Summary
Problem
Local platform users who can write to TYPO3’s mail‑file spool directory can craft a file that the system will automatically deserialize without any class restrictions. This flaw allows an attacker to inject and execute arbitrary PHP code in the public scope of the web server.
The vulnerability is triggered when TYPO3 is configured with $GLOBALS['TYPO3_CONF_VARS']['MAIL']['transport_spool_type'] = 'file'; and a scheduler task or cron job runs the command mailer:spool:send. The spool‑send operation performs the insecure deserialization that is at the core of this issue.
Credits
Thanks to Vitaly Simonovich for reporting this issue, and to TYPO3 security team members Elias Häußler and Oliver Hader for fixing it.
References
Impact
Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Update to TYPO3 versions 10.4.55 ELTS, 11.5.49 ELTS, 12.4.41 LTS, 13.4.23 LTS, 14.0.2 that fix the problem described.
Frequently Asked Questions
- What is CVE-2026-0859? CVE-2026-0859 is a medium-severity insecure deserialization vulnerability in typo3/cms-core (composer), affecting versions >= 14.0.0, <= 14.0.1. It is fixed in 14.0.2, 13.4.23, 12.4.41, 11.5.49, 10.4.55. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
- Which versions of typo3/cms-core are affected by CVE-2026-0859? typo3/cms-core (composer) versions >= 14.0.0, <= 14.0.1 is affected.
- Is there a fix for CVE-2026-0859? Yes. CVE-2026-0859 is fixed in 14.0.2, 13.4.23, 12.4.41, 11.5.49, 10.4.55. Upgrade to this version or later.
- Is CVE-2026-0859 exploitable, and should I be worried? Whether CVE-2026-0859 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
- What actually determines whether CVE-2026-0859 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.
- How do I fix CVE-2026-0859?
- Upgrade
typo3/cms-coreto 14.0.2 or later - Upgrade
typo3/cms-coreto 13.4.23 or later - Upgrade
typo3/cms-coreto 12.4.41 or later - Upgrade
typo3/cms-coreto 11.5.49 or later - Upgrade
typo3/cms-coreto 10.4.55 or later
- Upgrade