CWE-795: Only Filtering Special Elements at a Specified Location

Description

The product receives data from an upstream component, but only accounts for special elements at a specified location, thereby missing remaining special elements that may exist before sending it to a downstream component.

Submission Date :

Dec. 4, 2009, midnight

Modification Date :

2023-06-29 00:00:00+00:00

Organization :

MITRE

Extended Description

A filter might only account for instances of special elements when they occur:

relative to a marker (e.g. "at the beginning/end of string; the second argument"), or
at an absolute position (e.g. "byte number 10").
This may leave special elements in the data that did not match the filter position, but still may be dangerous.

Example Vulnerable Codes

Example - 1

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


my $Username = GetUntrustedInput();$Username =~ s/^\.\.\///;my $filename = "/home/user/" . $Username;ReadAndSendFile($filename);

Since the regular expression is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. 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-22).

Example - 2

The following code takes untrusted input and uses a substring function to filter a 3-character "../" element located at the 0-index position of the input string. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.


$Username = substr($Username, 3);
my $Username = GetUntrustedInput();if (substr($Username, 0, 3) eq '../') {}my $filename = "/home/user/" . $Username;ReadAndSendFile($filename);

Since the if function is only looking for a substring of "../" between the 0 and 2 position, it only removes that specific "../" element. So an input value such as:

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

will have the first "../" filtered, 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-22).

Related Weaknesses

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.

Visit http://cwe.mitre.org/ for more details.