CWE-241 Base Draft

Improper Handling of Unexpected Data Type

This vulnerability occurs when software fails to properly validate or safely process data that arrives in an unexpected format. For example, the system might expect a numeric input but receives text…

Definition

What is CWE-241?

This vulnerability occurs when software fails to properly validate or safely process data that arrives in an unexpected format. For example, the system might expect a numeric input but receives text instead, leading to crashes, errors, or security issues.
At its core, this weakness is about broken assumptions in your code's data handling. Developers often write logic expecting data in a specific format—like integers, strings, or structured objects. When the incoming data doesn't match that type (e.g., a user submits 'abc' where a number is required), the application may throw unhandled exceptions, behave unpredictably, or expose internal system details through error messages. To prevent this, implement strict input validation and type checking at all system boundaries. Use strong typing in your programming language where possible, and always sanitize and verify data from external sources like user inputs, APIs, or files before processing. Defensive coding practices, such as using try-catch blocks and designing functions to handle a range of input types gracefully, are essential to maintain stability and security when faced with unexpected data.
Real-world impact

Real-world CVEs caused by CWE-241

  • CVE-1999-1156

    FTP server crash via PORT command with non-numeric character.

  • CVE-2004-0270

    Anti-virus product has assert error when line length is non-numeric.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
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-241

  • Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
  • Implementation Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Detection signals

How to detect CWE-241

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-241 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-241?

This vulnerability occurs when software fails to properly validate or safely process data that arrives in an unexpected format. For example, the system might expect a numeric input but receives text instead, leading to crashes, errors, or security issues.

How serious is CWE-241?

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-241?

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

How can I prevent CWE-241?

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and…

How does Plexicus detect and fix CWE-241?

Plexicus's SAST engine matches the data-flow signature for CWE-241 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-241?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/241.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-241

CWE-228 Parent

Improper Handling of Syntactically Invalid Structure

This vulnerability occurs when software fails to properly reject or process input that doesn't follow the expected format or structure,…

CWE-166 Sibling

Improper Handling of Missing Special Element

This vulnerability occurs when software expects a specific delimiter, terminator, or other special marker in its input but fails to…

CWE-167 Sibling

Improper Handling of Additional Special Element

This vulnerability occurs when software receives data from another component but fails to properly process or validate unexpected special…

CWE-168 Sibling

Improper Handling of Inconsistent Special Elements

This vulnerability occurs when software fails to correctly process input containing conflicting or mismatched special elements like…

CWE-229 Sibling

Improper Handling of Values

This vulnerability occurs when software fails to correctly process situations where input contains too few values, too many values, or…

CWE-233 Sibling

Improper Handling of Parameters

This vulnerability occurs when software fails to correctly process input that contains an unexpected number of parameters, missing fields,…

CWE-237 Sibling

Improper Handling of Structural Elements

This vulnerability occurs when an application fails to properly validate, sanitize, or interpret the complex internal parts of structured…

Resources

Further reading

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