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".

Primary

Implementation
REALIZATION: This weakness is caused during implementation of an architectural security tactic.

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

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
Input Validation

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 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.

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
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.

Implementation
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).

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.

my $Username = GetUntrustedInput();$Username =~ s/\.\.\///;my $filename = "/home/user/" . $Username;ReadAndSendFile($filename);

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:

../../../etc/passwd

will have the first "../" stripped, resulting in:

../../etc/passwd

This value is then concatenated with the /home/user/ directory:

/home/user/../../etc/passwd

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.

char *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);

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].

CVE-2001-0677
Read arbitrary files from mail client by providing a special MIME header that is internally used to store pathnames for attachments.
https://www.cve.org/CVERecord?id=CVE-2001-0677

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.
https://www.cve.org/CVERecord?id=CVE-2000-0703

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

Special Elements (Characters or Reserved Words)

Custom Special Character Injection

SFP24
Tainted input to command

Discouraged
This CWE entry is a level-1 Class (i.e., a child of a Pillar). It might have lower-level children that would be more appropriate
Examine children of this entry to see if there is a better fit

For many years, there have been significant subtree
overlap challenges between CWE-138 (and descendants) and
CWE-74 (and descendants) due to variances in the "facets" or
"dimensions" of abstraction. Under CWE-138, entries are
hierarchically organized around the "type of special
element" that is not neutralized. Under CWE-74,
hierarchical organization is around the "type of
data/command" that is affected. This multi-faceted
challenge will require extensive research and significant
changes that have not been able to be resolved as of CWE
4.19.

This weakness can be related to interpretation conflicts or interaction errors in intermediaries (such as proxies or application firewalls) when the intermediary's model of an endpoint does not account for protocol-specific special elements.
See this entry's children for different types of special elements that have been observed at one point or another. However, it can be difficult to find suitable CVE examples. In an attempt to be complete, CWE includes some types that do not have any associated observed example.
This weakness is probably under-studied for proprietary or custom formats. It is likely that these issues are fairly common in applications that use their own custom format for configuration files, logs, meta-data, messaging, etc. They would only be found by accident or with a focused effort based on an understanding of the format.

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Description, Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Description, Relationships, Other_Notes, Taxonomy_Mappings

CWE Content Team
MITRE
2009-03-10
updated Description, Name

CWE Content Team
MITRE
2009-07-27
updated Applicable_Platforms, Description, Observed_Examples, Other_Notes, Potential_Mitigations, Relationship_Notes, Relationships, Research_Gaps, Taxonomy_Mappings, Weakness_Ordinalities

CWE Content Team
MITRE
2009-12-28
updated Relationships

CWE Content Team
MITRE
2010-04-05
updated Description, Name

CWE Content Team
MITRE
2010-12-13
updated Description

CWE Content Team
MITRE
2011-03-29
updated Potential_Mitigations

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Common_Consequences, Relationships

CWE Content Team
MITRE
2014-07-30
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2017-01-19
updated Relationships

CWE Content Team
MITRE
2017-05-03
updated Potential_Mitigations

CWE Content Team
MITRE
2017-11-08
updated Modes_of_Introduction, Potential_Mitigations, Relationships

CWE Content Team
MITRE
2020-02-24
updated Potential_Mitigations, Relationships

CWE Content Team
MITRE
2020-06-25
updated Potential_Mitigations

CWE Content Team
MITRE
2021-10-28
updated Relationships

CWE Content Team
MITRE
2022-04-28
updated Related_Attack_Patterns

CWE Content Team
MITRE
2023-01-31
updated Description, Potential_Mitigations

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2024-02-29
4.14
2024-02-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Demonstrative_Examples, Maintenance_Notes

Special Elements (Characters or Reserved Words)
Failure to Sanitize Special Elements
Improper Sanitization of Special Elements