CVE-2026-86431

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Source
https://cve.org/CVERecord?id=CVE-2026-86431
Import Source
https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-86431.json
JSON Data
https://api.test.osv.dev/v1/vulns/CVE-2026-86431
Aliases
Downstream
Published
2026-09-07T12:53:51Z
Modified
2026-09-17T03:47:31Z
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",
    "unresolved_ranges":  [
        {
            "extracted_events":  [
                {
                    "introduced":  "2.7.0"
                },
                {
                    "fixed":  "2.9.1"
                }
            ],
            "source":  "AFFECTED_FIELD"
        },
        {
            "extracted_events":  [
                {
                    "introduced":  "2.7.0"
                },
                {
                    "fixed":  "2.9.1"
                }
            ],
            "source":  "CPE_FIELD"
        }
    ]
}
References

Affected packages

Git / github.com/thephpleague/commonmark

Affected ranges

Type
GIT
Repo
https://github.com/thephpleague/commonmark
Events
Introduced
0 Unknown introduced commit / All previous commits are affected
Fixed
43207253ea5f14867c77c697cd3838c446cadcea
Database specific
Show details 
{
    "source":  "REFERENCES"
}

Affected versions

0.*
0.1.0
0.1.1
0.1.2
0.10.0
0.11.0
0.11.1
0.11.2
0.11.3
0.12.0
0.13.0
0.13.1
0.13.2
0.13.3
0.13.4
0.14.0
0.15.0
0.15.1
0.15.2
0.15.3
0.15.4
0.15.5
0.15.6
0.15.7
0.16.0
0.17.0
0.17.2
0.17.4
0.18.1
0.2.0
0.2.1
0.3.0
0.4.0
0.5.0
0.5.1
0.6.0
0.6.1
0.7.0
0.7.1
0.8.0
0.9.0
2.*
2.0.0
2.0.0-beta1
2.0.0-beta2
2.0.0-beta3
2.0.0-rc1
2.0.0-rc2
2.0.1
2.0.2
2.1.0
2.2.0
2.3.0
2.4.0
2.5.0
2.6.0

Database specific

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