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Base · Medium

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

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 d...

CWE-795 · Base Level

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.

A filter might only account for instances of special elements when they occur: This may leave special elements in the data that did not match the filter position, but still may be dangerous.

Potential Impact

Integrity

Unexpected State

Demonstrative Examples

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.
Bad
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:
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-22).
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.
Bad
my $Username = GetUntrustedInput();if (substr($Username, 0, 3) eq '../') {$Username = substr($Username, 3);}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:
Attack
../../../etc/passwd
will have the first "../" filtered, 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-22).

CWE-791: Incomplete Filtering of Special Elements

The product receives data from an upstream component, but does not completely filter special elements before sending it ...

Frequently Asked Questions

What is CWE-795?

CWE-795 (Only Filtering Special Elements at a Specified Location) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. 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 d...

How can CWE-795 be exploited?

Attackers can exploit CWE-795 (Only Filtering Special Elements at a Specified Location) to unexpected state. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-795?

Follow secure coding practices, conduct code reviews, and use automated security testing tools (SAST/DAST) to detect this weakness early in the development lifecycle.

What is the severity of CWE-795?

CWE-795 is classified as a Base-level weakness (Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.