CWE WEAKNESSES  /  CWE-138

CWE-138

Improper Neutralization of Special Elements

What it is

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".

Impact

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 does not match against the allowlist. Properly encode your output, and quote any elements that have
• [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 performing input validation, consider all potentially relevant properties, including length, type of input, the full r
• [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 whit

Real-world CVE 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.

Related weaknesses

Source: MITRE CWE. View on cwe.mitre.org →