441
Common Consequences
Scope Impact Likelihood
Integrity Unexpected State
Potential Mitigations
Phase: Architecture and Design
Strategy = Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have
alternate names.
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 (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.
Demonstrative Examples
Example 1:
Windows provides the MultiByteToWideChar(), WideCharToMultiByte(), UnicodeToBytes(), and
BytesToUnicode() functions to convert between arbitrary multibyte (usually ANSI) character strings
and Unicode (wide character) strings. The size arguments to these functions are specified in
different units, (one in bytes, the other in characters) making their use prone to error.
In a multibyte character string, each character occupies a varying number of bytes, and therefore
the size of such strings is most easily specified as a total number of bytes. In Unicode, however,
characters are always a fixed size, and string lengths are typically given by the number of
characters they contain. Mistakenly specifying the wrong units in a size argument can lead to a
buffer overflow.
The following function takes a username specified as a multibyte string and a pointer to a structure
for user information and populates the structure with information about the specified user. Since
Windows authentication uses Unicode for usernames, the username argument is first converted
from a multibyte string to a Unicode string.
Example Language: C (Bad)
void getUserInfo(char *username, struct _USER_INFO_2 info){
WCHAR unicodeUser[UNLEN+1];
MultiByteToWideChar(CP_ACP, 0, username, -1, unicodeUser, sizeof(unicodeUser));
CWE Version 4.14
CWE-177: Improper Handling of URL Encoding (Hex Encoding)
CWE-177: Improper Handling of URL Encoding (Hex Encoding)