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MyBB CVE-2026-45123

| EUVDEUVD-2026-61051 MEDIUM
Server-Side Request Forgery (SSRF) (CWE-918)
2026-08-18 GitHub_M
PHP SSRF Mybb
4.3
CVSS 3.1 · Vendor: GitHub_M
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Severity by source

Vendor (GitHub_M) PRIMARY
4.3 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
vuln.today AI
4.3 MEDIUM

Network-accessible SSRF requiring a valid forum account (PR:L); confidentiality impact limited to internal service probing with no integrity or availability effect.

3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

Primary rating from Vendor (GitHub_M).

CVSS VectorVendor: GitHub_M

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

Lifecycle Timeline

3
Patch available
Aug 18, 2026 - 17:17 EUVD
Source Code Evidence Fetched
Aug 18, 2026 - 16:51 vuln.today
Analysis Generated
Aug 18, 2026 - 16:51 vuln.today

DescriptionCVE.org

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.

AnalysisAI

Server-side request forgery in MyBB prior to 1.8.40 allows authenticated forum users to bypass the remote host blocklist by supplying IPv6 targets, causing the application to issue unintended internal network requests. The flaw is a fail-open design: fetch_remote_file() skips blocklist validation entirely when get_ip_by_hostname() returns an empty array - which it always did for IPv6 hostnames because the resolver only extracted 'ip' keys, not 'ipv6' keys. …

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Vulnerability AssessmentAI

Exploitation Exploitation requires a valid MyBB forum account (PR:L per the CVSS vector) - unauthenticated or guest access is insufficient to reach the `fetch_remote_file()` code path. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The vendor-assigned CVSS 3.1 score of 4.3 (Medium) with vector AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N accurately reflects the real-world risk profile. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario Full exploit scenario with step-by-step reproduction available after sign-in.
Remediation Upgrade MyBB to version 1.8.40, available at https://github.com/mybb/mybb/releases/tag/mybb_1840 and documented at https://mybb.com/versions/1.8.40. … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-45123 vulnerability details – vuln.today
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