CVE-2026-86431
commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
Description
league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.
INFO
Published Date :
Sept. 7, 2026, 1:20 p.m.
Last Modified :
Oct. 8, 2026, 4:17 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 4.0 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Update league/commonmark to version 2.9.1 or later.
- Ensure AttributesExtension is configured securely.
- Avoid processing untrusted Markdown with the extension enabled.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-86431.
| URL | Resource |
|---|---|
| https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea | Patch |
| https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8 | Exploit Mitigation Vendor Advisory |
| https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass | Patch Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-86431 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-86431
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-86431 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-86431 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by [email protected]
Oct. 08, 2026
Action Type Old Value New Value Added Affected Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/86xxx/CVE-2026-86431.json">CVE-2026-86431</a> -
Initial Analysis by [email protected]
Sep. 09, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:* versions from (including) 2.7.0 up to (excluding) 2.9.1 Added Reference Type VulnCheck: https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea Types: Patch Added Reference Type VulnCheck: https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8 Types: Exploit, Mitigation, Vendor Advisory Added Reference Type VulnCheck: https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass Types: Patch, Third Party Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Sep. 08, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-86431', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-09-08T13:30:04.102356Z'} -
New CVE Received by [email protected]
Sep. 07, 2026
Action Type Old Value New Value Added Description league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N Added CWE CWE-79 Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/86xxx/CVE-2026-86431.json">CVE-2026-86431</a> Added Reference https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea Added Reference https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8 Added Reference https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass