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MantisBT is Vulnerable to Stored XSS in Custom Field Textarea Values

Moderate severity GitHub Reviewed Published May 9, 2026 in mantisbt/mantisbt • Updated May 11, 2026

Package

composer mantisbt/mantisbt (Composer)

Affected versions

<= 2.28.1

Patched versions

2.28.2

Description

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

Workarounds

The default Content-Security Policy will block script execution.

References

Credits

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

References

@dregad dregad published to mantisbt/mantisbt May 9, 2026
Published to the GitHub Advisory Database May 11, 2026
Reviewed May 11, 2026
Last updated May 11, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-39960

GHSA ID

GHSA-qj6w-v29q-4rgx

Source code

Credits

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