MyBB up to 1.8.39 Captcha contact.php invalidate_captcha captchaimage authentication replay

Summaryinfo

A vulnerability was found in MyBB up to 1.8.39. It has been rated as critical. This affects the function captcha::invalidate_captcha of the file contact.php of the component Captcha. The manipulation of the argument captchaimage leads to authentication replay. This vulnerability is documented as CVE-2026-45734. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in MyBB up to 1.8.39 and classified as critical. This issue affects the function captcha::invalidate_captcha of the file contact.php of the component Captcha. The manipulation of the argument captchaimage with an unknown input leads to a authentication replay vulnerability. Using CWE to declare the problem leads to CWE-294. A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Impacted is confidentiality, integrity, and availability. The summary by CVE is:

MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through challenge replay. The successful validation paths in contact.php, member.php?action=do_resendactivation, member.php?action=do_lostpw, member.php?action=do_emailuser, and sendthread.php?action=do_sendtofriend do not call captcha::invalidate_captcha() for the MyBB Default CAPTCHA selected by the captchaimage setting. A valid response can therefore be reused until a non-vulnerable endpoint invalidates it, an incorrect response is submitted, or the challenge expires. This issue is fixed in version 1.8.40.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-45734 since 05/13/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/18/2026). The attack technique deployed by this issue is T1040 according to MITRE ATT&CK.

By approaching the search of inurl:contact.php it is possible to find vulnerable targets with Google Hacking.

Upgrading to version 1.8.40 eliminates this vulnerability.

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Productinfo

Type

• Content Management System

Name

• MyBB

Version

• 1.8.0
• 1.8.1
• 1.8.2
• 1.8.3
• 1.8.4
• 1.8.5
• 1.8.6
• 1.8.7
• 1.8.8
• 1.8.9
• 1.8.10
• 1.8.11
• 1.8.12
• 1.8.13
• 1.8.14
• 1.8.15
• 1.8.16
• 1.8.17
• 1.8.18
• 1.8.19
• 1.8.20
• 1.8.21
• 1.8.22
• 1.8.23
• 1.8.24
• 1.8.25
• 1.8.26
• 1.8.27
• 1.8.28
• 1.8.29
• 1.8.30
• 1.8.31
• 1.8.32
• 1.8.33
• 1.8.34
• 1.8.35
• 1.8.36
• 1.8.37
• 1.8.38
• 1.8.39

License

• open-source

Website

• Product: https://mybb.com/

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