CWE-182 Base Draft

Collapse of Data into Unsafe Value

This vulnerability occurs when an application's data filtering or transformation process incorrectly merges or simplifies information, producing a result that violates security rules. Essentially,…

Definition

What is CWE-182?

This vulnerability occurs when an application's data filtering or transformation process incorrectly merges or simplifies information, producing a result that violates security rules. Essentially, safe input gets collapsed into a dangerous value.
Collapse of Data into Unsafe Value happens when security checks are applied to individual pieces of data in isolation, but the process that combines or reduces this data (like trimming, canonicalizing, or aggregating) creates a new, composite value that bypasses those original checks. For example, filtering separate 'script' and 'alert' strings might pass validation, but if they are later concatenated, they form a working XSS payload. The core failure is that validation logic doesn't account for how safe components can become dangerous when merged. Developers can prevent this by validating the final, assembled data in the exact context where it will be used, not just its individual parts. Treat any data aggregation or transformation step as a potential new input source that requires its own security evaluation. Implementing allow-list validation on the complete output string and using context-aware encoding libraries are key defensive strategies.
Real-world impact

Real-world CVEs caused by CWE-182

  • CVE-2004-0815

    "/.////" in pathname collapses to absolute path.

  • CVE-2005-3123

    "/.//..//////././" is collapsed into "/.././" after ".." and "//" sequences are removed.

  • CVE-2002-0325

    ".../...//" collapsed to "..." due to removal of "./" in web server.

  • CVE-2002-0784

    chain: HTTP server protects against ".." but allows "." variants such as "////./../.../". If the server removes "/.." sequences, the result would collapse into an unsafe value "////../" (CWE-182).

  • CVE-2005-2169

    MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".

  • CVE-2001-1157

    XSS protection mechanism strips a sequence that is nested in another sequence.

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-182

  • Architecture and Design Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
  • 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.
  • Canonicalize the name to match that of the file system's representation of the name. This can sometimes be achieved with an available API (e.g. in Win32 the GetFullPathName function).
Detection signals

How to detect CWE-182

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

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Frequently asked questions

Frequently asked questions

What is CWE-182?

This vulnerability occurs when an application's data filtering or transformation process incorrectly merges or simplifies information, producing a result that violates security rules. Essentially, safe input gets collapsed into a dangerous value.

How serious is CWE-182?

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-182?

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

How can I prevent CWE-182?

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names. 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…

How does Plexicus detect and fix CWE-182?

Plexicus's SAST engine matches the data-flow signature for CWE-182 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-182?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/182.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-182

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CWE-172 Sibling

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CWE-228 Sibling

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CWE-240 Sibling

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Resources

Further reading

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