1050
Effectiveness = High
Potential Mitigations
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 (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.[REF-422] [REF-423]
Phase: Implementation
When running on a web server that supports case-insensitive filenames, perform caseinsensitive
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
CWE Version 4.14
CWE-434: Unrestricted Upload of File with Dangerous Type
CWE-434: Unrestricted Upload of File with Dangerous Type