The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

Improper encoding or escaping can allow attackers to change the commands that are sent to another component, inserting malicious commands instead.
Most products follow a certain protocol that uses structured messages for communication between components, such as queries or commands. These structured messages can contain raw data interspersed with metadata or control information. For example, "GET /index.html HTTP/1.1" is a structured message containing a command ("GET") with a single argument ("/index.html") and metadata about which protocol version is being used ("HTTP/1.1").
If an application uses attacker-supplied inputs to construct a structured message without properly encoding or escaping, then the attacker could insert special characters that will cause the data to be interpreted as control information or metadata. Consequently, the component that receives the output will perform the wrong operations, or otherwise interpret the data incorrectly.

Primary

Resultant

Output Sanitization

Output Validation

Output Encoding

Implementation

Operation

High

Integrity
Modify Application Data
The communications between components can be modified in unexpected ways. Unexpected commands can be executed, bypassing other security mechanisms. Incoming data can be misinterpreted.

Integrity
Confidentiality
Availability
Access Control
Execute Unauthorized Code or Commands
The communications between components can be modified in unexpected ways. Unexpected commands can be executed, bypassing other security mechanisms. Incoming data can be misinterpreted.

Confidentiality
Bypass Protection Mechanism
The communications between components can be modified in unexpected ways. Unexpected commands can be executed, bypassing other security mechanisms. Incoming data can be misinterpreted.

Automated Static Analysis
This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.
Moderate
This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Automated Dynamic Analysis
This weakness can be detected using dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Architecture and Design
Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
For example, consider using the ESAPI Encoding control [REF-45] or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error.
Alternately, use built-in functions, but consider using wrappers in case those functions are discovered to have a vulnerability.

Architecture and Design
Parameterization

If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
For example, stored procedures can enforce database query structure and reduce the likelihood of SQL injection.

Architecture and Design
Implementation
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies.

Architecture and Design
In some cases, input validation may be an important strategy when output encoding is not a complete solution. For example, you may be providing the same output that will be processed by multiple consumers that use different encodings or representations. In other cases, you may be required to allow user-supplied input to contain control information, such as limited HTML tags that support formatting in a wiki or bulletin board. When this type of requirement must be met, use an extremely strict allowlist to limit which control sequences can be used. Verify that the resulting syntactic structure is what you expect. Use your normal encoding methods for the remainder of the input.

Architecture and Design
Use input validation as a defense-in-depth measure to reduce the likelihood of output encoding errors (see CWE-20).

Requirements
Fully specify which encodings are required by components that will be communicating with each other.

Implementation
When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.

This code displays an email address that was submitted as part of a form.

<% String email = request.getParameter("email"); %>...Email Address: <%= email %>

The value read from the form parameter is reflected back to the client browser without having been encoded prior to output, allowing various XSS attacks (CWE-79).

Consider a chat application in which a front-end web application communicates with a back-end server. The back-end is legacy code that does not perform authentication or authorization, so the front-end must implement it. The chat protocol supports two commands, SAY and BAN, although only administrators can use the BAN command. Each argument must be separated by a single space. The raw inputs are URL-encoded. The messaging protocol allows multiple commands to be specified on the same line if they are separated by a "|" character.
First let's look at the back end command processor code

$inputString = readLineFromFileHandle($serverFH);

# generate an array of strings separated by the "|" character.
@commands = split(/\|/, $inputString);
foreach $cmd (@commands) {

# separate the operator from its arguments based on a single whitespace
($operator, $args) = split(/ /, $cmd, 2);
$args = UrlDecode($args);if ($operator eq "BAN") {ExecuteBan($args);}elsif ($operator eq "SAY") {ExecuteSay($args);}}

The front end web application receives a command, encodes it for sending to the server, performs the authorization check, and sends the command to the server.

$inputString = GetUntrustedArgument("command");($cmd, $argstr) = split(/\s+/, $inputString, 2);

# removes extra whitespace and also changes CRLF's to spaces
$argstr =~ s/\s+/ /gs;
$argstr = UrlEncode($argstr);if (($cmd eq "BAN") && (! IsAdministrator($username))) {die "Error: you are not the admin.\n";}

# communicate with file server using a file handle
$fh = GetServerFileHandle("myserver");
print $fh "$cmd $argstr\n";

It is clear that, while the protocol and back-end allow multiple commands to be sent in a single request, the front end only intends to send a single command. However, the UrlEncode function could leave the "|" character intact. If an attacker provides:

SAY hello world|BAN user12

then the front end will see this is a "SAY" command, and the $argstr will look like "hello world | BAN user12". Since the command is "SAY", the check for the "BAN" command will fail, and the front end will send the URL-encoded command to the back end:

SAY hello%20world|BAN%20user12

The back end, however, will treat these as two separate commands:

SAY hello worldBAN user12

Notice, however, that if the front end properly encodes the "|" with "%7C", then the back end will only process a single command.

This example takes user input, passes it through an encoding scheme, then lists the contents of the user's home directory based on the user name.

sub GetUntrustedInput {return($ARGV[0]);}
sub encode {my($str) = @_;$str =~ s/\&/\&amp;/gs;$str =~ s/\"/\&quot;/gs;$str =~ s/\'/\&apos;/gs;$str =~ s/\</\&lt;/gs;$str =~ s/\>/\&gt;/gs;return($str);}
sub doit {my $uname = encode(GetUntrustedInput("username"));print "<b>Welcome, $uname!</b><p>\n";system("cd /home/$uname; /bin/ls -l");
}

The programmer attempts to encode dangerous characters, however the denylist for encoding is incomplete (CWE-184) and an attacker can still pass a semicolon, resulting in a chain with OS command injection (CWE-78).
Additionally, the encoding routine is used inappropriately with command execution. An attacker doesn't even need to insert their own semicolon. The attacker can instead leverage the encoding routine to provide the semicolon to separate the commands. If an attacker supplies a string of the form:

' pwd

then the program will encode the apostrophe and insert the semicolon, which functions as a command separator when passed to the system function. This allows the attacker to complete the command injection.

CVE-2021-41232
Chain: authentication routine in Go-based agile development product does not escape user name (CWE-116), allowing LDAP injection (CWE-90)
https://www.cve.org/CVERecord?id=CVE-2021-41232

CVE-2008-4636
OS command injection in backup software using shell metacharacters in a filename; correct behavior would require that this filename could not be changed.
https://www.cve.org/CVERecord?id=CVE-2008-4636

CVE-2008-0769
Web application does not set the charset when sending a page to a browser, allowing for XSS exploitation when a browser chooses an unexpected encoding.
https://www.cve.org/CVERecord?id=CVE-2008-0769

CVE-2008-0005
Program does not set the charset when sending a page to a browser, allowing for XSS exploitation when a browser chooses an unexpected encoding.
https://www.cve.org/CVERecord?id=CVE-2008-0005

CVE-2008-5573
SQL injection via password parameter; a strong password might contain "&"
https://www.cve.org/CVERecord?id=CVE-2008-5573

CVE-2008-3773
Cross-site scripting in chat application via a message subject, which normally might contain "&" and other XSS-related characters.
https://www.cve.org/CVERecord?id=CVE-2008-3773

CVE-2008-0757
Cross-site scripting in chat application via a message, which normally might be allowed to contain arbitrary content.
https://www.cve.org/CVERecord?id=CVE-2008-0757

22
Improper Output Handling

IDS00-J
Sanitize untrusted data passed across a trust boundary
Exact

IDS05-J
Use a subset of ASCII for file and path names

IDS00-J
Prevent SQL injection
Imprecise

IDS33-PL
Sanitize untrusted data passed across a trust boundary
Exact

Allowed-with-Review
This CWE entry is a Class and might have Base-level children that would be more appropriate
Examine children of this entry to see if there is a better fit

This weakness is primary to all weaknesses related to injection (CWE-74) since the inherent nature of injection involves the violation of structured messages.

CWE-116 and CWE-20 have a close association because, depending on the nature of the structured message, proper input validation can indirectly prevent special characters from changing the meaning of a structured message. For example, by validating that a numeric ID field should only contain the 0-9 characters, the programmer effectively prevents injection attacks.
However, input validation is not always sufficient, especially when less stringent data types must be supported, such as free-form text. Consider a SQL injection scenario in which a last name is inserted into a query. The name "O'Reilly" would likely pass the validation step since it is a common last name in the English language. However, it cannot be directly inserted into the database because it contains the "'" apostrophe character, which would need to be escaped or otherwise neutralized. In this case, stripping the apostrophe might reduce the risk of SQL injection, but it would produce incorrect behavior because the wrong name would be recorded.

The usage of the "encoding" and "escaping" terms varies widely. For example, in some programming languages, the terms are used interchangeably, while other languages provide APIs that use both terms for different tasks. This overlapping usage extends to the Web, such as the "escape" JavaScript function whose purpose is stated to be encoding. The concepts of encoding and escaping predate the Web by decades. Given such a context, it is difficult for CWE to adopt a consistent vocabulary that will not be misinterpreted by some constituency.
This is a data/directive boundary error in which data boundaries are not sufficiently enforced before it is sent to a different control sphere.
While many published vulnerabilities are related to insufficient output encoding, there is such an emphasis on input validation as a protection mechanism that the underlying causes are rarely described. Within CVE, the focus is primarily on well-understood issues like cross-site scripting and SQL injection. It is likely that this weakness frequently occurs in custom protocols that support multiple encodings, which are not necessarily detectable with automated techniques.

CWE Community
2006-07-19
Draft 3
2006-07-19
Submitted by members of the CWE community to extend early CWE versions

Sean Eidemiller
Cigital
2008-07-01
added/updated demonstrative examples

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Name, Relationships

CWE Content Team
MITRE
2009-01-12
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Demonstrative_Examples, Description, Likelihood_of_Exploit, Name, Observed_Examples, Potential_Mitigations, References, Relationship_Notes, Relationships, Research_Gaps, Terminology_Notes, Theoretical_Notes

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

CWE Content Team
MITRE
2009-05-27
updated Related_Attack_Patterns

CWE Content Team
MITRE
2009-07-27
updated Demonstrative_Examples

CWE Content Team
MITRE
2009-10-29
updated Relationships

CWE Content Team
MITRE
2009-12-28
updated Demonstrative_Examples, Potential_Mitigations

CWE Content Team
MITRE
2010-02-16
updated Detection_Factors, Potential_Mitigations, References, Taxonomy_Mappings

CWE Content Team
MITRE
2010-04-05
updated Potential_Mitigations

CWE Content Team
MITRE
2010-06-21
updated Potential_Mitigations

CWE Content Team
MITRE
2011-03-29
updated Relationship_Notes, Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2011-09-13
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated References, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-06-23
updated References

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

CWE Content Team
MITRE
2015-12-07
updated Relationships

CWE Content Team
MITRE
2017-01-19
updated Relationships

CWE Content Team
MITRE
2017-05-03
updated Related_Attack_Patterns

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Common_Consequences, Demonstrative_Examples, Likelihood_of_Exploit, References, Taxonomy_Mappings

CWE Content Team
MITRE
2018-03-27
updated References

CWE Content Team
MITRE
2019-01-03
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2019-06-20
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-06-25
updated Applicable_Platforms, Demonstrative_Examples, Potential_Mitigations

CWE Content Team
MITRE
2021-03-15
updated Relationships, Terminology_Notes

CWE Content Team
MITRE
2021-10-28
updated Relationships

CWE Content Team
MITRE
2022-10-13
updated Observed_Examples

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated References, Relationships, Time_of_Introduction

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2024-07-16
4.15
2024-07-16
updated Applicable_Platforms

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated References

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Demonstrative_Examples, Relationships, Weakness_Ordinalities

Output Validation
Incorrect Output Sanitization
Insufficient Output Sanitization