# CVE-2026-107843: Contao registration module unvalidated mail resend

Source: https://junglewise.ai/threats/cve-2026-107843-contao-the-registration-module-re-sends-activation-mails  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-09. Updated: 2026-10-09  
License: CC BY 4.0 with a link back

CVE-2026-107843 · Severity: medium · CVSS 5.3 · Published 2026-10-09

Contao's registration module, used to manage member signups on websites, can be abused to send unlimited emails to any address without proper validation. An attacker can spam a target email with activation messages and determine whether an unconfirmed account exists for that address, leaking membership information the site should keep private.

Technologies: [Contao Core Bundle](https://junglewise.ai/threats/technologies/core-bundle), [contao/core-bundle (Packagist)](https://junglewise.ai/threats/technologies/contao-core-bundle). Vendors: [Contao](https://junglewise.ai/threats/vendors/contao), [Packagist](https://junglewise.ai/threats/vendors/packagist).

## Quick answers

**Is CVE-2026-107843 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-107843 being exploited in the wild.

**How severe is CVE-2026-107843?** Medium severity, with a CVSS score of 5.3, published 9 October 2026.

**Which products and versions are affected?** Contao Core Bundle 4.1.0 to 5.3.49; 5.4.0-RC1 to 5.7.11; contao/core-bundle (Packagist) ECOSYSTEM:introduced=4.1.0,fixed=5.3.50.

**Is there a fix for CVE-2026-107843?** Yes. contao/core-bundle (Packagist): upgrade to 5.3.50 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-107843 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-107843?** Check whether you run Contao Core Bundle or contao/core-bundle (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- contao/core-bundle (Packagist): upgrade to 5.3.50 or later

  ```sh
  composer require contao/core-bundle:^5.3.50
  ```


## Technical details

The ModuleRegistration::compile() method accepts any POST request to pages with the registration module and calls resendActivationMail() without validating the FORM_SUBMIT parameter or captcha result. OptInToken::send() lacks rate limiting, allowing one email per HTTP request. An attacker can trigger this on unconfirmed registrations (tl_member.disable=1 with pending reg-* token) to enumerate valid accounts and perform mail flooding from the site's own sender.

## Affected products

- Contao core-bundle 4.1.0 to 5.3.49; 5.4.0-RC1 to 5.7.11

## Identifiers

- CVE-2026-107843
- GHSA-mfxh-vp55-7gc6

## Timeline

- 2026-08-25: disclosed
- 2026-10-09: advisory

## References

- [https://api.github.com/users/HDWSec](https://api.github.com/users/HDWSec)

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