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

Code Examples & CVEs

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.

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:

BadPerl

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

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Detect imprecise mappings involving CWE-793

Review mappings to CWE-793, Only Filtering One Instance of a Special Element, 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-793 through an SSDF control plan

Use NIST SSDF practices to make prevention of CWE-793, Only Filtering One Instance of a Special Element, 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-793 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-793, Only Filtering One Instance of a Special Element, 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 One Instance of a Special Element

Recorded impacts include Unexpected State; related weaknesses include CWE-792. Use these source-defined anchors to turn CWE-793 into implementation, review, and verification checks for the affected component.

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

Validate CWE-793 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-793, Only Filtering One Instance of a Special Element. 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-793: Only Filtering One Instance of a Special Element?+

CWE-793: Only Filtering One Instance of a Special Element is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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.

What are the security consequences of Only Filtering One Instance of a Special Element?+

If exploited, CWE-793 (Only Filtering One Instance of a Special Element) it can compromise Integrity, leading to outcomes such as Unexpected State.

Which programming languages are affected by Only Filtering One Instance of a Special Element?+

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

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