# 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.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Oct 2026
- **Updated:** 13 Oct 2026
- **Also known as:** GHSA-mfxh-vp55-7gc6

## 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.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-204 (Observable Response Discrepancy)

## Which versions are affected by CVE-2026-107843?

4 open source packages ship an affected version, including contao/core-bundle.


| Package | Ecosystem | 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.

## Is CVE-2026-107843 being exploited?

There is no known exploitation of CVE-2026-107843 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-107843 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-107843 in the wild and no public exploit code.

Affected packages and fixed versions:
- contao/core-bundle (Packagist): affected from 4.1.0; fixed in 5.3.50
- contao/core-bundle (Packagist): affected from 4.1.0; fixed in 5.3.50
- contao/core-bundle (Packagist): affected from 5.4.0-RC1; fixed in 5.7.12
- contao/core-bundle (Packagist): affected from 5.4.0-RC1; fixed in 5.7.12

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-107843/ (Markdown: https://insights.sbomify.com/CVE-2026-107843/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-107843
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-107843
- OSV: https://osv.dev/vulnerability/CVE-2026-107843

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-107843/
Summaries are generated and may contain mistakes; the official description above is the record.