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CWE-434: Unrestricted Upload of File with Dangerous Type (4.17)

Weakness ID: 434

Description

Alternate Terms

Used in vulnerability databases and elsewhere, but it is insufficiently precise. The phrase could be interpreted as the lack of restrictions on the size or number of uploaded files, which is a resource consumption issue.

Common Consequences

Execute Unauthorized Code or Commands

Arbitrary code execution is possible if an uploaded file is interpreted and executed as code by the recipient. This is especially true for web-server extensions such as .asp and .php because these file types are often treated as automatically executable, even when file system permissions do not specify execution. For example, in Unix environments, programs typically cannot run unless the execute bit is set, but PHP programs may be executed by the web server without directly invoking them on the operating system.

Potential Mitigations

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.[

] [

]

Architecture and Design

Strategy: 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. [

]

Implementation

Strategy: 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 (

) before allowing .html or .htm file types.

Implementation

Strategy: 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.[

] [

]

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

. 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

Strategy: Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [

]. 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

Strategy: 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

Note: 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.

Relationships

Relevant to the view "Research Concepts" (View-1000)

Relevant to the view "Software Development" (View-699)

Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (View-1003)

Relevant to the view "Architectural Concepts" (View-1008)

Modes Of Introduction

Applicable Platforms

ASP.NET (Sometimes Prevalent)

PHP (Often Prevalent)

Class: Not Language-Specific (Undetermined Prevalence)

Web Server (Sometimes Prevalent)

Likelihood Of Exploit

Demonstrative Examples

Example 1

The following code intends to allow a user to upload a picture to the web server. The HTML code that drives the form on the user end has an input field of type "file".

(good code)

Example Language: HTML 

<form action="upload_picture.php" method="post" enctype="multipart/form-data">

Choose a file to upload:

<input type="file" name="filename"/>

<br/>

<input type="submit" name="submit" value="Submit"/>

</form>

Once submitted, the form above sends the file to upload_picture.php on the web server. PHP stores the file in a temporary location until it is retrieved (or discarded) by the server side code. In this example, the file is moved to a more permanent pictures/ directory.

(bad code)

Example Language: PHP 

// uploaded is going to be saved.

$target = "pictures/" . basename($_FILES['uploadedfile']['name']);

// Move the uploaded file to the new location.

if(move_uploaded_file($_FILES['uploadedfile']['tmp_name'], $target))

{

echo "The picture has been successfully uploaded.";

}

else

{

echo "There was an error uploading the picture, please try again.";

}

The problem with the above code is that there is no check regarding type of file being uploaded. Assuming that pictures/ is available in the web document root, an attacker could upload a file with the name:

Since this filename ends in ".php" it can be executed by the web server. In the contents of this uploaded file, the attacker could use:

(attack code)

Example Language: PHP 

<?php

system($_GET['cmd']);

?>

Once this file has been installed, the attacker can enter arbitrary commands to execute using a URL such as:

http://server.example.com/upload_dir/malicious.php?cmd=ls%20-l

which runs the "ls -l" command - or any other type of command that the attacker wants to specify.

Example 2

The following code demonstrates the unrestricted upload of a file with a Java servlet and a path traversal vulnerability. The action attribute of an HTML form is sending the upload file request to the Java servlet.

(good code)

Example Language: HTML 

<form action="FileUploadServlet" method="post" enctype="multipart/form-data">

Choose a file to upload:

<input type="file" name="filename"/>

<br/>

<input type="submit" name="submit" value="Submit"/>

</form>

When submitted the Java servlet's doPost method will receive the request, extract the name of the file from the Http request header, read the file contents from the request and output the file to the local upload directory.

(bad code)

Example Language: Java 

public class FileUploadServlet extends HttpServlet {

...

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

String contentType = request.getContentType();

// the starting position of the boundary header

int ind = contentType.indexOf("boundary=");

String boundary = contentType.substring(ind+9);

String pLine = new String();

String uploadLocation = new String(UPLOAD_DIRECTORY_STRING); //Constant value

// verify that content type is multipart form data

if (contentType != null && contentType.indexOf("multipart/form-data") != -1) {

// extract the filename from the Http header

BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));

...

pLine = br.readLine();

String filename = pLine.substring(pLine.lastIndexOf("\\"), pLine.lastIndexOf("\""));

...

// output the file to the local upload directory

try {

BufferedWriter bw = new BufferedWriter(new FileWriter(uploadLocation+filename, true));

for (String line; (line=br.readLine())!=null; ) {

if (line.indexOf(boundary) == -1) {

bw.write(line);
bw.newLine();
bw.flush();

}

} //end of for loop

bw.close();

} catch (IOException ex) {...}

// output successful upload response HTML page

}

// output unsuccessful upload response HTML page

else

{...}

}

...

}

This code does not perform a check on the type of the file being uploaded (CWE-434). This could allow an attacker to upload any executable file or other file with malicious code.

Additionally, the creation of the BufferedWriter object is subject to relative path traversal (CWE-23). Since the code does not check the filename that is provided in the header, an attacker can use "../" sequences to write to files outside of the intended directory. Depending on the executing environment, the attacker may be able to specify arbitrary files to write to, leading to a wide variety of consequences, from code execution, XSS (CWE-79), or system crash.

Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Weakness Ordinalities

Primary

(where the weakness exists independent of other weaknesses)

This can be primary when there is no check for the file type at all.

Resultant

(where the weakness is typically related to the presence of some other weaknesses)

This can be resultant when use of double extensions (e.g. ".php.gif") bypasses a check.

Resultant

(where the weakness is typically related to the presence of some other weaknesses)

This can be resultant from client-side enforcement (

); some products will include web script in web clients to check the filename, without verifying on the server side.

Detection Methods

Dynamic Analysis with Automated Results Interpretation

According to SOAR, the following detection techniques may be useful:

Cost effective for partial coverage:

• Web Application Scanner
• Web Services Scanner
• Database Scanners

Effectiveness: SOAR Partial

Dynamic Analysis with Manual Results Interpretation

According to SOAR, the following detection techniques may be useful:

Cost effective for partial coverage:

• Fuzz Tester
• Framework-based Fuzzer

Effectiveness: SOAR Partial

Manual Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Focused Manual Spotcheck - Focused manual analysis of source
• Manual Source Code Review (not inspections)

Effectiveness: High

Automated Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Source code Weakness Analyzer
• Context-configured Source Code Weakness Analyzer

Effectiveness: High

Architecture or Design Review

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

• Formal Methods / Correct-By-Construction

Cost effective for partial coverage:

• Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)

Effectiveness: High

Functional Areas

Affected Resources

Memberships

Vulnerability Mapping Notes

(this CWE ID may be used to map to real-world vulnerabilities)

Rationale

Comments

Notes

Relationship

This can have a chaining relationship with incomplete denylist / permissive allowlist errors when the product tries, but fails, to properly limit which types of files are allowed (CWE-183, CWE-184).

This can also overlap multiple interpretation errors for intermediaries, e.g. anti-virus products that do not remove or quarantine attachments with certain file extensions that can be processed by client systems.

Taxonomy Mappings

References

Content History

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