SSRF in Mybb
CVE-2026-45123
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 fetch_remote_file() fails open when get_ip_by_hostname() 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.
Vulnerability class: SSRF (Server-Side Request Forgery)
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.
Affected products
- Mybb — versions < 1.8.40
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM)
- security-advisories@github.com (x_refsource_MISC)
- security-advisories@github.com (x_refsource_MISC)
- security-advisories@github.com (x_refsource_MISC)
Frequently asked questions
- What is CVE-2026-45123?
- CVE-2026-45123 is a medium-severity vulnerability in Mybb, classified under Server-Side Request Forgery (SSRF). CVSS score: 4.3/10. Published 2026-08-18.
- How severe is CVE-2026-45123?
- Medium severity. CVSS v3 base score is 4.3 out of 10.