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Variant · Low-Medium

CWE-793: Only Filtering One Instance of a Special Element

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

CWE-793 · Variant Level

Description

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

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

Potential Impact

Integrity

Unexpected State

Demonstrative Examples

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
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).

CWE-792: Incomplete Filtering of One or More Instances of Special Elements

The product receives data from an upstream component, but does not completely filter one or more instances of special el...

Frequently Asked Questions

What is CWE-793?

CWE-793 (Only Filtering One Instance of a Special Element) 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 filters a single instance of a special element before sending it to a downstream component.

How can CWE-793 be exploited?

Attackers can exploit CWE-793 (Only Filtering One Instance of a Special Element) to unexpected state. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-793?

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-793?

CWE-793 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.