RCE in Mybb

CVE-2026-45117

MyBB is free and open source forum software. From 1.8.13 until 1.8.40, the installer module does not properly escape user-supplied database configuration values written to the configuration file, resulting in PHP code injection and remote code execution when the installer is available. install/index.php processes the values with addcslashes(), but the $characters argument added in MyBB 1.8.13 does not include the backslash character, allowing crafted input to escape the generated PHP string. The uniquely identifying implementation details include introduced in MyBB 1.8.13. This issue is fixed in version 1.8.40.

Vulnerability class: RCE (Remote Code Execution)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.011 (65.7th percentile), scored .

Low probability of exploitation in the next 30 days, but still tracked. 66th percentile — 65.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-45117: held from 0.008 to 0.011.

EPSS over last 30 days · oldest: 0.008 · newest: 0.011 · change: +0.004

Affected products

  • Mybb — versions < 1.8.40

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-45117?
CVE-2026-45117 is a critical-severity vulnerability in Mybb, classified under Code Injection. CVSS score: 9.8/10. Published 2026-08-18.
How severe is CVE-2026-45117?
Critical severity. CVSS v3 base score is 9.8 out of 10.