The product does not properly handle when an input uses an alternate encoding that is valid for the control sphere to which the input is being sent.

What CVEs are caused by CWE-173?

8 recorded CVEs are attributed to CWE-173, including CVE-2019-9636, CVE-2026-10050, CVE-2026-19611.

What are the consequences of CWE-173?

Exploiting CWE-173 can lead to: Bypass Protection Mechanism.

Is CWE-173 actively exploited?

8 recorded CVEs are caused by CWE-173; none are currently in CISA's KEV catalog of actively exploited flaws.

CWE-173: Improper Handling of Alternate Encoding

How to prevent it

Practical mitigations for CWE-173, grouped by where in the lifecycle they apply.

Architecture and Design

Input Validation

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

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

Output Encoding

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

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.

CWE-173: Improper Handling of Alternate Encoding

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

References

Track CWE-173 CVEs

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Related weaknesses

Parent

Can precede

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

References

Track CWE-173 CVEs