CWE-118 Class Incomplete

Incorrect Access of Indexable Resource ('Range Error')

This vulnerability occurs when software fails to properly check the boundaries of an indexed resource, like an array, buffer, or file, before accessing it. This allows reading from or writing to…

Definition

What is CWE-118?

This vulnerability occurs when software fails to properly check the boundaries of an indexed resource, like an array, buffer, or file, before accessing it. This allows reading from or writing to memory locations outside the intended range.
Think of this as a 'fence' problem. Your code has a defined area it's allowed to work with, like an array with 10 elements. A range error happens when the program tries to access the 11th or the -1st element because it skipped a crucial boundary check. This is a common root cause for crashes, unexpected behavior, and serious security flaws like buffer overflows. For developers, preventing this means rigorously validating all indices and pointers before use. Always ensure the index is greater than or equal to zero and less than the total length of the resource. Use safe language features and libraries that perform automatic bounds checking, and be especially vigilant with low-level operations in languages like C and C++, where manual management is required.
Real-world impact

Real-world CVEs caused by CWE-118

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

    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-118

  • 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-118

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-118 and opens a fix PR in under 60 seconds.

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Frequently asked questions

Frequently asked questions

What is CWE-118?

This vulnerability occurs when software fails to properly check the boundaries of an indexed resource, like an array, buffer, or file, before accessing it. This allows reading from or writing to memory locations outside the intended range.

How serious is CWE-118?

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-118?

MITRE lists the following affected platforms: Not Technology-Specific.

How can I prevent CWE-118?

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-118?

Plexicus's SAST engine matches the data-flow signature for CWE-118 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-118?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/118.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-118

CWE-664 Parent

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CWE-1229 Sibling

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CWE-1250 Sibling

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CWE-1329 Sibling

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CWE-221 Sibling

Information Loss or Omission

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CWE-372 Sibling

Incomplete Internal State Distinction

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CWE-400 Sibling

Uncontrolled Resource Consumption

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CWE-404 Sibling

Improper Resource Shutdown or Release

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CWE-410 Sibling

Insufficient Resource Pool

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Resources

Further reading

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