The product receives input from an upstream component, but it does not handle or incorrectly handles when an expected special element is missing.

What CVEs are caused by CWE-166?

2 recorded CVEs are attributed to CWE-166, including CVE-2026-21218, CVE-2024-38091.

What are the consequences of CWE-166?

Exploiting CWE-166 can lead to: DoS: Crash, Exit, or Restart.

Is CWE-166 actively exploited?

2 recorded CVEs are caused by CWE-166; none are currently in CISA's KEV catalog of actively exploited flaws.

CWE-166: Improper Handling of Missing Special Element

Developers should anticipate that special elements will be removed in the input vectors of their product. Use an appropriate combination of denylists and allowlists to ensure only valid, expected and appropriate input is processed by the system.

Implementation

Input Validation

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

Input Validation

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.

When it is introduced

CWE-166: Improper Handling of Missing Special Element

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Illustrative examples

Terminology & mappings

Mapped taxonomies

Frequently asked questions

What is CWE-166?

What CVEs are caused by CWE-166?

How do you prevent CWE-166?

What are the consequences of CWE-166?

Is CWE-166 actively exploited?

References

Stay ahead of CWE-166

When it is introduced

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Illustrative examples

Related weaknesses

Parent

Terminology & mappings

Mapped taxonomies

Frequently asked questions

What is CWE-166?

What CVEs are caused by CWE-166?

How do you prevent CWE-166?

What are the consequences of CWE-166?

Is CWE-166 actively exploited?

References

Stay ahead of CWE-166