CVE-2026-45123 mybb CVE debrief

Who should care

MyBB administrators and security teams should assess exposure and apply the patch. The vulnerability has a CVSS score of 4.3 and is considered medium severity. MyBB administrators and security teams should assess exposure and apply the patch. The vulnerability allows potential unauthorized requests to remote hosts and bypassing of host restrictions via crafted remote targets.

Why it matters

CVE-2026-45123 is a server-side request forgery vulnerability in MyBB forum software prior to version 1.8.40. The vulnerability allows a crafted remote target to bypass host restrictions due to improper handling of IPv6 addresses. MyBB administrators and security teams should assess exposure and apply the patch. The vulnerability has a CVSS score of 4.3 and is considered medium severity.

• Potential unauthorized requests to remote hosts
• Bypassing of host restrictions via crafted remote targets
• Verification of MyBB version and remote request configurations
• Assessment of IPv6 address handling in remote requests

Technical summary

The remote requests feature in MyBB prior to 1.8.40 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, and verification in fetch_remote_file() fails open when get_ip_by_hostname() returns no result, allowing a crafted remote target to bypass the host restriction. The vulnerability allows potential unauthorized requests to remote hosts and bypassing of host restrictions via crafted remote targets.

Defensive priority

Medium priority for MyBB administrators and security teams to assess exposure and apply the patch.

Recommended defensive actions

• Assess exposure by checking if the MyBB installation is using a version prior to 1.8.40
• Apply the patch by upgrading to MyBB version 1.8.40 or later
• Review remote request configurations to ensure proper handling of IPv6 addresses
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, including its description, CVSS score, and references to patches and advisories. The vulnerability exists due to the remote requests feature not correctly handling 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. MyBB administrators and security teams should assess exposure and apply the patch.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.