EUVD-2026-61006Severity by source
AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N
Network-delivered stored XSS exploitable by any registered member, requiring victim page view; scope change and high C/I reflect cross-context session hijacking potential.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
Lifecycle Timeline
4DescriptionCVE.org
MyBB is free and open source forum software. Prior to 1.8.40, the user datahandler does not properly validate checkbox and multiselect profile field types, resulting in stored JavaScript code injection. UserDataHandler::verify_profile_fields() only performs the specialized validation when is_array($profile_fields[$field]) is true. A non-array profile_fields[fidX] value instead of the expected profile_fields[fidX][] shape falls through to generic text handling and is stored without verification. The affected value is then rendered directly by member.php and inc/functions_post.php rather than processed by the MyCode parser. The uniquely identifying implementation details include inc/datahandlers/user.php. This issue is fixed in version 1.8.40.
AnalysisAI
Stored cross-site scripting in MyBB before 1.8.40 allows any registered forum member to inject persistent JavaScript into checkbox and multiselect profile fields by submitting a scalar string value instead of the expected array shape, bypassing type-specific sanitization in UserDataHandler::verify_profile_fields(). The unsanitized payload is written to the database and rendered verbatim in member.php and inc/functions_post.php when other users view the attacker's profile or posts displaying profile fields. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | The attacker must hold a valid MyBB forum account at member privilege level or above (PR:L); unauthenticated exploitation is not possible. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 8.7 score is well-supported by the vector metrics: network-accessible, low-complexity exploitation requiring only a registered forum account (PR:L), with victim page view as the sole friction (UI:R). … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to MyBB 1.8.40 immediately; this is the vendor-released patch confirmed by the GitHub release tag at https://github.com/mybb/mybb/releases/tag/mybb_1840, the security advisory at https://github.com/mybb/mybb/security/advisories/GHSA-4p6g-p3qh-559v, and commit c32f0c22baab704a68b8d58fcdd2f26fadbbe19b. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, inventory all MyBB deployments and confirm installed versions; any instance before 1.8.40 is confirmed vulnerable. …
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External POC / Exploit Code
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