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Description
The software allows the attacker to upload or transfer files of dangerous types that can be automatically processed within the product’s environment.
Modes of Introduction:
– Implementation
Likelihood of Exploit: Medium
Related Weaknesses
CWE-669
CWE-669
CWE-351
CWE-436
CWE-430
Consequences
Integrity, Confidentiality, Availability: 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.
Potential Mitigations
Phase: Architecture and Design
Description:
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
Description:
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
Description:
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
Description:
Phase: Architecture and Design
Description:
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
Description:
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
Description:
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
Description:
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
Description:
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
Description:
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 Design, Operation
Description:
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 Design, Operation
Effectiveness: Limited
Description:
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.
CVE References
- CVE-2001-0901
- Web-based mail product stores “.shtml” attachments that could contain SSI
- 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
