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MyBB CVE-2026-45125

| EUVDEUVD-2026-61011 MEDIUM
Improper Neutralization of CRLF Sequences ('CRLF Injection') (CWE-93)
2026-08-18 GitHub_M
PHP Code Injection Mybb
5.3
CVSS 3.1 · Vendor: GitHub_M
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Severity by source

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

AC:H reflects the non-default guest cansendemail prerequisite outside attacker control; PR:N retains the unauthenticated guest path; I:L for header injection with no confidentiality or availability impact.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
4.0 AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/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
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
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 Email User controller does not sanitize sender names correctly, resulting in mail header injection. member.php?action=do_emailuser accepts the fromname HTTP parameter for guests or the stored username for authenticated users when the cansendemail group permission is enabled. When mail_handler is set to the default PHP mail value, the sender name is used without sanitization in Return-Path and Reply-To headers, allowing arbitrary headers to be injected with CRLF sequences. This issue is fixed in version 1.8.40.

AnalysisAI

Mail header injection in MyBB's Email User feature (prior to 1.8.40) allows CRLF sequences embedded in the unsanitized fromname HTTP parameter to inject arbitrary SMTP headers - including Bcc, Reply-To, and Return-Path - into outgoing forum emails. The attack surface is conditional on two prerequisites: the mail_handler must be set to PHP (the default) and the cansendemail group permission must be enabled for the attacker's user group, including guests for the unauthenticated path. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Vulnerability AssessmentAI

Exploitation Two configuration prerequisites must be simultaneously present. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The NVD CVSS score of 5.3 (Medium, AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N) reflects a fully remote, unauthenticated, low-complexity attack with limited integrity impact. … 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 to MyBB 1.8.40, which is the vendor-confirmed patched release available at https://mybb.com/versions/1.8.40 and https://github.com/mybb/mybb/releases/tag/mybb_1840. … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-45125 vulnerability details – vuln.today
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