Improper Handling of Inconsistent Special Elements

Description

This vulnerability occurs when software fails to correctly process input containing conflicting or mismatched special elements like characters, symbols, or reserved keywords.

Extended Description

This flaw typically manifests when paired elements, such as brackets, quotes, or tags, appear in an invalid sequence or structure. For example, the software might incorrectly handle closing brackets that appear before their opening counterparts, or it might fail when special delimiters are improperly nested within each other, breaking the expected hierarchy. From a security perspective, inconsistent handling of these elements can create parsing discrepancies that attackers exploit to bypass validation, inject malicious code, or cause the application to crash. Developers should implement strict, context-aware parsing logic that validates the order, nesting, and pairing of all special elements within the input stream, rather than processing them in isolation.

Common Consequences 1

Scope: AvailabilityAccess ControlNon-Repudiation

Impact: DoS: Crash, Exit, or RestartBypass Protection MechanismHide Activities

Potential Mitigations 3

Developers should anticipate that inconsistent special elements will be injected/manipulated 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.

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.