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.
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".
Potential Impact
Confidentiality, Integrity, Availability, Other
Execute Unauthorized Code or Commands, Alter Execution Logic, DoS: Crash, Exit, or Restart
Demonstrative Examples
my $Username = GetUntrustedInput();$Username =~ s/\.\.\///;my $filename = "/home/user/" . $Username;ReadAndSendFile($filename);../../../etc/passwd../../etc/passwd/home/user/../../etc/passwdchar *foo;
foo=malloc(sizeof(char)*5);
foo[0]='a';
foo[1]='a';
foo[2]=fgetc(stdin);
foo[3]='c';
foo[4]='\0';
printf("%c %c %c %c %c \n",foo[0],foo[1],foo[2],foo[3],foo[4]);
printf("%s\n",foo);Mitigations & Prevention
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 does not match against the allowlist. Properly encode your output, and quote any elements that have special meaning to the component with which you are communicating.
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 relat
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.
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.
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 qu
Real-World CVE Examples
| CVE ID | Description |
|---|---|
| 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. |
Related Weaknesses
Taxonomy Mappings
- PLOVER: — Special Elements (Characters or Reserved Words)
- PLOVER: — Custom Special Character Injection
- Software Fault Patterns: SFP24 — Tainted input to command
Frequently Asked Questions
What is CWE-138?
CWE-138 (Improper Neutralization of Special Elements) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Class-level weakness. 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 th...
How can CWE-138 be exploited?
Attackers can exploit CWE-138 (Improper Neutralization of Special Elements) to execute unauthorized code or commands, alter execution logic, dos: crash, exit, or restart. This weakness is typically introduced during the Implementation phase of software development.
How do I prevent CWE-138?
Key mitigations include: 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) th
What is the severity of CWE-138?
CWE-138 is classified as a Class-level weakness (High abstraction). It has been observed in 4 real-world CVEs.