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Vulnerability · Published Oct 9, 2026 · Assigned by GitHub_M

CVE-2026-107843Contao: The registration module re-sends activation mails on any unauthenticated POST, with no throttle and no captcha check

Medium 5.3

In plain English

A medium-severity resource allocation without limits in contao from contao. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 19% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.3%top 80.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

Contao is an Open Source CMS. From version 4.1.0 until 5.3.50 and 5.7.12, ModuleRegistration::compile() enters its follow-up registration branch on any POST to a page containing the registration module without verifying FORM_SUBMIT or the preceding captcha result. resendActivationMail() can then invoke OptInToken::send() without rate limiting, allowing an unauthenticated attacker to cause repeated activation emails to be sent to an address with a pending registration and to determine whether that pending registration exists. The branch is reachable only when reg_activate is enabled and the target has an unconfirmed registration and opt-in token. This issue is fixed in versions 5.3.50 and 5.7.12.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Packagistcontao/core-bundle5.3.50GHSA-mfxh-vp55-7gc6
Packagistcontao/core-bundle5.7.12GHSA-mfxh-vp55-7gc6

Affected products (as reported)

VendorProductVersions
contao contao >= 4.1.0, < 5.3.50>= 5.4.0-RC1, < 5.7.12

Weakness: CWE-204, CWE-770 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

References