passsive
Confidentiality (VC): High
Integrity (VI): High
Availability (VA): None
Confidentiality (SC): Low
Integrity (SI): Low
Availability (SA): None
Automatable (AU): Yes

Base Score 4.0
7.40
Severity 4.0
HIGH
Vector 3.x
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N CVSS v3.1 Severity and Metrics:

Base Score: 5.40 MEDIUM
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Attack Vector (AV): Network
Attack Complexity (AC): Low
Privileges Required (PR): Low
User Interaction (UI): Required
Scope (S): Changed
Confidentiality (C): Low
Integrity (I): Low
Availability (A): None

Base Score 3.x
5.40
Severity 3.x
MEDIUM

Vulnerable products and versions

CPE From Up to
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.17.0 (including) 3.17.21 (excluding)
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.18.0 (including) 3.18.15 (excluding)
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.19.0 (including) 3.19.12 (excluding)
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.20.0 (including) 3.20.8 (excluding)
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.21.0 (including) 3.21.6 (excluding)
cpe:2.3:a:github:enterprise_server:*:*:*:*:*:*:*:* 3.22.0 (including) 3.22.1 (excluding)

To consult the complete list of CPE names with products and versions, see this page


References to Advisories, Solutions, and Tools