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['HTTPREFERER'] 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 $forceredirect is true, allowing script execution when a victim selects the link. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the Admin CP Recovery Codes module does not validate requests correctly, allowing same-site attackers to rotate a victim administrator's recovery codes with a specially crafted URL. The Admin CP Home, Preferences, Recovery Codes action=recoverycodes page regenerates Two-Factor Authentication recovery codes in mybbadminoptions.recoverycodes on GET requests without request forgery protection. The uniquely identifying implementation details include admin/modules/home/preferences.php. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the Mod CP Report Center does not check permissions consistently, allowing moderators without report-management permission to mark reports as resolved. The modcp.php?action=doreports Mark Selected as Read handler is reachable with canmodcp even without canmanagereportedcontent or canmanagereportedposts. When no forums are in scope, $flistreports is empty and the UPDATE mybbreportedcontent query executes without the expected permission-based limitation. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not verify private event status consistently, allowing users with viewing and moderation permissions to access and moderate private events. The private-event check used by getevents() in inc/functionscalendar.php and the event action is missing from the remaining calendar.php actions, despite the limited-access behavior described in inc/languages/english/calendar.lang.php. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the User CP Buddy/Ignore List component does not validate reciprocal buddy-list updates correctly. The usercp.php?action=doeditlists delete handler removes the selected entry from the acting user's list and then updates mybbusers.buddylist for the target account. The reciprocal update searches for the deleted target UID instead of the acting user's UID and uses the unchecked arraysearch() return value as an array key. A false result can be converted to index 0, removing the target account's first stored buddy while leaving the actual reciprocal entry unchanged. The uniquely identifying implementation details include false converted to index 0. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through challenge replay. The successful validation paths in contact.php, member.php?action=doresendactivation, member.php?action=dolostpw, member.php?action=doemailuser, and sendthread.php?action=dosendtofriend do not call captcha::invalidatecaptcha() for the MyBB Default CAPTCHA selected by the captchaimage setting. A valid response can therefore be reused until a non-vulnerable endpoint invalidates it, an incorrect response is submitted, or the challenge expires. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not validate moderation permissions for the destination calendar when moving events. A user with moderation permission for the source calendar can move an event to a calendar where the user has only viewing permission because the domove action in calendar.php does not check canmoderateevents for the target calendar. The uniquely identifying implementation details include calendar event move, source calendar moderation permission, and destination calendar viewing permission. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the Admin CP UTF-8 Conversion module does not validate certain requests correctly, allowing same-site attackers to alter table encoding and deny service with a specially crafted URL. The do=all control flow in admin/modules/tools/systemhealth.php performs ALTER TABLE operations, column rewrite phases, and fulltext index rebuilds on GET requests for the database table named by the table parameter without request verification. The uniquely identifying implementation details include GET requests without request verification. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the Admin CP Security Questions module does not validate the anti-CSRF token correctly, allowing same-site attackers to enable or disable registration challenge questions with a specially crafted URL. The controller processes GET requests for the disable and enable actions to modify mybbquestions.active without verifying the mypostkey token attached by the user interface. The uniquely identifying implementation details include admin/modules/config/questions.php. This issue is fixed in version 1.8.40.

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::verifyprofilefields() only performs the specialized validation when isarray($profilefields[$field]) is true. A non-array profilefields[fidX] value instead of the expected profilefields[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/functionspost.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.

MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not check permissions consistently when listing calendars, allowing authenticated users to access titles of calendars that are otherwise inaccessible. The affected calendar-selection paths in calendar.php perform permission checks against an invalid calendar context before returning calendar titles. The uniquely identifying implementation details include titles of inaccessible calendars, and invalid calendar permission context. This issue is fixed in version 1.8.40.

Impact The registration component does not validate the text-based Security Question CAPTCHA correctly, allowing attackers to bypass the challenge via a specially crafted value.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Details The public Registration workflow (member.php?action=doregister) accepts a hidden field questionid — expected to match the question session identifier (mybbquestionsessions.sid) — and validates the challenge answer without a fail-closed fallback for invalid identifiers. If the value is blank, forged, or expired, the request continues without a question-related error.

Patches MyBB 1.8.(...) resolves this issue with the following changes:

- Commit: https://github.com/mybb/mybb/commit/ - .patch: https://github.com/mybb/mybb/commit/.patch

References - Release Notes: https://mybb.com/versions/1.8.(...)/

For more information Go to mybb.com/security to report possible security concerns or to learn more about security research at MyBB.

Contact The security team can be reached at security@mybb.com.

MyBB is free and open source forum software. Prior to 1.8.40, the remote requests feature does not correctly handle IPv6 addresses, resulting in a server-side request forgery vulnerability. The default disallowed remote hosts list does not include IPv6 addresses. Verification in fetchremotefile() fails open when getipbyhostname() returns no result because that function does not return IPv6 results, allowing a crafted remote target to bypass the host restriction. The uniquely identifying implementation details include fail-open verification, and inc/functions.php. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the ACP Users View Manager module does not validate requests correctly, allowing same-site attackers to change a victim administrator's default user list view by embedding a specially crafted URL. The Set as Default control named setdefault in Admin CP, Users & Groups, Users, View Manager changes the administrator's default view on GET requests without request forgery protection. The uniquely identifying implementation details include Users & Groups → Users → View Manager, and admin/inc/functionsviewmanager.php. This issue is fixed in version 1.8.40.

MyBB is free and open source forum software. Prior to 1.8.40, the ACP Mass Mail module does not validate certain requests correctly, allowing same-site attackers to create draft entries from archived entries by embedding a specially crafted URL. The Resend route in Admin CP, Users & Groups, Mass Mail, Mass Mailing Archive duplicates archived mailing entries into new drafts with mybbmassemails.status set to 0 on GET requests without request forgery protection. The uniquely identifying implementation details include Users & Groups → Mass Mail → Mass Mailing Archive, mybbmassemails.status = 0, and admin/modules/user/massmail.php. This issue is fixed in version 1.8.40.

Rejected reason: This CVE ID has been rejected or withdrawn by its CVE Numbering Authority. This CVE was assigned on the basis that the permission system allows a limited administrator to grant privileges exceeding their own authorization scope, potentially constituting an insecure default configuration. Following a dispute, the MITRE TL-Root determined the behavior reflects documented and intended product design rather than a security vulnerability.

MyBB Timeline Plugin 1.0 contains cross-site scripting vulnerabilities that allow attackers to inject malicious scripts through thread titles, post content, and user profile fields like Location and Bio. Attackers can also exploit a cross-site request forgery vulnerability in the timeline.php profile action to change a user's cover picture by crafting malicious forms that execute when victims visit affected profiles.

MyBB Recent threads 17.0 contains a persistent cross-site scripting vulnerability that allows attackers to inject malicious scripts by creating threads with crafted subject lines. Attackers can create threads with script tags in the subject parameter to execute arbitrary JavaScript in the browsers of all users viewing the index page.

MyBB Last User's Threads in Profile Plugin 1.2 contains a persistent cross-site scripting vulnerability that allows attackers to inject malicious scripts by crafting thread subjects with script tags. Attackers can create threads with script payloads in the subject field that execute when users visit the attacker's profile page.

MyBB My Arcade Plugin 1.3 contains a persistent cross-site scripting vulnerability that allows authenticated users to inject malicious scripts through arcade game score comments. Attackers can add crafted HTML and JavaScript payloads in the comment field that execute when other users view or edit the comment.

MyBB Downloads Plugin 2.0.3 contains a persistent cross-site scripting vulnerability that allows regular members to inject malicious scripts through the download title field. Attackers can submit a new download with HTML/JavaScript code in the title parameter, which executes when administrators validate the download in downloads.php.

MyBB Like Plugin 3.0.0 contains a stored cross-site scripting vulnerability. Authenticated attackers can inject script payloads into post or thread subjects; when other users view a profile that displays the attacker's liked posts, the unsanitized subject is rendered, executing the script in the viewer's browser.

MyBB Delete Account Plugin 1.4 contains a cross-site scripting vulnerability in the account deletion reason input field. Attackers can inject malicious scripts that will execute in the admin interface when viewing delete account reasons.

MyBB Trending Widget Plugin 1.2 contains a cross-site scripting vulnerability that allows attackers to inject malicious scripts through thread titles. Attackers can modify thread titles with script payloads that will execute when other users view the trending widget.

MyBB Thread Redirect Plugin 0.2.1 contains a cross-site scripting vulnerability in the custom text input field for thread redirects. Attackers can inject malicious SVG scripts that will execute when other users view the thread, allowing arbitrary script execution.

myBB Forums 1.8.26 contains a stored cross-site scripting vulnerability in the forum announcement system that allows authenticated administrators to inject malicious scripts when creating announcements. Attackers can exploit this vulnerability by inserting script payloads in the announcement title field when adding announcements through the 'Forums and Posts' > 'Forum Announcements' interface, causing arbitrary JavaScript to execute when the announcement is displayed on the forum.

MyBB 1.8.32 contains a chained vulnerability that allows authenticated administrators to bypass avatar upload restrictions and execute arbitrary code. Attackers can modify upload path settings, upload a malicious PHP-embedded image file, and execute commands through the language configuration editing interface.

myBB Forums 1.8.26 contains a stored cross-site scripting vulnerability in the forum management system that allows authenticated administrators to inject malicious scripts when creating new forums. Attackers can exploit this vulnerability by inserting script payloads in the forum title field when adding new forums through the 'Forums and Posts' > 'Forum Management' interface, causing arbitrary JavaScript to execute when the forum listing is viewed.

myBB Forums 1.8.26 contains a stored cross-site scripting vulnerability in the template management system that allows authenticated administrators to inject malicious scripts when creating new templates. Attackers can exploit this vulnerability by inserting script payloads in the template title field when adding new templates through the 'Templates and Style' > 'Templates' > 'Manage Templates' > 'Global Templates' interface, causing arbitrary JavaScript to execute when the template is viewed.

myBB version 1.6.4 was distributed with an unauthorized backdoor embedded in the source code. The backdoor allowed remote attackers to execute arbitrary PHP code by injecting payloads into a specially crafted collapsed cookie. This vulnerability was introduced during packaging and was not part of the intended application logic. Exploitation requires no authentication and results in full compromise of the web server under the context of the web application.