Summary
Kunstmaan CMS: MediaBundle extension blacklist bypass allows authenticated administrators to upload executable PHP files leading to remote code execution
The MediaBundle blocks dangerous upload extensions with a blacklist that was matched
case-sensitively, while the stored filename was lowercased afterwards. A file uploaded
as webshell.pHp therefore bypassed the blacklist and was written to the web-accessible
upload directory as webshell.php, where the web server executed it. Any authenticated
backend user with access to the media section could obtain remote code execution.
Details
FileHandler::getFilePath() rewrote blacklisted extensions to .txt using a
case-sensitive regex, and only then lowercased the extension when building the stored
name, so the check ran against the attacker-controlled casing and the normalisation
happened after it.
Two further weaknesses contributed:
- The default blacklist contained only
phpandhtaccess, leaving other
server-executable extensions (phtml,php5,phar,shtml,cgi, …) unblocked
regardless of casing. - Configured blacklist values were interpolated into the regex unescaped.
Workarounds
If you cannot upgrade, configure the web server to refuse to execute scripts in the
media upload directory (for example a location block in nginx or php_flag engine off
in Apache).
Impact
An authenticated user with access to the admin media section can upload a file that the
web server executes as PHP. The uploaded file is reachable over HTTP without
authentication, giving arbitrary code execution as the web server user.
The application accepts file uploads without adequately restricting the file type or content. Typical impact: remote code execution if the uploaded file can be served and executed on the server.
CVE-2026-104890 has a CVSS score of 7.2 (High). The vector is network-reachable, high 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 (7.3.2); upgrading removes the vulnerable code path.
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.
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Fixed in kunstmaan/media-bundle 7.3.2. The extension is now normalised before it is
checked and compared with in_array(); the default blacklist is expanded to the full
set of server-executable extensions; and a new opt-in allowed_extensions option allows
projects to enforce a strict allow-list.
Note that the patch does not rename files already stored on disk. Sites should audit
their media upload directory for existing files with an executable extension.
Frequently Asked Questions
- What is CVE-2026-104890? CVE-2026-104890 is a high-severity unrestricted upload of dangerous file types vulnerability in kunstmaan/media-bundle (composer), affecting versions < 7.3.2. It is fixed in 7.3.2. The application accepts file uploads without adequately restricting the file type or content.
- How severe is CVE-2026-104890? CVE-2026-104890 has a CVSS score of 7.2 (High). 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.
- Which packages are affected by CVE-2026-104890?
kunstmaan/media-bundle(composer) (versions < 7.3.2)kunstmaan/bundles-cms(composer) (versions < 7.3.2)
- Is there a fix for CVE-2026-104890? Yes. CVE-2026-104890 is fixed in 7.3.2. Upgrade to this version or later.
- Is CVE-2026-104890 exploitable, and should I be worried? Whether CVE-2026-104890 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-2026-104890 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-2026-104890?
- Upgrade
kunstmaan/media-bundleto 7.3.2 or later - Upgrade
kunstmaan/bundles-cmsto 7.3.2 or later
- Upgrade