CVE-2026-107843 Contao: The registration module re-sends activation mails
Unauthenticated attackers can trigger repeated activation emails and verify registration status in Contao CMS when activation is enabled, causing resource exhaustion and information disclosure.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-107843?
CVE-2026-107843 is a medium-severity denial of service vulnerability in contao, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-107843?
4 open source packages ship an affected version, including contao/core-bundle.
| Package | Affected | Fixed in |
|---|---|---|
contao/core-bundle Packagist | From 4.1.0 | 5.3.50 |
contao/core-bundle Packagist | From 4.1.0 | 5.3.50 |
contao/core-bundle Packagist | From 5.4.0-RC1 | 5.7.12 |
contao/core-bundle Packagist | From 5.4.0-RC1 | 5.7.12 |
How do I fix CVE-2026-107843?
Upgrade contao/core-bundle to a fixed version for your release line: 5.3.50 or 5.7.12.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-107843 being exploited?
There is no known exploitation of CVE-2026-107843 in the wild and no public exploit code.
Sources
Also known as GHSA-mfxh-vp55-7gc6.