CVE-2026-107843

Contao: The registration module re-sends activation mails

ModuleRegistration::compile() reaches its follow-up registration branch on any POST to a page carrying the module. That branch checks neither FORM_SUBMIT nor the captcha result computed immediately above it, and resendActivationMail() leads to OptInToken::send(), which has no rate limit at all.

Anyone on the internet can make a Contao installation send unlimited mail to an address of their choosing, from the site's own sender and reputation, at one outbound message per HTTP request. That is both a nuisance for the recipient and a deliverability risk for the site operator. The same request is a reliable account oracle for "this address has a pending registration on this site", which is exactly the sort of membership fact a public site is usually expected not to disclose.

Honest bound. The target must have an unconfirmed registration, that is tl_member.disable = 1 together with an unconfirmed reg- opt-in token. An attacker can create that state for an arbitrary address, since registration requires no ownership proof, but on a site where reg_activate is off the branch is unreachable.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-107843 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.3.50, 5.7.12); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-107843? CVE-2026-107843 is a medium-severity allocation of resources without limits or throttling vulnerability in contao/core-bundle (composer), affecting versions >= 4.1.0, < 5.3.50. It is fixed in 5.3.50, 5.7.12. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-107843? CVE-2026-107843 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of contao/core-bundle are affected by CVE-2026-107843? contao/core-bundle (composer) versions >= 4.1.0, < 5.3.50 is affected.
4. Is there a fix for CVE-2026-107843? Yes. CVE-2026-107843 is fixed in 5.3.50, 5.7.12. Upgrade to this version or later.
5. Is CVE-2026-107843 exploitable, and should I be worried? Whether CVE-2026-107843 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-107843 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-107843?
   • Upgrade contao/core-bundle to 5.3.50 or later
   • Upgrade contao/core-bundle to 5.7.12 or later

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