CWE Reference

CWE-138: Improper Neutralization of Special Elements

Official CWE-138 CWE context with Glexia analysis, remediation guidance, related CVEs, and ATT&CK context.

Release 4.20weaknessDraft

Glexia's Take

CWE-138: Improper Neutralization of Special Elements

Improper Neutralization of Special Elements represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Executive Impact

Confidentiality,Integrity,Availability,Other: Execute Unauthorized Code or Commands,Alter Execution Logic,DoS: Crash, Exit, or Restart

Developer Pattern

CWE-138 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.

Confidence

high confidence from CWE-138, 4.20.

Official CWE Definition

CWE-138: Improper Neutralization of Special Elements

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".

Type: weakness
Abstraction: Class
Status: Draft
Source: MITRE CWE definition

Developer And Remediation Guidance

How teams prevent and detect this weakness

Causes

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:,will have the first "../" stripped, resulting in:,This value is then concatenated with the /home/user/ directory:,which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-23).
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].

Remediation

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 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.
Implementation: [object Object]
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 inputs after they have been checked.
Implementation: 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 (CWE-88).

Detection

Code review
SAST
DAST
Focused regression tests

Mappings

Related CVEs, CWEs, and ATT&CK context

Related CWEs

Related CVEs

Related CVE mappings appear after CVE records are cross-indexed.

Open CWE CVE mapping

ATT&CK Relevance

ATT&CK relevance is shown only when reviewed or responsibly inferred.