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

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.

Code Examples & CVEs

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.

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:

BadPerl

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

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.

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:

BadPerl

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

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Detect imprecise mappings involving CWE-795

Review mappings to CWE-795, Only Filtering Special Elements at a Specified Location, with MITRE's mapping and navigation criteria. Flag any selection that points to a View or Category, remains at a higher abstraction than the available evidence supports, or ignores relevant parent, child, peer, and alternative entries. Treat those findings as mapping-quality defects, correct them before publication, and retain the comparison trail for repeatable audits.

CWE Mapping and Navigation Guidance — MITRE CWE

Prevent CWE-795 through an SSDF control plan

Use NIST SSDF practices to make prevention of CWE-795, Only Filtering Special Elements at a Specified Location, an explicit development outcome. Translate the CWE definition into security requirements and design constraints, choose safer implementation patterns and toolchain checks, and define acceptance evidence before release. Feed every confirmed occurrence back into the requirements, design review, and coding rules so the same root cause is removed across the product rather than patched in one location.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Triage CWE-795 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-795, Only Filtering Special Elements at a Specified Location, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Use MITRE taxonomy anchors to review Only Filtering Special Elements at a Specified Location

Recorded impacts include Unexpected State; related weaknesses include CWE-791. Use these source-defined anchors to turn CWE-795 into implementation, review, and verification checks for the affected component.

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

Validate CWE-795 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-795, Only Filtering Special Elements at a Specified Location. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-795: Only Filtering Special Elements at a Specified Location?+

CWE-795: Only Filtering Special Elements at a Specified Location is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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: 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.

What are the security consequences of Only Filtering Special Elements at a Specified Location?+

If exploited, CWE-795 (Only Filtering Special Elements at a Specified Location) it can compromise Integrity, leading to outcomes such as Unexpected State.

Which programming languages are affected by Only Filtering Special Elements at a Specified Location?+

CWE-795 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-795 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

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

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Detection Methods