CWE-793 Variant Incomplete

Only Filtering One Instance of a Special Element

This vulnerability occurs when an application processes incoming data but only removes or neutralizes one occurrence of a dangerous element, leaving other identical or similar elements untouched…

Definition

What is CWE-793?

This vulnerability occurs when an application processes incoming data but only removes or neutralizes one occurrence of a dangerous element, leaving other identical or similar elements untouched before passing the data along.
Imagine a scenario where user input contains multiple instances of a malicious script tag or a special command character. If the security filter is designed to catch and remove only the first (or last) instance it finds, all subsequent instances will pass through unchanged. This creates a deceptive security gap where the code appears to be sanitizing input but is actually performing an incomplete, single-operation cleanup. This type of flaw is often location-dependent, meaning the filter's effectiveness depends on whether it targets the initial or final element in a sequence. Developers might mistakenly assume their filter runs in a loop or uses a global replacement pattern, but a one-time replacement leaves the door open for attackers who can simply embed multiple harmful elements to bypass the defense.
Real-world impact

Real-world CVEs caused by CWE-793

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

  1. 1

    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.

  2. 2

    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:

  3. 3

    will have the first "../" stripped, resulting in:

  4. 4

    This value is then concatenated with the /home/user/ directory:

  5. 5

    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).

Vulnerable code example

Vulnerable Perl

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.

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

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:

Attacker payload 
../../../etc/passwd
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-793

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-793

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-793 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-793?

This vulnerability occurs when an application processes incoming data but only removes or neutralizes one occurrence of a dangerous element, leaving other identical or similar elements untouched before passing the data along.

How serious is CWE-793?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-793?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-793?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-793?

Plexicus's SAST engine matches the data-flow signature for CWE-793 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-793?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/793.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-793

CWE-792 Parent

Incomplete Filtering of One or More Instances of Special Elements

This vulnerability occurs when an application receives external data but fails to properly neutralize all instances of potentially…

CWE-794 Sibling

Incomplete Filtering of Multiple Instances of Special Elements

This vulnerability occurs when an application receives data from a source but fails to remove or neutralize every instance of a…

Resources

Further reading

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