EUVD-2026-61021Severity by source
AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
Remote, low-complexity, unauthenticated delivery but requires a victim click (UI:R); reflected XSS impact is scope-changing into the browser (S:C) yet realistically bounded to the victim's session, so C:L/I:L rather than H.
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 Contact module does not validate a redirect URL or protocol correctly, resulting in an open redirect and reflected JavaScript code injection. contact.php accepts the redirect target from the from HTTP parameter in $mybb->input['from'] or the Referer HTTP header in $_SERVER['HTTP_REFERER'] and passes it to redirect() without sufficient verification. A javascript: URI becomes the target of the Click here if you don't want to wait any longer link because $force_redirect is true, allowing script execution when a victim selects the link. This issue is fixed in version 1.8.40.
AnalysisAI
Reflected JavaScript injection and open redirect in MyBB forum software before 1.8.40 lets a remote attacker execute script in a victim's browser session. The Contact module (contact.php) takes a redirect target from the 'from' request parameter or the Referer header and passes it to redirect() without validating the URL scheme, so a javascript: URI can be delivered as the destination of the post-submission 'Click here if you don't want to wait any longer' link. CVSS is 9.3; no public exploit identified at time of analysis, and the flaw requires the victim to click the crafted link.
Technical ContextAI
MyBB is a widely used PHP-based open-source forum application. The root cause is CWE-83 (Improper Neutralization of Scripts in Attributes / failure to sanitize a redirect target), a variant of cross-site scripting arising from unvalidated URL scheme handling. contact.php reads the redirect destination from $mybb->input['from'] (the 'from' HTTP parameter) or from $_SERVER['HTTP_REFERER'] and forwards it to the framework's redirect() helper. Because $force_redirect is true, MyBB renders a manual fallback link whose href is the attacker-controlled value; the code accepts a javascript: pseudo-protocol URI rather than restricting to http/https or relative paths, converting an open redirect into DOM-context script execution. The affected component is identified by CPE cpe:2.3:a:mybb:mybb:*, i.e. the core MyBB product across versions prior to 1.8.40.
RemediationAI
Vendor-released patch: 1.8.40 - upgrade MyBB to 1.8.40, which corrects contact.php to validate the redirect URL and reject non-http(s) schemes; follow the release notes at https://github.com/mybb/mybb/releases/tag/mybb_1840 and the advisory GHSA-wf92-5q5h-qr53. If an immediate upgrade is not possible, restrict or disable the Contact module (Admin CP setting to turn off the contact form / 'Contact' link) to remove the vulnerable code path, at the cost of losing the site's contact functionality. As a broader compensating control, deploy a Content-Security-Policy that forbids inline and javascript: URI execution (e.g. a restrictive script-src without 'unsafe-inline'), which blunts script execution but may break themes or plugins relying on inline scripts; a WAF rule blocking 'from' parameters and Referer values containing 'javascript:' can also reduce exposure but is bypass-prone and should not be treated as a substitute for the patch.
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