Weaknesses in this attack-focused category do not properly filter and interpret special elements in user-controlled input which could cause adverse effect on the software behavior and integrity.
Precise terminology for the underlying weaknesses does not exist.
Therefore, these weaknesses use the terminology associated with the
Time of Introduction
Technical Impact: Unexpected state
Developers should anticipate that special elements will be
injected/removed/manipulated in the input vectors of their software
system. Use an appropriate combination of black lists and whitelists to
ensure only valid, expected and appropriate input is processed by the
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input
validation strategy, i.e., use a whitelist of acceptable inputs that
strictly conform to specifications. Reject any input that does not
strictly conform to specifications, or transform it into something that
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
(i.e., do not rely on a blacklist). A blacklist 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, blacklists can be useful for detecting potential
attacks or determining which inputs are so malformed that they should be
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 whitelist (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 (CWE-88).
Strategy: 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 whitelist validation schemes by introducing dangerous inputs after they have been checked.
Customized languages and grammars, even those that are specific to a
particular product, are potential sources of weaknesses that are related to
special elements. However, most researchers concentrate on the most commonly
used representations for data transmission, such as HTML and SQL. Any
representation that is commonly used is likely to be a rich source of
weaknesses; researchers are encouraged to investigate previously unexplored
Mapped Taxonomy Name
Mapped Node Name
Common Special Element Manipulations
Software Fault Patterns
Tainted input to command
The list of children for this entry is far from complete.