CWE-790: Improper Filtering of Special Elements

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
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
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
will have the first "../" stripped, resulting in:
4
This value is then concatenated with the /home/user/ directory:
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

// 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.

CWE-790

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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

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Improper Filtering of Special Elements

What is CWE-790?

Real-world CVEs caused by CWE-790

Step-by-step attacker path

Vulnerable Perl

Secure pseudo

How to prevent CWE-790

How to detect CWE-790

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-790

Improper Neutralization of Special Elements

Improper Neutralization of Delimiters

Improper Neutralization of Input Terminators

Improper Neutralization of Input Leaders

Improper Neutralization of Quoting Syntax

Improper Neutralization of Escape, Meta, or Control Sequences

Improper Neutralization of Comment Delimiters

Improper Neutralization of Macro Symbols

Improper Neutralization of Substitution Characters

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`