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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 = Output Encoding
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).
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.
Observed Examples
Reference Description
CVE-2002-0542 The mail program processes special "~" escape sequence even when not in
interactive mode.
https://www.cve.org/CVERecord?id=CVE-2002-0542
CVE-2000-0703 Setuid program does not filter escape sequences before calling mail program.
https://www.cve.org/CVERecord?id=CVE-2000-0703
CVE-2002-0986 Mail function does not filter control characters from arguments, allowing mail
message content to be modified.
https://www.cve.org/CVERecord?id=CVE-2002-0986
CVE-2003-0020 Multi-channel issue. Terminal escape sequences not filtered from log files.
https://www.cve.org/CVERecord?id=CVE-2003-0020
CVE-2003-0083 Multi-channel issue. Terminal escape sequences not filtered from log files.
https://www.cve.org/CVERecord?id=CVE-2003-0083
CVE-2003-0021 Terminal escape sequences not filtered by terminals when displaying files.
https://www.cve.org/CVERecord?id=CVE-2003-0021
CVE-2003-0022 Terminal escape sequences not filtered by terminals when displaying files.
https://www.cve.org/CVERecord?id=CVE-2003-0022
CVE-2003-0023 Terminal escape sequences not filtered by terminals when displaying files.
https://www.cve.org/CVERecord?id=CVE-2003-0023
CVE-2003-0063 Terminal escape sequences not filtered by terminals when displaying files.
https://www.cve.org/CVERecord?id=CVE-2003-0063
CVE-2000-0476 Terminal escape sequences not filtered by terminals when displaying files.
https://www.cve.org/CVERecord?id=CVE-2000-0476
CVE-2001-1556 MFV. (multi-channel). Injection of control characters into log files that allow
information hiding when using raw Unix programs to read the files.
https://www.cve.org/CVERecord?id=CVE-2001-1556
CWE Version 4.14
CWE-151: Improper Neutralization of Comment Delimiters
CWE-151: Improper Neutralization of Comment Delimiters