Multiple Interpretations of UI Input

Description

This vulnerability occurs when a user interface can interpret the same input in multiple ways, but automatically chooses a less secure option without warning the user.

Extended Description

This flaw often appears in features like search bars, file uploaders, or command panels where a single input string can be parsed differently. For example, an application might treat a user's entry as either a harmless data lookup or a system command, but it silently defaults to the more powerful—and risky—interpretation. This creates a security gap because the user is unaware their simple action could trigger a privileged operation. To prevent this, developers should implement clear input disambiguation. The UI should either ask the user to confirm their intent when ambiguity is detected, or consistently apply a strict, security-focused parsing rule by default. Adding user prompts or visual cues for risky interpretations shifts security from a hidden assumption to a shared responsibility with the end-user.

Common Consequences 1

Scope: Other

Impact: Varies by Context

Potential Mitigations 2

Phase: Implementation

Strategy: 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.

Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize). Make sure that the application does not decode the same input twice (Double Decoding of the Same Data). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.