CWE-138: Improper Neutralization of Special Elements

Description:
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component.

Extended Description:
Most languages and protocols have their own special elements such as characters and reserved words. These special elements can carry control implications. If product does not prevent external control or influence over the inclusion of such special elements, the control flow of the program may be altered from what was intended. For example, both Unix and Windows interpret the symbol < ("less than") as meaning "read input from a file".

Affected languages: Not Language-Specific
Parent weaknesses: CWE-707

Consequences:
  - Confidentiality: Execute Unauthorized Code or Commands

Mitigations:
  - [Implementation] Developers should anticipate that special elements (e.g. delimiters, symbols) will be injected into input vectors of their product. One defense is to create an allowlist (e.g. a regular expression) that defines valid input according to the requirements specifications. Strictly filter any input that 
  - [Implementation] 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 pe
  - [Implementation] Use and specify an appropriate output encoding to ensure that the special elements are well-defined. A normal byte sequence in one encoding could be a special element in another.
  - [Implementation] 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 i

Demonstrative examples:
The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.
Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:
my $Username = GetUntrustedInput(); $Username =~ s/\.\.\///; my $filename = "/home/user/" . $Username; ReadAndSendFile($filename);
../../../etc/passwd

The following example assigns some
               character values to a list of characters and prints
               them each individually, and then as a string. The third
               character value is intended to be an integer taken from
               user input and converted to an int. The first print
               statement will print each character separated by a
               space.
However, if a NULL byte is read from stdin
               by fgetc, then it will return 0. When foo is printed as
               a string, the 0 at character foo[2] will act as a NULL
               terminator, and the second printf() statement will
			   not print foo[3].
char *foo; 
				   foo=malloc(sizeof(char)*5); 
				   foo[0]='a'; 
				   foo[1]='a'; 
				   foo[2]=fgetc(stdin); 
				   foo

Real-world examples:
  - CVE-2001-0677: Read arbitrary files from mail client by providing a special MIME header that is internally used to store pathnames for attachments.
  - CVE-2000-0703: Setuid program does not cleanse special escape sequence before sending data to a mail program, causing the mail program to process those sequences.
  - CVE-2003-0020: Multi-channel issue. Terminal escape sequences not filtered from log files.
  - CVE-2003-0083: Multi-channel issue. Terminal escape sequences not filtered from log files.