CWE-796 Variant Incomplete

Only Filtering Special Elements Relative to a Marker

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations (like the start or end of a string). It fails to detect and…

Definition

What is CWE-796?

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations (like the start or end of a string). It fails to detect and remove the same dangerous elements if they appear elsewhere in the data, allowing them to pass through to critical system components.
Imagine a security filter designed to strip out SQL comment sequences (like '--') only if they appear at the very beginning of a user-provided string. If an attacker injects that same sequence in the middle of the input (e.g., `admin'--`), the filter misses it entirely. The downstream database, receiving the unfiltered input, then interprets the sequence as legitimate SQL, potentially leading to unauthorized access or data manipulation. This flaw stems from an incomplete validation logic that makes a dangerous assumption: that malicious payloads only appear in predictable spots. Effective sanitization must scan and clean the *entire* input, regardless of position. Developers should use context-aware encoding or parameterized interfaces instead of relying on positional filtering, as attackers will always probe for and exploit these blind spots.
Real-world impact

Real-world CVEs caused by CWE-796

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 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.

  2. 2

    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:

  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-22).

Vulnerable code example

Vulnerable Perl

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.

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

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:

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-796

  • 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-796

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-796 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-796?

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations (like the start or end of a string). It fails to detect and remove the same dangerous elements if they appear elsewhere in the data, allowing them to pass through to critical system components.

How serious is CWE-796?

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-796?

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

How can I prevent CWE-796?

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-796?

Plexicus's SAST engine matches the data-flow signature for CWE-796 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-796?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/796.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-796

CWE-795 Parent

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…

CWE-797 Sibling

Only Filtering Special Elements at an Absolute Position

This vulnerability occurs when software checks for dangerous characters or patterns only at a fixed, hardcoded location in input data.…

Resources

Further reading

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