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The security vulnerability identified in Contao involves a critical flaw within the ModuleRegistration component.
Specifically, the ModuleRegistration::compile() method processes incoming requests and reaches its follow-up registration branch.
This critical branch is triggered on any HTTP POST request directed to a page carrying the registration module.
Upon inspection, the affected branch completely fails to perform essential security validations.
It neither checks the FORM_SUBMIT parameter nor validates the security captcha result computed immediately above it.
Because these validation checks are entirely bypassed during the execution flow, unauthenticated visitors can proceed further.
The execution flow subsequently invokes the resendActivationMail() function without enforcing any security barriers.
This function directly leads to the OptInToken::send() method execution.
Crucially, the OptInToken::send() method lacks any form of rate limiting or throttling mechanisms whatsoever.
As a result of this design oversight, malicious actors can exploit the endpoint to repeatedly trigger email delivery.
An attacker can script automated requests to make a vulnerable Contao installation send unlimited emails.
These emails are dispatched to any arbitrary email address chosen by the attacker.
They are sent directly from the site’s own legitimate sender address and trusted server reputation.
The amplification factor allows exactly one outbound message to be sent per standard HTTP request.
This creates a severe deliverability risk for the site operator and an extreme nuisance for the recipient.
Furthermore, the mechanism acts as a reliable account oracle.
It allows external entities to verify whether a specific email address has a pending registration on the target site.
This discloses membership status facts that public websites are normally expected to keep confidential.
To reach this specific execution branch, the target account must satisfy certain precise state preconditions.
The target email must have an unconfirmed registration status, represented in the database as tl_member.disable equals 1.
Additionally, it must be paired with an unconfirmed registration opt-in token.
While attackers can easily create this unconfirmed state for any arbitrary address since initial registration requires no ownership proof, mitigation is possible.
Specifically, on sites where registration activation (reg_activate) is disabled, the vulnerable branch remains entirely unreachable.
DailyCVE Form:
Platform: Contao CMS
Version: Up to 5.3.49
Vulnerability: Unrestricted email resending
Severity: Moderate
Date: August 24 2026
Prediction: Already patched today
What Undercode Say
Bash Commands and Codes
Example command simulating registration POST requests curl -X POST "https://vulnerable-target.com/registration" \ -d "[email protected]" \ -d "FORM_SUBMIT=tl_registration"
// Relevant logic pattern within ModuleRegistration::compile()
if ($this->Input->getMethod() === 'POST') {
// Missing verification checks on FORM_SUBMIT and captcha
$this->resendActivationMail($email);
}
Exploit: (Educational Purposes!)
1. Target a Contao instance running an affected version with an active public registration module. 2. Submit an unauthenticated POST request to initialize an unconfirmed registration state for an arbitrary address. 3. Send automated follow-up POST requests that bypass missing captcha and FORM_SUBMIT validation checks. 4. Trigger the uncontrolled invocation of OptInToken::send() to flood the recipient mailbox and enumerate account states.
Protection: from this CVE
- Update the Contao core bundle to version 5.3.50, 5.7.12, or later to enforce proper validation and rate limiting. - Disable frontend member registration modules if registration functionality is not mandatory for the web application. - Implement strict rate limiting policies at the WAF or web server level for incoming POST requests targeting registration endpoints.
Impact
- Unlimited outbound email delivery abuse directed at arbitrary recipients using the host server's reputation. - Severe risk of domain blacklisting and degradation of email deliverability for genuine site notifications. - Disclosure of private pending membership status information via reliable account oracle behavior.
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Sources:
Reported By: github.com
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