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Variant · Low-Medium

CWE-797: Only Filtering Special Elements at an Absolute Position

The product receives data from an upstream component, but only accounts for special elements at an absolute position (e.g. "byte number 10"), thereby missing remaining special elements that may exist...

CWE-797 · Variant Level

Description

The product receives data from an upstream component, but only accounts for special elements at an absolute position (e.g. "byte number 10"), thereby missing remaining special elements that may exist before sending it to a downstream component.

Potential Impact

Integrity

Unexpected State

Demonstrative Examples

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-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, th...

Frequently Asked Questions

What is CWE-797?

CWE-797 (Only Filtering Special Elements at an Absolute Position) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product receives data from an upstream component, but only accounts for special elements at an absolute position (e.g. "byte number 10"), thereby missing remaining special elements that may exist...

How can CWE-797 be exploited?

Attackers can exploit CWE-797 (Only Filtering Special Elements at an Absolute Position) to unexpected state. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-797?

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-797?

CWE-797 is classified as a Variant-level weakness (Low-Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.