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Unrestricted Upload of File with Dangerous Type

Draft Base
Structure: Simple
Description

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

The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.
Common Consequences 1
Scope: IntegrityConfidentialityAvailability

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 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.
Detection Methods 5
Dynamic Analysis with Automated Results InterpretationSOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Web Application Scanner Web Services Scanner Database Scanners
Dynamic Analysis with Manual Results InterpretationSOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Fuzz Tester Framework-based Fuzzer
Manual Static Analysis - Source CodeHigh
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Focused Manual Spotcheck - Focused manual analysis of source Manual Source Code Review (not inspections)
Automated Static Analysis - Source CodeHigh
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Source code Weakness Analyzer Context-configured Source Code Weakness Analyzer
Architecture or Design ReviewHigh
According to SOAR [REF-1479], 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.)
Potential Mitigations 12
Phase: 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]
Phase: 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.
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. [REF-423]
Phase: 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.
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 (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) before allowing .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.[REF-422] [REF-423]
Phase: Implementation
When running on a web server that supports case-insensitive filenames, perform case-insensitive evaluations of the extensions that are provided.
Phase: 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 Client-Side Enforcement of Server-Side Security. 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.
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.
Phase: Architecture and DesignOperation

Strategy: 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.
Phase: Architecture and DesignOperation

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 Creation of chroot Jail Without Changing Working Directory and other weaknesses related to jails.

Effectiveness: Limited
Demonstrative Examples 2
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".

Code Example:

Good
HTML
html
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.

Code Example:

Bad
PHP


// Define the target location where the picture being*


php


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:
Code Example:
Attack
bash
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:
Code Example:
Attack
PHP
php
Once this file has been installed, the attacker can enter arbitrary commands to execute using a URL such as:
Code Example:
Attack
bash
which runs the "ls -l" command - or any other type of command that the attacker wants to specify.
ID : DX-22
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.
Code Example:
Good
HTML
html
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.
Code Example:
Bad
Java
java
This code does not perform a check on the type of the file being uploaded (Unrestricted Upload of File with Dangerous Type). 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 (Relative Path Traversal). 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 (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')), or system crash.
  
Observed Examples 11
CVE-2023-5227PHP-based FAQ management app does not check the MIME type for uploaded images
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
  
References 7
Dynamic File Uploads, Security and You
Richard Stanway (r1CH)
https://web.archive.org/web/20090208005456/http://shsc.info/FileUploadSecurity(2025-07-24)
ID: REF-422
8 Basic Rules to Implement Secure File Uploads
Johannes Ullrich
28-12-2009
https://www.sans.org/blog/8-basic-rules-to-implement-secure-file-uploads/(2023-04-07)
ID: REF-423
Top 25 Series - Rank 8 - Unrestricted Upload of Dangerous File Type
Johannes Ullrich
SANS Software Security Institute
25-02-2010
https://www.sans.org/blog/top-25-series-rank-8-unrestricted-upload-of-dangerous-file-type/(2023-04-07)
ID: REF-424
Least Privilege
Sean Barnum and Michael Gegick
14-09-2005
https://web.archive.org/web/20211209014121/https://www.cisa.gov/uscert/bsi/articles/knowledge/principles/least-privilege(2023-04-07)
ID: REF-76
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Automated Source Code Security Measure (ASCSM)
Object Management Group (OMG)
01-2016
http://www.omg.org/spec/ASCSM/1.0/
ID: REF-962
State-of-the-Art Resources (SOAR) for Software Vulnerability Detection, Test, and Evaluation
Gregory Larsen, E. Kenneth Hong Fong, David A. Wheeler, and Rama S. Moorthy
07-2014
https://www.ida.org/-/media/feature/publications/s/st/stateoftheart-resources-soar-for-software-vulnerability-detection-test-and-evaluation/p-5061.ashx(2025-09-05)
ID: REF-1479
 
  
  
 
 
Likelihood of Exploit 
 
 
 
 Medium 
 
 
  
 
 
Applicable Platforms 
 
 
 
 
 
Languages:
 
 
 ASP.NET : SometimesPHP : OftenNot Language-Specific : Undetermined 
 
 
Technologies:
 
 
 Web Server : Sometimes 
 
 
 
  
 
 
Modes of Introduction 
 
 
 
 
 
  Implementation  
 
  Architecture and Design  
 
 
 
  
Related Attack Patterns 
  
Alternate Terms 
Unrestricted File Upload
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.
  
 
 
Functional Areas 
 
 
 
     
  1. File Processing
    2.  
 
 
  
 
 
Affected Resources 
 
 
 
     
  1. File or Directory
    2.  
 
 
  
 
 
Related Weaknesses 
 
 
 
 
  ChildOf:
 Incorrect Resource Transfer Between Spheres (CWE-669) 
  ChildOf:
 Incorrect Resource Transfer Between Spheres (CWE-669) 
  PeerOf:
 Insufficient Type Distinction (CWE-351) 
  PeerOf:
 Interpretation Conflict (CWE-436) 
  PeerOf:
 Deployment of Wrong Handler (CWE-430) 
 
 
 
 
Taxonomy Mapping 
  • PLOVER
  • OWASP Top Ten 2007
  • OMG ASCSM
  
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 (Permissive List of Allowed Inputs, Incomplete List of Disallowed Inputs).


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.