CWE-790 Class Incomplete

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 characters or control elements before passing that data to…

Definition

What is CWE-790?

This vulnerability occurs when an application accepts data from a source but fails to properly sanitize or incorrectly filters out special characters or control elements before passing that data to another system component.
At its core, this weakness is about a broken chain of trust in data handling. An application often assumes data from an upstream component (like a user, another service, or a database) is safe, but it must actively validate and neutralize potentially dangerous elements—such as quotes, brackets, script tags, or command delimiters—before that data flows downstream. If this filtering step is missing or flawed, the downstream component interprets these special elements as part of its own commands or code, leading to security breaches. For developers, this means you cannot rely on the source of your data. You must implement context-aware filtering or encoding at the point where data is used. For example, data destined for SQL needs parameterized queries, data for HTML output needs HTML entity encoding, and data for system commands needs strict allow-list validation. The fix isn't a single filter; it's applying the correct defense for each specific output context to prevent injections, cross-site scripting (XSS), and command execution attacks.
Real-world impact

Real-world CVEs caused by CWE-790

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-790

  • 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-790

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-790 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-790?

This vulnerability occurs when an application accepts data from a source but fails to properly sanitize or incorrectly filters out special characters or control elements before passing that data to another system component.

How serious is CWE-790?

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-790?

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

How can I prevent CWE-790?

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-790?

Plexicus's SAST engine matches the data-flow signature for CWE-790 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-790?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/790.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-790

CWE-138 Parent

Improper Neutralization of Special Elements

This vulnerability occurs when an application accepts external input but fails to properly sanitize special characters or syntax that have…

CWE-140 Sibling

Improper Neutralization of Delimiters

This vulnerability occurs when an application fails to properly handle or sanitize delimiter characters within data inputs, allowing them…

CWE-147 Sibling

Improper Neutralization of Input Terminators

This vulnerability occurs when an application accepts external input but fails to properly handle special characters that downstream…

CWE-148 Sibling

Improper Neutralization of Input Leaders

This vulnerability occurs when an application fails to properly validate or handle input that begins with special control characters or…

CWE-149 Sibling

Improper Neutralization of Quoting Syntax

This vulnerability occurs when an application fails to properly validate or escape quote characters (like single ' or double " quotes) in…

CWE-150 Sibling

Improper Neutralization of Escape, Meta, or Control Sequences

This vulnerability occurs when an application fails to properly sanitize or escape special character sequences in user-supplied input…

CWE-151 Sibling

Improper Neutralization of Comment Delimiters

This vulnerability occurs when an application accepts user input and fails to properly sanitize characters that can be interpreted as…

CWE-152 Sibling

Improper Neutralization of Macro Symbols

This vulnerability occurs when an application accepts user input containing macro symbols (like those used in templates or configuration…

CWE-153 Sibling

Improper Neutralization of Substitution Characters

This vulnerability occurs when an application accepts user input and fails to properly sanitize special characters that can trigger…

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Further reading

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