CWE-791 Base Incomplete

Incomplete Filtering of Special Elements

This vulnerability occurs when an application accepts data from a source but fails to properly clean or neutralize all special characters or commands before passing that data to another system…

Definition

What is CWE-791?

This vulnerability occurs when an application accepts data from a source but fails to properly clean or neutralize all special characters or commands before passing that data to another system component.
Think of this as a faulty security checkpoint. The application performs some filtering on incoming data—like removing certain quotes or semicolons—but the filter's rules are incomplete. Attackers can craft inputs using alternative encodings, unexpected character combinations, or overlooked special elements to bypass these partial defenses. This incomplete sanitization leaves a gap between what the filter catches and what the downstream component actually interprets as a command, leading to injection flaws. For developers, the core issue is relying on a denylist or a partial allowlist approach. Effective prevention requires a positive security model: validate and encode data based on the specific context where it will be used (like SQL, OS commands, or HTML). Always use parameterized queries, trusted APIs, and standardized encoding libraries instead of attempting to manually filter or escape characters, as this approach is notoriously error-prone and difficult to maintain.
Real-world impact

Real-world CVEs caused by CWE-791

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-791

  • 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-791

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-791 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-791?

This vulnerability occurs when an application accepts data from a source but fails to properly clean or neutralize all special characters or commands before passing that data to another system component.

How serious is CWE-791?

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-791?

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

How can I prevent CWE-791?

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-791?

Plexicus's SAST engine matches the data-flow signature for CWE-791 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-791?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/791.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-791

CWE-790 Parent

Improper Filtering of Special Elements

This vulnerability occurs when an application accepts data from a source but fails to properly sanitize or incorrectly filters out special…

CWE-792 Child

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-795 Child

Only Filtering Special Elements at a Specified Location

This vulnerability occurs when a security filter only checks for dangerous input patterns at specific, predefined locations within the…

Resources

Further reading

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