CVE-2026-86431

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Source
https://cve.org/CVERecord?id=CVE-2026-86431
Import Source
https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-86431.json
JSON Data
https://api.osv.dev/v1/vulns/CVE-2026-86431
Aliases
Downstream
Published
2026-09-07T12:53:51Z
Modified
2026-09-10T03:30:44Z
Severity
  • 6.9 (Medium) CVSS_V4 - CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N CVSS Calculator
Summary
commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
Details

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.

Database specific 
{
    "cna_assigner": "VulnCheck",
    "cwe_ids": [
        "CWE-79"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json"
}
References

Affected packages

Git / github.com/thephpleague/commonmark

Affected ranges

Type
GIT
Repo
https://github.com/thephpleague/commonmark
Events
Introduced
6fbb36d44824ed4091adbcf4c7d4a3923cdb3405
Fixed
43207253ea5f14867c77c697cd3838c446cadcea
Database specific
Show details 
{
    "extracted_events": [
        {
            "introduced": "2.7.0"
        },
        {
            "fixed": "2.9.1"
        }
    ],
    "source": [
        "AFFECTED_FIELD",
        "REFERENCES"
    ]
}

Database specific

source 
"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-86431.json"