Base weakness

Draft

CWE-434: Unrestricted Upload of File with Dangerous Type

Also known as: Unrestricted File Upload

The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.

2,532

Recorded CVEs

With this primary weakness

KEVs

In the CISA KEV catalog

Medium

Likelihood of exploit

Per MITRE

Base

Abstraction

MITRE classification

Overview

CWE-434 (Unrestricted Upload of File with Dangerous Type) is a base-level software weakness catalogued by MITRE in the Common Weakness Enumeration (CWE). It describes a recurring type of mistake that can lead to exploitable security vulnerabilities.

How to prevent it

Practical mitigations for CWE-434, grouped by where in the lifecycle they apply.

Architecture and Design

Generate a new, unique filename for an uploaded file instead of using the user-supplied filename, so that no external input is used at all.[REF-422] [REF-423]

Architecture and Design

Enforcement by Conversion

When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.

Architecture and Design

Consider storing the uploaded files outside of the web document root entirely. Then, use other mechanisms to deliver the files dynamically. [REF-423]

Implementation

Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

For example, limiting filenames to alphanumeric characters can help to restrict the introduction of unintended file extensions.

Architecture and Design

Define a very limited set of allowable extensions and only generate filenames that end in these extensions. Consider the possibility of XSS (CWE-79) before allowing .html or .htm file types.

Implementation

Input Validation

Ensure that only one extension is used in the filename. Some web servers, including some versions of Apache, may process files based on inner extensions so that "filename.php.gif" is fed to the PHP interpreter.[REF-422] [REF-423]

Implementation

When running on a web server that supports case-insensitive filenames, perform case-insensitive evaluations of the extensions that are provided.

Architecture and Design

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

Implementation

Do not rely exclusively on sanity checks of file contents to ensure that the file is of the expected type and size. It may be possible for an attacker to hide code in some file segments that will still be executed by the server. For example, GIF images may contain a free-form comments field.

Implementation

Do not rely exclusively on the MIME content type or filename attribute when determining how to render a file. Validating the MIME content type and ensuring that it matches the extension is only a partial solution.

Architecture and Design

Operation

Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

Architecture and Design

Operation

Sandbox or Jail

Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.

OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.

This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.

Be careful to avoid CWE-243 and other weaknesses related to jails.

Effectiveness: Limited — The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

CWE-434: Unrestricted Upload of File with Dangerous Type

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Dynamic Analysis with Automated Results Interpretation

Dynamic Analysis with Manual Results Interpretation

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

Code examples

Illustrative examples

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-434?

What CVEs are caused by CWE-434?

Is CWE-434 part of the OWASP Top 10?

How do you prevent CWE-434?

How is CWE-434 detected?

What are the consequences of CWE-434?

Is CWE-434 actively exploited?

References

Stay ahead of CWE-434

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Dynamic Analysis with Automated Results Interpretation

Dynamic Analysis with Manual Results Interpretation

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

Code examples

Illustrative examples

Related weaknesses

Parent

Peer

Related

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-434?

What CVEs are caused by CWE-434?

Is CWE-434 part of the OWASP Top 10?

How do you prevent CWE-434?

How is CWE-434 detected?

What are the consequences of CWE-434?

Is CWE-434 actively exploited?

References

Stay ahead of CWE-434