The software 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.
Extended Description
Improper encoding or escaping can allow attackers to change the commands
that are sent to another component, inserting malicious commands
instead.
Most software follows 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.
Alternate Terms
Output Sanitization
Output Validation
Output Encoding
Terminology Notes
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. Of course, 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.
Time of Introduction
Architecture and Design
Implementation
Operation
Applicable Platforms
Languages
All
Technology Classes
Database-Server: (Often)
Web-Server: (Often)
Common Consequences
Scope
Effect
Integrity
Confidentiality
Authorization
The communications between components can be modified in unexpected
ways. Unexpected commands can be executed, bypassing other security
mechanisms. Incoming data can be misinterpreted
Likelihood of Exploit
Very High
Demonstrative Examples
Example 1
Here a value read from an HTML form parameter is reflected back to
the client browser without having been encoded prior to output.
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.
Back End: Command Processor Code
(Bad Code)
Perl
$inputString = readLineFromFileHandle($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);
}
}
Front End: Web Application
In this code, the web application receives a command, encodes it for
sending to the server, performs the authorization check, and sends the
command to the server.
(Bad Code)
Perl
$inputString = GetUntrustedArgument("command");
($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";
Diagnosis
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:
(Attack)
SAY hello world|BAN user12
then the front end will see this is a "SAY" command, and the $argstr
will look like "hello world | BAN user12". Since the command is "SAY",
the check for the "BAN" command will fail, and the front end will send
the URL-encoded command to the back end:
SAY hello%20world|BAN%20user12
The back end, however, will treat these as two separate commands: "SAY
hello world" and "BAN user12". Notice, however, that if the front end
properly encodes the "|" with "%7C", then the back end will only process
a single command.
Example 3
This example takes user input, passes it through an encoding scheme
and then creates a directory specified by the user.
(Bad Code)
Perl
sub GetUntrustedInput {
return($ARGV[0]);
}
sub encode {
my($str) = @_;
$str =~ s/\&/\&/gs;
$str =~ s/\"/\"/gs;
$str =~ s/\'/\'/gs;
$str =~ s/\</\</gs;
$str =~ s/\>/\>/gs;
return($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
blacklist for encoding is incomplete (CWE-184) and an attacker can still
pass a semicolon, resulting in a chain with command injection
(CWE-77).
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:
(Attack)
' 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.
OS command injection in backup software using
shell metacharacters in a filename; correct behavior would require that this
filename could not be changed.
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.
Cross-site scripting in chat application via a
message, which normally might be allowed to contain arbitrary
content.
Potential Mitigations
Phase
Description
Architecture and Design
Use languages, libraries, or frameworks that make it easier to
generate properly encoded output.
Examples include the ESAPI Encoding control.
Alternately, use built-in functions, but consider using wrappers in
case those functions are discovered to have a vulnerability.
Architecture and Design
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 whitelist 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.
Testing
Implementation
Use automated static analysis tools that target this type of weakness.
Many modern techniques use data flow analysis to minimize the number of
false positives. This is not a perfect solution, since 100% accuracy and
coverage are not feasible.
Testing
Use 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.
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 handled. 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.
Research Gaps
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.
Theoretical Notes
This is a data/directive boundary error in which data boundaries are not
sufficiently enforced before it is sent to a different control
sphere.
Description
