Improper Handling of Additional Special Element

Description

This vulnerability occurs when software receives data from another component but fails to properly process or validate unexpected special characters or control elements within that input.

Extended Description

This flaw typically arises in parsers, interpreters, or data processors that expect a specific format. When an extra delimiter, escape sequence, or control character appears unexpectedly, the system might misinterpret boundaries, execute unintended commands, or corrupt data structures. This often leads to injection attacks, crashes, or logic errors. Developers can prevent this by implementing strict input validation that rejects or sanitizes unexpected special elements before processing. Use well-tested parsing libraries with clear specifications for handling edge cases, and design data handlers to be robust against malformed input by failing securely rather than making dangerous assumptions.

Common Consequences 1

Scope: Integrity

Impact: Unexpected State

Potential Mitigations 4

Developers should anticipate that extra special elements will be injected 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: Output Encoding

While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments. The most conservative approach is to escape or filter all characters that do not pass an extremely strict allowlist (such as everything that is not alphanumeric or white space). If some special characters are still needed, such as white space, wrap each argument in quotes after the escaping/filtering step. Be careful of argument injection (Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')).

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.

Observed Examples 3

CVE-2000-0116Extra "<" in front of SCRIPT tag bypasses XSS prevention.

CVE-2001-1157Extra "<" in front of SCRIPT tag.

CVE-2002-2086"<script" - probably a cleansing error