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Vulnerability record · CVE-2021-27889 · published 15 March 2021

CVE-2021-27889: Mybb cross-site scripting vulnerability

Mybb · Mybb

Cross-site Scripting (XSS) vulnerability in MyBB before 1.8.26 via Nested Auto URL when parsing messages.

6.1 CVSS 3.1 Medium EPSS 5.1% · top 8.0% CWE-79 · Cross-site scripting
6.1CVSS 3.1 base score, v2 4.3
5.1%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
6References, 4 tagged exploit
17 Jun 2026Last modified by NVD

Description

Cross-site Scripting (XSS) vulnerability in MyBB before 1.8.26 via Nested Auto URL when parsing messages.

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

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://packetstormsecurity.com/files/161908/MyBB-1.8.25-Remote-Command-Execution.html ExploitThird Party AdvisoryVDB Entry
https://blog.sonarsource.com/mybb-remote-code-execution-chain ExploitThird Party Advisory
https://github.com/mybb/mybb/security/advisories/GHSA-xhj7-3349-mqcm PatchThird Party Advisory
http://packetstormsecurity.com/files/161908/MyBB-1.8.25-Remote-Command-Execution.html ExploitThird Party AdvisoryVDB Entry
https://blog.sonarsource.com/mybb-remote-code-execution-chain ExploitThird Party Advisory
https://github.com/mybb/mybb/security/advisories/GHSA-xhj7-3349-mqcm PatchThird Party Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

10.0CVE-2011-10018Mybb code injection vulnerabilitymyBB version 1.6.4 was distributed with an unauthorized backdoor embedded in the source code. The backdoor allowed remote attackers to execute arbitr…EPSS 2.0%10.0CVE-2015-8974Mybb merge system sql injection vulnerabilitySQL injection vulnerability in the Group Promotions module in the admin control panel in MyBB (aka MyBulletinBoard) before 1.6.18 and 1.8.x before 1.…EPSS 2.1%10.0CVE-2015-2786Mybb vulnerabilityUnspecified vulnerability in MyBB (aka MyBulletinBoard) before 1.8.4 has unknown attack vectors related to "Group join request notifications sent to …EPSS 1.4%10.0CVE-2011-5133Mybb vulnerabilityUnspecified vulnerability in MyBB before 1.6.5 has unknown impact and attack vectors, related to an "unparsed user avatar in the buddy list."EPSS 1.7%10.0CVE-2006-0218Mybb vulnerabilityMultiple unspecified vulnerabilities in MyBulletinBoard (MyBB) before 1.0.2 have unspecified impact and attack vectors, related to (1) admin/moderate…EPSS 1.2%9.8CVE-2020-22612Mybb code injection vulnerabilityInstaller RCE on settings file write in MyBB before 1.8.22.EPSS 0.73%9.8CVE-2017-16780Mybb cross-site request forgery vulnerabilityThe installer in MyBB before 1.8.13 allows remote attackers to execute arbitrary code by writing to the configuration file.EPSS 5.8%9.8CVE-2016-9402Mybb merge system sql injection vulnerabilitySQL injection vulnerability in the moderation tool in MyBB (aka MyBulletinBoard) before 1.8.7 and MyBB Merge System before 1.8.7 might allow remote a…EPSS 2.1%

Source: NIST National Vulnerability Database (record CVE-2021-27889), CISA KEV, FIRST EPSS (scores of 2026-09-29). This page is refreshed as NVD updates the record.