Unrestricted Upload of File with Dangerous Type
Weakness ID: 434 (Weakness Base)Status: Draft
+ Description

Description Summary

The software allows the attacker to upload or transfer files of dangerous types that can be automatically processed within the product's environment.
+ Alternate Terms
Unrestricted File Upload:

The "unrestricted file upload" term is 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.

+ Time of Introduction
  • Implementation
  • Architecture and Design
+ Applicable Platforms

Languages

ASP.NET: (Sometimes)

PHP: (Often)

Language-independent

Architectural Paradigms

Web-based

Technology Classes

Web-Server: (Sometimes)

+ Common Consequences
ScopeEffect
Integrity

Technical Impact: 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 .asp and .php extensions uploaded to web servers 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.

+ Likelihood of Exploit

Medium to High

+ 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 
// Define the target location where the picture being
// 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:

(Attack)
 
malicious.php

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)
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:

(Attack)
 
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 HTML code is the same as in the previous example with the action attribute of the form sending the upload file request to the Java servlet instead of the PHP code.

(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
{...}
}
...
}

As with the previous example this code does not perform a check on the type of the file being uploaded. 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-22, CWE-23). 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.

+ Observed Examples
ReferenceDescription
CVE-2001-0901Web-based mail product stores ".shtml" attachments that could contain SSI
CVE-2002-1841PHP upload does not restrict file types
CVE-2005-1868upload and execution of .php file
CVE-2005-1881upload file with dangerous extension
CVE-2005-0254program does not restrict file types
CVE-2004-2262improper type checking of uploaded files
CVE-2006-4558Double "php" extension leaves an active php extension in the generated filename.
CVE-2006-6994ASP program allows upload of .asp files by bypassing client-side checks
CVE-2005-3288ASP file upload
CVE-2006-2428ASP file upload
+ Potential Mitigations

Phase: Architecture and Design

Generate your own filename for an uploaded file instead of the user-supplied filename, so that no external input is used at all.

Phase: Architecture and Design

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

Phase: 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 you allow .html or .htm file types.

Phase: 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.

Phase: Implementation

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

Phase: 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.

Phase: 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.

+ Weakness Ordinalities
OrdinalityDescription
Primary

This can be primary when there is no check at all.

Resultant

This is frequently resultant when use of double extensions (e.g. ".php.gif") bypasses a sanity check.

This can be resultant from client-side enforcement (CWE-602); some products will include web script in web clients to check the filename, without verifying on the server side.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory429Handler Errors
Development Concepts (primary)699
ChildOfCategoryCategory632Weaknesses that Affect Files or Directories
Resource-specific Weaknesses (primary)631
ChildOfWeakness ClassWeakness Class669Incorrect Resource Transfer Between Spheres
Research Concepts (primary)1000
ChildOfCategoryCategory714OWASP Top Ten 2007 Category A3 - Malicious File Execution
Weaknesses in OWASP Top Ten (2007) (primary)629
ChildOfCategoryCategory8012010 Top 25 - Insecure Interaction Between Components
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
PeerOfWeakness BaseWeakness Base351Insufficient Type Distinction
Research Concepts1000
PeerOfWeakness BaseWeakness Base430Deployment of Wrong Handler
Research Concepts1000
PeerOfWeakness BaseWeakness Base436Interpretation Conflict
Research Concepts1000
CanFollowWeakness ClassWeakness Class73External Control of File Name or Path
Research Concepts1000
CanFollowWeakness BaseWeakness Base183Permissive Whitelist
Research Concepts1000
CanFollowWeakness BaseWeakness Base184Incomplete Blacklist
Research Concepts1000
+ Relationship Notes

This can have a chaining relationship with incomplete blacklist / permissive whitelist 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 filter attachments with certain file extensions that can be processed by client systems.

+ Research Gaps

PHP applications are most targeted, but this likely applies to other languages that support file upload, as well as non-web technologies. ASP applications have also demonstrated this problem.

+ Affected Resources
  • File/Directory
+ Functional Areas
  • File Processing
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERUnrestricted File Upload
OWASP Top Ten 2007A3CWE More SpecificMalicious File Execution
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
122Exploitation of Authorization
+ References
Richard Stanway (r1CH). "Dynamic File Uploads, Security and You". <http://shsc.info/FileUploadSecurity>.
Johannes Ullrich. "8 Basic Rules to Implement Secure File Uploads". 2009-12-28. <http://blogs.sans.org/appsecstreetfighter/2009/12/28/8-basic-rules-to-implement-secure-file-uploads/>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Alternate Terms, Relationships, Other Notes, Taxonomy Mappings
2009-01-12CWE Content TeamMITREInternal
updated Relationships
2009-12-28CWE Content TeamMITREInternal
updated Applicable Platforms, Functional Areas, Likelihood of Exploit, Potential Mitigations, Time of Introduction
2010-02-16
(Critical)
CWE Content TeamMITREInternal
converted from Compound Element to Weakness
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