CWE-758 Class Incomplete

Reliance on Undefined, Unspecified, or Implementation-Defined Behavior

This weakness occurs when software depends on specific behaviors of an API, data structure, or system component that are not formally guaranteed by its specification. The code assumes these…

Definition

What is CWE-758?

This weakness occurs when software depends on specific behaviors of an API, data structure, or system component that are not formally guaranteed by its specification. The code assumes these behaviors will always work a certain way, but they might change or fail under different conditions.
Relying on undefined or platform-specific behavior creates fragile code that can break unexpectedly. For example, a function might return results in a particular order on your development machine, but a different compiler or operating system version could alter that order, causing logic errors or crashes. This often surfaces during porting to a new environment or during complex interactions between components. To avoid this, developers should strictly use documented, standardized behaviors and avoid assumptions about internal implementation details that aren't part of the official contract.
Real-world impact

Real-world CVEs caused by CWE-758

  • CVE-2006-1902

    Change in C compiler behavior causes resultant buffer overflows in programs that depend on behaviors that were undefined in the C standard.

How attackers exploit it

Step-by-step attacker path

  1. 1

    This code assumes a particular function will always be found at a particular address. It assigns a pointer to that address and calls the function.

  2. 2

    The same function may not always be found at the same memory address. This could lead to a crash, or an attacker may alter the memory at the expected address, leading to arbitrary code execution.

  3. 3

    The following function returns a stack address.

Vulnerable code example

Vulnerable C

This code assumes a particular function will always be found at a particular address. It assigns a pointer to that address and calls the function.

Vulnerable C 
int (*pt2Function) (float, char, char)=0x08040000;
  int result2 = (*pt2Function) (12, 'a', 'b');
```
// Here we can inject code to execute.*
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-758

  • 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-758

Fuzzing High

Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.

Plexicus auto-fix

Plexicus auto-detects CWE-758 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-758?

This weakness occurs when software depends on specific behaviors of an API, data structure, or system component that are not formally guaranteed by its specification. The code assumes these behaviors will always work a certain way, but they might change or fail under different conditions.

How serious is CWE-758?

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-758?

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

How can I prevent CWE-758?

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-758?

Plexicus's SAST engine matches the data-flow signature for CWE-758 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-758?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/758.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-758

CWE-710 Parent

Improper Adherence to Coding Standards

This weakness occurs when developers don't consistently follow established coding standards and best practices, which can introduce…

CWE-1041 Sibling

Use of Redundant Code

This weakness occurs when a codebase contains identical or nearly identical logic duplicated across multiple functions, methods, or…

CWE-1044 Sibling

Architecture with Number of Horizontal Layers Outside of Expected Range

This occurs when a software system is built with either too many or too few distinct architectural layers, falling outside a recommended…

CWE-1048 Sibling

Invokable Control Element with Large Number of Outward Calls

This weakness occurs when a single function, method, or callable code block makes an excessively high number of calls to other objects or…

CWE-1059 Sibling

Insufficient Technical Documentation

This weakness occurs when a software or hardware product lacks comprehensive technical documentation. Missing or incomplete details about…

CWE-1061 Sibling

Insufficient Encapsulation

This weakness occurs when a software component exposes too much of its internal workings, such as data structures or implementation logic.…

CWE-1065 Sibling

Runtime Resource Management Control Element in a Component Built to Run on Application Servers

This weakness occurs when an application built to run on a managed application server bypasses the server's high-level APIs and instead…

CWE-1066 Sibling

Missing Serialization Control Element

This weakness occurs when a class or data structure is marked as serializable but lacks the required control methods to properly handle…

CWE-1068 Sibling

Inconsistency Between Implementation and Documented Design

This weakness occurs when the actual code implementation deviates from the intended design described in its official documentation,…

Resources

Further reading

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