How severe is CVE-2026-39960?

CVE-2026-39960 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-39960?

CVE-2026-39960 affects the following packages: mantisbt/mantisbt (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-39960?

Update mantisbt/mantisbt to 2.28.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39960 is resolved across your whole dependency graph.

How do I detect CVE-2026-39960 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mantisbt/mantisbt, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-39960 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-39960 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-39960 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-39960?

CVE-2026-39960 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 19th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-39960?

CVE-2026-39960 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-39960 published, and has it been updated?

CVE-2026-39960 was published on May 20, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2026-39960 — mantisbt/mantisbt

MEDIUMFix: mantisbt/mantisbt@5fec0f4

CVE-2026-39960 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in mantisbt/mantisbt. A fix is available for mantisbt/mantisbt — see the affected versions and patch details below.

MantisBT is Vulnerable to Stored XSS through Custom Field Textarea Values

Also known asGHSA-qj6w-v29q-4rgx

Published

May 20, 2026

Updated

Aug 12, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Sep 26, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-39960.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs19th percentile — riskier than 19% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-39960 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐘mantisbt/mantisbt

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Session theft leading to admin account takeover, full project data access.

Precondition: A textarea-type custom field must be configured for the project
Attacker: Authenticated user with bug report permission (low privilege)
Victim: Any user viewing the bug edit form, including administrators

Patches

5fec0f448b7a7d7d539a6adb6dccceac4e4e4ab7

Workarounds

The default Content-Security Policy will block script execution.

References

https://mantisbt.org/bugs/view.php?id=37003
This is related to CVE-2024-34081.

Credits

Thanks to the following security researchers for independently discovering and responsibly reporting the issue, and providing a patch to fix it.

Thanks to Nozomu Sasaki (Paul) (@morimori-dev)
Tristan Madani (@TristanInSec) from Talence Security

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`mantisbt/mantisbt`	all versions	2.28.2`composer require mantisbt/mantisbt:^2.28.2`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mantisbt/mantisbt, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mantisbt/mantisbt to 2.28.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39960 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

Improper escaping of a textarea custom field's contents in the Update Issue page (bug_update_page.php) allows an attacker to inject HTML and, if CSP settings permit, execute arbitrary JavaScript when the page is loaded. ### Impact Session theft leading to admin account takeover, full project data access. - Precondition: A textarea-type custom field must be configured for the project - Attacker: Authenticated user with bug report permission (low privilege) - Victim: Any user viewing the bug edit form, including administrators ### Patches - 5fec0f448b7a7d7d539a6adb6dccceac4e4e4ab7 ### Workar