CWE-646: Reliance on File Name or Extension of Externally-Supplied File

VariantIncompleteExploit Likelihood: High

The product allows a file to be uploaded, but it relies on the file name or extension of the file to determine the appropriate behaviors. This could be used by attackers to cause the file to be misclassified and processed in a dangerous fashion.

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Extended Description

An application might use the file name or extension of a user-supplied file to determine the proper course of action, such as selecting the correct process to which control should be passed, deciding what data should be made available, or what resources should be allocated. If the attacker can cause the code to misclassify the supplied file, then the wrong action could occur. For example, an attacker could supply a file that ends in a ".php.gif" extension that appears to be a GIF image, but would be processed as PHP code. In extreme cases, code execution is possible, but the attacker could also cause exhaustion of resources, denial of service, exposure of debug or system data (including application source code), or being bound to a particular server side process. This weakness may be due to a vulnerability in any of the technologies used by the web and application servers, due to misconfiguration, or resultant from another flaw in the application itself.

Technical Details

Structure
Simple
Vulnerability Mapping
ALLOWED

Applicable To

Languages
Not Language-Specific
Platforms

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-646 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-646, Reliance on File Name or Extension of Externally-Supplied File, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Reliance on File Name or Extension of Externally-Supplied File during Architecture and Design

MITRE associates mitigation with Architecture and Design; recorded impacts include Read Application Data, DoS: Crash, Exit, or Restart, and Gain Privileges or Assume Identity; the entry maps to CAPEC-209. Use these source-defined anchors to turn CWE-646 into implementation, review, and verification checks for the affected component.

CWE-646: Reliance on File Name or Extension of Externally-Supplied File — MITRE CWE

Triage CWE-646 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-646, Reliance on File Name or Extension of Externally-Supplied File, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-646

Apply MITRE's full root-cause mapping guidance when using CWE-646, Reliance on File Name or Extension of Externally-Supplied File. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-646 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-646, Reliance on File Name or Extension of Externally-Supplied File. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-646: Reliance on File Name or Extension of Externally-Supplied File?+

CWE-646: Reliance on File Name or Extension of Externally-Supplied File is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product allows a file to be uploaded, but it relies on the file name or extension of the file to determine the appropriate behaviors. This could be used by attackers to cause the file to be misclassified and processed in a dangerous fashion. An application might use the file name or extension of a user-supplied file to determine the proper course of action, such as selecting the correct process to which control should be passed, deciding what data should be made available, or what resources should be allocated. If the attacker can cause the code to misclassify the supplied file, then the wrong action could occur. For example, an attacker could supply a file that ends in a ".php.gif" extension that appears to be a GIF image, but would be processed as PHP code. In extreme cases, code execution is possible, but the attacker could also cause exhaustion of resources, denial of service, exposure of debug or system data (including application source code), or being bound to a particular server side process. This weakness may be due to a vulnerability in any of the technologies used by the web and application servers, due to misconfiguration, or resultant from another flaw in the application itself.

What are the security consequences of Reliance on File Name or Extension of Externally-Supplied File?+

If exploited, CWE-646 (Reliance on File Name or Extension of Externally-Supplied File) it can compromise Confidentiality, Availability and Access Control, leading to outcomes such as Read Application Data, DoS: Crash, Exit, or Restart and Gain Privileges or Assume Identity.

How do you prevent or mitigate Reliance on File Name or Extension of Externally-Supplied File?+

Recommended mitigations for CWE-646 include: Make decisions on the server side based on file content and not on file name or extension.

Which programming languages are affected by Reliance on File Name or Extension of Externally-Supplied File?+

CWE-646 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-646 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

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