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Vulnerability · Published Aug 18, 2026 · Assigned by GitHub_M

CVE-2026-45734MyBB: Default CAPTCHA missing invalidation

Medium 5.3

In plain English

A medium-severity security flaw in mybb from mybb. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 46% of all known vulnerabilities).

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.6%top 53.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

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.

Affected products (as reported)

VendorProductVersions
mybb mybb < 1.8.40

Weakness: CWE-837 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

References