CVE-2020-13671 is an un-restricted upload of file vulnerability in Drupal Drupal core. CISA added it to the Known Exploited Vulnerabilities catalogue on 18 January 2022, which means attackers have used it in real attacks. Improper sanitization in the extension file names is present in Drupal core.
What this vulnerability means in Drupal core
This CVE is about unsafe file upload handling in Drupal core, specifically improper sanitization of extension names. That matters because file upload flaws often become execution or content-placement paths, especially when an attacker can influence what the server stores, serves, or later interprets as code rather than data.
For vulnerable web applications, the danger is not just that a file is accepted, but that the application may trust the filename, extension, or upload metadata too much. When that trust fails, attackers can smuggle active content, bypass extension checks, or place payloads where they can be reached through the web layer.
Why unrestricted file upload is dangerous
Unrestricted upload issues are dangerous because they sit at the boundary between user input and server-side execution. In a content management system, an upload path can affect site integrity, availability, and sometimes full application compromise if the uploaded object is later executed, parsed, or processed in an unsafe way.
Attackers often pair file upload flaws with other web weaknesses, such as misconfigured MIME handling, weak storage permissions, or exposed upload directories. A filename sanitization failure can be enough to undermine controls that were intended to restrict what types of content users may submit.
The practical lesson is that extension filtering alone is fragile. Security depends on server-side validation of type, storage location, handling rules, and post-upload access, not on the client or the filename supplied by the user.
How organisations assess exposure and limit blast radius
Exposure starts with knowing where Drupal core is deployed, which internet-facing instances accept uploads, and whether those instances process user-supplied files in ways that increase risk. The key question is not only whether the CVE exists in the estate, but whether the affected site allows unauthenticated or low-trust upload paths.
Security teams should confirm version and patch status against the vendor advisory and cross-check public exploitation status in the NIST National Vulnerability Database and the CVE Program. For prioritisation, a vulnerability already in CISA KEV deserves faster attention than an abstract catalogue entry.
When patching is delayed, limit upload exposure by reducing who can upload, isolating upload storage from executable paths, and tightening web-server handling rules. A vulnerability-management workflow is useful here because it ties asset inventory, verification, and remediation tracking to the real exposure surface rather than to the CVE name alone. See Vulnerability Management Guide for the operational model behind that prioritisation.
Detection, response, and post-patch review
Detection should focus on suspicious upload activity, newly created files in upload directories, unusual filename patterns, and any follow-on requests for recently uploaded content. If an attacker has already used the flaw, the issue may persist even after patching because the uploaded file, web shell, or modified content remains on disk.
Post-patch review should include a search for unexpected file types, changed permissions, unfamiliar admin activity, and evidence that the site served or executed content from an upload path. That review matters because fixing the code path does not automatically remove attacker-created artefacts or undo stored changes.
For teams that want a broader view of how file upload weaknesses are abused in practice, the Web Application Attacks Guide is a useful companion for understanding common exploitation patterns around upload handling, traversal, and related web attack paths.
Risk and Threat Considerations
File upload flaws are attractive to attackers because they can convert a normal content feature into a durable foothold. In a CMS, that can lead to site defacement, payload staging, credential capture, or a broader compromise if uploaded content is reachable or processed with unsafe trust assumptions.
Failure mechanism: The application accepts a file whose extension or handling metadata is not properly sanitised, then stores or serves it in a way that preserves attacker control over how the server treats that object.
Impact: Attackers may gain code execution, plant persistent web content, bypass content controls, or use the compromised site as a launch point for further intrusion.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST SP 800-53 Rev 5, CIS Controls v8 and OWASP ASVS set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-53 Rev 5 | SI-3 — Malicious Code Protection | Upload flaws can introduce malicious content that security controls should inspect and block. |
| AC-3 — Access Enforcement | Upload locations and retrieval paths need access enforcement to limit who can place or reach risky files. | |
| AU-6 — Audit Review, Analysis, and Reporting | Post-exploitation review depends on logs that show suspicious uploads and follow-on file access. | |
| Recommendation — Inspect uploaded content and enforce safeguards against malicious files before they are stored or executed. Enforce access restrictions on upload and file-serving paths to reduce abuse of uploaded content. Review upload and web-access logs to identify suspicious file placement and post-compromise activity. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | The CVE is an application-layer upload weakness that fits prescriptive web-application hardening. |
| Recommendation — Harden the application’s upload handling and validate file controls during secure development and testing. | ||
| OWASP ASVS | V5 — File Handling | ASVS file-handling requirements directly address validation, storage, and retrieval of user-supplied files. |
| Recommendation — Apply file-handling requirements to validate type, restrict storage, and prevent unsafe execution paths. | ||
Practitioner Guidance
What to watch for: Treat upload endpoints as high-risk trust boundaries. Review whether users can influence file names, MIME types, storage paths, or public access to uploaded objects, and verify that server-side validation enforces the intended file policy.
Governance implication: Ownership of upload security should sit with the application and platform teams together, because the code fix, web-server configuration, and hosting layout all affect whether the flaw is actually contained.
Practitioner takeaway: For this CVE, patching is necessary but not sufficient, because any uploaded malicious artefact or misconfigured delivery path can remain exploitable after the software update.
Related resources from NHI Mgmt Group
- Why do Rust file upload endpoints still create traversal risk even when file extensions are restricted?
- CVE-2021-20022 (SonicWall Email Security Unrestricted Upload of File)
- CVE-2021-26828 (OpenPLC ScadaBR Unrestricted Upload of File with Dangerous Type)
- CVE-2024-7694 (TeamT5 ThreatSonar Anti-Ransomware Unrestricted Upload of File with Dangerous Type)
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Reviewed and updated by the NHIMG editorial team on October 17, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org