CVE-2026-107843: Contao The Registration Module Re-sends Activation Vulnerability

MODERATE
Advisory Identifier: GHSA-mfxh-vp55-7gc6
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CVSS 5.3
CWE-204 CWE-770
Published
Advisory Summary

CVE-2026-107843 is a MODERATE (CVSS 5.3) the registration module re-sends activation vulnerability (CWE-204: Observable Response Discrepancy) affecting Contao libraries in the Packagist ecosystem. The issue involves the registration module re-sends activation mails. Remediation is available by upgrading affected packages to version 5.3.50, 5.7.12.

CVE-2026-107843 at a Glance

CVE Identifier CVE-2026-107843
GHSA Identifier GHSA-mfxh-vp55-7gc6
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-204: Observable Response Discrepancy
Affected Product Contao
Package Ecosystem Packagist
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-107843

To remediate this vulnerability, upgrade all affected Contao dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

contao/core-bundle fixed: 5.3.50 
Upgrade contao/core-bundle to 5.3.50
contao/core-bundle fixed: 5.7.12 
Upgrade contao/core-bundle to 5.7.12
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for contao/core-bundle

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-107843, inspect your project dependencies for Contao:

inspect dependencies for contao/core-bundle

Analyze the terminal output to locate any version of Contao in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 5.3.50), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-204: Observable Response Discrepancy, where the product provides different responses to incoming requests in a way that reveals internal state information to an unauthorized actor outside of the intended control sphere. In Contao, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

`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. ### Impact 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.

CVE-2026-107843 Affected Contao Versions

2 component(s)
Fixed Versions:
5.3.505.7.12
Product / Package Ecosystem Affected Versions Fixed Version
contao/core-bundle
 Packagist
≥ 4.1.0
< 5.3.50
5.3.50
contao/core-bundle
 Packagist
≥ 5.4.0-RC1
< 5.7.12
5.7.12

References

GitHub Advisory / Wiki ADVISORY
https://github.com/contao/contao/security/advisories/GHSA-mfxh-vp55-7gc6
Security Fix Commit FIX COMMIT
https://github.com/contao/contao/commit/2ea6117f9049db7221679251cfc41e67d941a74b
Contao Source Repository REPOSITORY
https://github.com/contao/contao
Contao Release RELEASE
https://github.com/contao/contao/releases/tag/5.3.50
Contao Release RELEASE
https://github.com/contao/contao/releases/tag/5.7.12

Frequently Asked Questions

What is CVE-2026-107843?
CVE-2026-107843 is a security vulnerability in Contao involving The registration module re-sends activation mails under CWE-204: Observable Response Discrepancy.
Which Contao versions are affected?
Affected versions include releases prior to 5.3.50, 5.7.12 across affected packages: contao/core-bundle.
What version fixes CVE-2026-107843?
Upgrading to 5.3.50, 5.7.12 (or the latest patched release for your respective branch) resolves the vulnerability.
Is contao/core-bundle affected?
Yes, contao/core-bundle is affected in releases prior to 5.3.50, 5.7.12. Upgrade your dependency to 5.3.50, 5.7.12 or later.
What is the CVSS score of CVE-2026-107843?
CVE-2026-107843 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N.
What CWE is associated with CVE-2026-107843?
CVE-2026-107843 is classified under CWE-204: Observable Response Discrepancy (The product provides different responses to incoming requests in a way that reveals internal state information to an unauthorized actor outside of the intended control sphere.).
How can I check Packagist dependencies for CVE-2026-107843?
Run the dependency tree command: 'inspect dependencies for contao/core-bundle'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mfxh-vp55-7gc6",
  "modified" : "2026-10-09T20:54:04Z",
  "published" : "2026-10-09T20:54:04Z",
  "aliases" : [ "CVE-2026-107843" ],
  "summary" : "Contao: The registration module re-sends activation mails",
  "details" : "`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.\n\n### Impact\n\nAnyone 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.\n\n**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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "contao/core-bundle"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.1.0"
      }, {
        "fixed" : "5.3.50"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "contao/core-bundle"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.4.0-RC1"
      }, {
        "fixed" : "5.7.12"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/security/advisories/GHSA-mfxh-vp55-7gc6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/commit/2ea6117f9049db7221679251cfc41e67d941a74b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/contao/contao"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/releases/tag/5.3.50"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/contao/contao/releases/tag/5.7.12"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-204", "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-09T20:54:04Z",
    "nvd_published_at" : null
  }
}