Server-Side Request Forgery Vulnerability in MyBB Forum Software
CVE-2026-45123

4.3MEDIUM

Key Information:

Vendor

Mybb

Status
Mybb
Vendor
CVE Published:
18 August 2026

What is CVE-2026-45123?

MyBB, a free and open-source forum software, is susceptible to a server-side request forgery (SSRF) vulnerability due to improper handling of IPv6 addresses in the remote requests feature. Prior to version 1.8.40, the default configuration does not adequately restrict IPv6 addresses from being accessed, allowing a crafted remote target to bypass host restrictions. The flaw occurs because the verification process in fetch_remote_file() fails open when get_ip_by_hostname() does not return an IPv6 result. This oversight can be exploited by malicious actors to manipulate server requests undetected. The issue has been rectified in MyBB version 1.8.40.

Affected Version(s)

mybb < 1.8.40

References

https://github.com/mybb/mybb/security/advisories/GHSA-56w...
x_refsource_CONFIRM
https://github.com/mybb/mybb/commit/9cdadbf66f4cef50019f1...
x_refsource_MISC
https://github.com/mybb/mybb/releases/tag/mybb_1840
x_refsource_MISC

CVSS V3.1

Score:
4.3
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
None
Availability:
Low
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.