CWE 793

Only Filtering One Instance of a Special Element

Weakness ID: 793 (Weakness Variant)

Status: Incomplete

Description

Description Summary

The software receives data from an upstream component, but only filters a single instance of a special element before sending it to a downstream component.

Extended Description

Incomplete filtering of this nature may be location-dependent, as in only the first or last element is filtered.

Demonstrative Examples

Example 1

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.

(Bad Code)

Example Language: Perl

my $Username = GetUntrustedInput();

$Username =~ s/\.\.\///;

my $filename = "/home/user/" . $Username;

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:

(Attack)

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

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

(Result)

../../etc/passwd

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

(Result)

/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).

Relationships