CWE-174 Variant Draft

Double Decoding of the Same Data

This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or neutralize security checks that happen after the first decode,…

Definition

What is CWE-174?

This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or neutralize security checks that happen after the first decode, leaving the system exposed.
Double decoding is dangerous because it breaks the expected data flow. Security mechanisms like input validation, sanitization, or intrusion detection are often placed after an initial decoding step, assuming the data is now in its canonical form. When a second, unexpected decode happens, it can transform the data again, rendering those intermediate protections useless and allowing malicious payloads to slip through. Developers can prevent this by establishing a strict, one-time decoding policy in a centralized location within the application's data pipeline. All incoming data should be decoded, validated, and sanitized once into a trusted, internal format before being passed to other components. This ensures security controls are applied to the final, operational form of the data and cannot be circumvented by further transformations.
Real-world impact

Real-world CVEs caused by CWE-174

  • CVE-2004-1315

    Forum software improperly URL decodes the highlight parameter when extracting text to highlight, which allows remote attackers to execute arbitrary PHP code by double-encoding the highlight value so that special characters are inserted into the result.

  • CVE-2004-1939

    XSS protection mechanism attempts to remove "/" that could be used to close tags, but it can be bypassed using double encoded slashes (%252F)

  • CVE-2001-0333

    Directory traversal using double encoding.

  • CVE-2004-1938

    "%2527" (double-encoded single quote) used in SQL injection.

  • CVE-2005-1945

    Double hex-encoded data.

  • CVE-2005-0054

    Browser executes HTML at higher privileges via URL with hostnames that are double hex encoded, which are decoded twice to generate a malicious hostname.

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-174

  • 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 Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.
  • 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-174

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-174 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-174?

This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or neutralize security checks that happen after the first decode, leaving the system exposed.

How serious is CWE-174?

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-174?

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

How can I prevent CWE-174?

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-174?

Plexicus's SAST engine matches the data-flow signature for CWE-174 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-174?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/174.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-174

CWE-675 Parent

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CWE-1341 Sibling

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CWE-764 Sibling

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CWE-765 Sibling

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Resources

Further reading

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