CWE-434: Unrestricted Upload of File with Dangerous Type
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
Scope
Effect
Integrity
Confidentiality
Availability
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".
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
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:
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.
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 {
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.
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.[R.434.1] [R.434.2]
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. [R.434.2]
Phase: Implementation
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input
validation strategy, i.e., use a whitelist 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
(i.e., do not rely on a blacklist). A blacklist 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, blacklists 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 (CWE-79) 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.[R.434.1] [R.434.2]
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 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.
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.
Phases: Architecture and Design; Operation
Strategy: Environment Hardening
Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.434.4]. 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.
Phases: 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
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.
Weakness Ordinalities
Ordinality
Description
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.
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 remove or quarantine 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.
[R.434.5] [REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 17, "File Uploading", Page 1068.. 1st Edition. Addison Wesley. 2006.
Description
