CVE-2022-21371

The following code could be for a social networking application in which each user's profile information is stored in a separate file. All files are stored in a single directory.

               
print "</ul>\n";
print "<li>$_</li>\n";

While the programmer intends to access files such as "/users/cwe/profiles/alice" or "/users/cwe/profiles/bob", there is no verification of the incoming user parameter. An attacker could provide a string such as:

               
../../../etc/passwd

The program would generate a profile pathname like this:

               
/users/cwe/profiles/../../../etc/passwd

When the file is opened, the operating system resolves the "../" during path canonicalization and actually accesses this file:

               
/etc/passwd

As a result, the attacker could read the entire text of the password file.

Notice how this code also contains an error message information leak (CWE-209) if the user parameter does not produce a file that exists: the full pathname is provided. Because of the lack of output encoding of the file that is retrieved, there might also be a cross-site scripting problem (CWE-79) if profile contains any HTML, but other code would need to be examined.

In the example below, the path to a dictionary file is read from a system property and used to initialize a File object.

               
File dictionaryFile = new File(filename);

However, the path is not validated or modified to prevent it from containing relative or absolute path sequences before creating the File object. This allows anyone who can control the system property to determine what file is used. Ideally, the path should be resolved relative to some kind of application or user home directory.

The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

               
ReadAndSendFile($filename);

Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:

               
../../../etc/passwd

will have the first "../" stripped, resulting in:

               
../../etc/passwd

This value is then concatenated with the /home/user/ directory:

               
/home/user/../../etc/passwd

which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-23).

The following code attempts to validate a given input path by checking it against a whitelist and once validated delete the given file. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".

               
}
f.delete()

An attacker could provide an input such as this:

               
/safe_dir/../important.dat

The software assumes that the path is valid because it starts with the "/safe_path/" sequence, but the "../" sequence will cause the program to delete the important.dat file in the parent directory

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.

               
</form>
<input type="submit" name="submit" value="Submit"/>

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.

               
}

                     
{...}// the starting position of the boundary header// verify that content type is multipart form data

                              // output the file to the local upload directory
bw.close();
}
bw.flush();
// output successful upload response HTML page
// output unsuccessful upload response HTML page
...

This code does not check the filename that is provided in the header, so 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 (CWE-79), or system crash.

Also, 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 (CWE-434).