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TYPO3CMS

CVE-2021-21355

HIGH · 8.6 CVSS v3.1 Published 2021-03-23
Fix available
A fix is available. Upgrade to 8.7.40 / 9.5.25 or later.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
TYPO3 is an open source PHP based web content management system. In TYPO3 before versions 8.7.40, 9.5.25, 10.4.14, 11.1.1, due to the lack of ensuring file extensions belong to configured allowed mime-types, attackers can upload arbitrary data with arbitrary file extensions - however, default _fileDenyPattern_ successfully blocked files like _.htaccess_ or _malicious.php_. Besides that, _UploadedFileReferenceConverter_ transforming uploaded files into proper FileReference domain model objects handles possible file uploads for other extensions as well - given those extensions use the Extbase MVC framework, make use of FileReference items in their direct or inherited domain model definitions and did not implement their own type converter. In case this scenario applies, _UploadedFileReferenceConverter_ accepts any file mime-type and persists files in the default location. In any way, uploaded files are placed in the default location _/fileadmin/user_upload/_, in most scenarios keeping the submitted filename - which allows attackers to directly reference files, or even correctly guess filenames used by other individuals, disclosing this information. No authentication is required to exploit this vulnerability. This is fixed in versions 8.7.40, 9.5.25, 10.4.14, 11.1.1.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In TYPO3 before versions 8.7.40, 9.5.25, 10.4.14, 11.1.1, the UploadedFileReferenceConverter (used by Extbase MVC framework) fails to validate that file extensions belong to configured allowed mime-types. This allows unauthenticated attackers to upload arbitrary data with any file extension to /fileadmin/user_upload/, potentially disclosing uploaded files due to predictable naming.

MitigationUpgrade TYPO3 to version 8.7.40, 9.5.25, 10.4.14, or 11.1.1 or later. Review custom extensions that use FileReference domain models to ensure they implement proper file type validation.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
TYPO3CMS
Affected:>= 8.0.0, < 8.7.40>= 9.0.0, < 9.5.25>= 10.0.0, < 10.4.14>= 11.0.0, < 11.1.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
High
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine installed TYPO3 version
    Locate the version file (e.g., composer.json, sysext/core/Classes/Version.php, or check the TYPO3 backend About module) and extract the version number
    Affected if The version falls within any of these ranges: 8.x < 8.7.40, 9.x < 9.5.25, 10.x < 10.4.14, or 11.x < 11.1.1
  2. Verify Extbase extension is active
    Check if the Extbase extension (EXT:extbase) is installed and loaded. This can be done via the TYPO3 backend Package Manager or by inspecting the PackageStates.php file in typo3conf/
    Affected if Extbase is installed and enabled, as the vulnerability exists in the UploadedFileReferenceConverter component used by the Extbase MVC framework
  3. Confirm file upload functionality is accessible
    Verify that the /fileadmin/user_upload/ directory exists and is writable on the server. Also check if any frontend forms or backend modules allow file uploads through Extbase controllers
    Affected if The upload directory exists and the system accepts file uploads via Extbase-based forms or controllers
  4. Review file extension validation configuration
    Inspect the $GLOBALS['TYPO3_CONF_VARS']['SYS']['fileExt'] configuration or FileStorage configuration to determine which extensions are allowed. The vulnerability allows bypass of this validation
    Affected if File extension validation is configured but the UploadedFileReferenceConverter fails to enforce it properly for uploaded files

You are affected if your TYPO3 version is below the fixed releases (8.7.40, 9.5.25, 10.4.14, 11.1.1) AND Extbase with file upload functionality is in use, allowing potentially malicious file types to be uploaded to /fileadmin/user_upload/.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to 8.7.40 / 9.5.25 / 10.4.14 or later
Fixed in 8.7.409.5.2510.4.14
Interim mitigation

Upgrade TYPO3 to version 8.7.40, 9.5.25, 10.4.14, or 11.1.1 or later. Review custom extensions that use FileReference domain models to ensure they implement proper file type validation.

Fix this in TYPO3 Scoped from the published advisory
  • Consultation3.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
17.0 hours of engineering $3,000
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-21355 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data