EUVD-2026-61005Severity by source
AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N
Attacker needs a registered account (PR:L), victim must actively interact with Buddy UI or PM popup (UI:R), JavaScript executes across scope boundary into victim session (S:C), enabling session hijacking (C:H, I:H); no server availability impact.
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 Buddy/Ignore component does not sanitize usernames correctly, allowing attackers to perform JavaScript code injection through a specially crafted username. The User CP Buddy/Ignore list and the Select Buddies list in Private Messages pass usernames through htmlspecialchars_uni(), which may leave single quotes unescaped. The payload is triggered when a victim chooses Yes in Please Confirm while removing the username in usercp.php, or selects the username through the onclick handler in the xmlhttp.php Select Buddies popup. The uniquely identifying implementation details include Private Messages Select Buddies list, and unescaped single quotes. This issue is fixed in version 1.8.40.
AnalysisAI
Stored cross-site scripting in MyBB's Buddy/Ignore component prior to version 1.8.40 allows authenticated attackers to inject arbitrary JavaScript by registering usernames that survive the htmlspecialchars_uni() sanitization pass with unescaped single quotes intact. The payload fires in the victim's browser during two specific user interactions: clicking 'Yes' in the removal confirmation dialog within usercp.php, or selecting the crafted username via the onclick handler in the xmlhttp.php Private Messages Select Buddies popup. …
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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 (low-privilege registered user) with a username containing a single-quote-based JavaScript payload. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 vector AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N and score of 8.7 accurately model the attack profile: network-accessible, low complexity, low-privilege account required, user interaction needed, with a scope change into the victim's authenticated session. … 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 - the vendor-released patch confirmed at https://github.com/mybb/mybb/releases/tag/mybb_1840 and https://mybb.com/versions/1.8.40. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Identify all systems running MyBB and document their current versions, then assess whether the Buddy/Ignore feature is actively used by checking user activity logs. …
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External POC / Exploit Code
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