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.

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

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, contao/core-bundle (Packagist). Vendors: Contao, 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
```
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

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