Unrestricted Upload of File with Dangerous TypeID: 434 | Date: (C)2012-05-14 (M)2022-10-10 |
Type: weakness | Status: DRAFT |
Abstraction Type: Base |
Description
The software allows the attacker to upload or transfer files of
dangerous types that can be automatically processed within the product's
environment.
Likelihood of Exploit: Medium to High
Applicable PlatformsLanguage: SometimesLanguage: ASP.NETLanguage: OftenLanguage: PHPLanguage Class: Language-independentArchitectural Paradigm: Web-basedTechnology Class: Web-ServerTechnology Class: Sometimes
Time Of Introduction
- Implementation
- Architecture and Design
Related Attack Patterns
Common Consequences
Scope | Technical Impact | Notes |
---|
IntegrityConfidentialityAvailability | 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. |
Detection MethodsNone
Potential Mitigations
Phase | Strategy | Description | Effectiveness | Notes |
---|
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.[R.434.1] [R.434.2] | | |
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.
[R.434.2] | | |
Implementation | 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. | | |
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.[R.434.1] [R.434.2] | | |
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 DesignOperation | 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. | | |
Architecture and DesignOperation | 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. | 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. |
RelationshipsThis 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.
Related CWE | Type | View | Chain |
---|
CWE-434 ChildOf CWE-813 | Category | CWE-809 | |
Demonstrative Examples (Details)
- 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. (Demonstrative Example Id DX-22)
- 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".
Observed Examples
- CVE-2001-0901 : Web-based mail product stores ".shtml" attachments that could contain SSI
- CVE-2002-1841 : PHP upload does not restrict file types
- CVE-2005-1868 : upload and execution of .php file
- CVE-2005-1881 : upload file with dangerous extension
- CVE-2005-0254 : program does not restrict file types
- CVE-2004-2262 : improper type checking of uploaded files
- CVE-2006-4558 : Double "php" extension leaves an active php extension in the generated filename.
- CVE-2006-6994 : ASP program allows upload of .asp files by bypassing client-side checks
- CVE-2005-3288 : ASP file upload
- CVE-2006-2428 : ASP file upload
For more examples, refer to CVE relations in the bottom box.
White Box Definitions None
Black Box Definitions None
Taxynomy Mappings
Taxynomy | Id | Name | Fit |
---|
PLOVER | | Unrestricted File Upload | |
OWASP Top Ten 2007 | A3 | Malicious File Execution | CWE_More_Specific |
References:
- Richard Stanway (r1CH) .Dynamic File Uploads, Security and You.
- Johannes Ullrich .8 Basic Rules to Implement Secure File
Uploads. 2009-12-28.
- Johannes Ullrich .Top 25 Series - Rank 8 - Unrestricted Upload of Dangerous File
Type. SANS Software Security Institute. 2010-02-25.
- Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
- Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 17, "File Uploading", Page 1068.'. Published on 2006.