CWE-94: Improper Control of Generation of Code ('Code Injection')
Improper Control of Generation of Code ('Code Injection')
Weakness ID: 94 (Weakness Class)
Status: Draft
Description
Description Summary
The software constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.
Extended Description
When software allows a user's input to contain code syntax, it might be possible for an attacker to craft the code in such a way that it will alter the intended control flow of the software. Such an alteration could lead to arbitrary code execution.
Injection problems encompass a wide variety of issues -- all mitigated in very different ways. For this reason, the most effective way to discuss these weaknesses is to note the distinct features which classify them as injection weaknesses. The most important issue to note is that all injection problems share one thing in common -- i.e., they allow for the injection of control plane data into the user-controlled data plane. This means that the execution of the process may be altered by sending code in through legitimate data channels, using no other mechanism. While buffer overflows, and many other flaws, involve the use of some further issue to gain execution, injection problems need only for the data to be parsed. The most classic instantiations of this category of weakness are SQL injection and format string vulnerabilities.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
Interpreted languages: (Sometimes)
Common Consequences
Scope
Effect
Access Control
Technical Impact: Bypass protection
mechanism
In some cases, injectable code controls authentication; this may lead
to a remote vulnerability.
Access Control
Technical Impact: Gain privileges / assume
identity
Injected code can access resources that the attacker is directly
prevented from accessing.
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands
Code injection attacks can lead to loss of data integrity in nearly
all cases as the control-plane data injected is always incidental to
data recall or writing. Additionally, code injection can often result in
the execution of arbitrary code.
Non-Repudiation
Technical Impact: Hide activities
Often the actions performed by injected control code are
unlogged.
Likelihood of Exploit
Medium
Demonstrative Examples
Example 1
This example attempts to write user messages to a message file and
allow users to view them.
While the programmer intends for the MessageFile to only include data,
an attacker can provide a message such as:
(Attack)
name=h4x0r
message=%3C?php%20system(%22/bin/ls%20-l%22);?%3E
which will decode to the following:
(Attack)
<?php system("/bin/ls -l");?>
The programmer thought they were just including the contents of a
regular data file, but PHP parsed it and executed the code. Now, this
code is executed any time people view messages.
Notice that XSS (CWE-79) is also possible in this situation.
Example 2
edit-config.pl: This CGI script is used to modify settings in a
configuration file.
(Bad Code)
Example
Language: Perl
use CGI qw(:standard);
sub config_file_add_key {
my ($fname, $key, $arg) = @_;
# code to add a field/key to a file goes here
}
sub config_file_set_key {
my ($fname, $key, $arg) = @_;
# code to set key to a particular file goes here
}
sub config_file_delete_key {
my ($fname, $key, $arg) = @_;
# code to delete key from a particular file goes
here
}
sub handleConfigAction {
my ($fname, $action) = @_;
my $key = param('key');
my $val = param('val');
# this is super-efficient code, especially if you have to
invoke
# any one of dozens of different functions!
my $code = "config_file_$action_key(\$fname, \$key,
\$val);";
eval($code);
}
$configfile = "/home/cwe/config.txt";
print header;
if (defined(param('action'))) {
handleConfigAction($configfile, param('action'));
}
else {
print "No action specified!\n";
}
The script intends to take the 'action' parameter and invoke one of a
variety of functions based on the value of that parameter -
config_file_add_key(), config_file_set_key(), or
config_file_delete_key(). It could set up a conditional to invoke each
function separately, but eval() is a powerful way of doing the same
thing in fewer lines of code, especially when a large number of
functions or variables are involved. Unfortunately, in this case, the
attacker can provide other values in the action parameter, such as:
add_key(",","); system("/bin/ls"); This would produce the following
string in handleConfigAction(): config_file_add_key(",",");
system("/bin/ls"); Any arbitrary Perl code could be added after the
attacker has "closed off" the construction of the original function
call, in order to prevent parsing errors from causing the malicious
eval() to fail before the attacker's payload is activated. This
particular manipulation would fail after the system() call, because the
"_key(\$fname, \$key, \$val)" portion of the string would cause an
error, but this is irrelevant to the attack because the payload has
already been activated.
chain: Resultant eval injection. An invalid value
prevents initialization of variables, which can be modified by attacker and
later injected into PHP eval statement.
PHP code from User-Agent HTTP header directly
inserted into log file implemented as PHP
script.
Potential Mitigations
Phase: Architecture and Design
Refactor your program so that you do not have to dynamically generate
code.
Phase: Architecture and Design
Run your code in a "jail" or similar sandbox environment that enforces
strict boundaries between the process and the operating system. This may
effectively restrict which code can be executed by your software.
Examples include the Unix chroot jail and AppArmor. In general,
managed code may provide some protection.
This may not be a feasible solution, and it only limits the impact to
the operating system; the rest of your application may still be subject
to compromise.
Be careful to avoid CWE-243 and other weaknesses related to jails.
Phase: Implementation
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input
validation strategy, i.e., use a whitelist 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
(i.e., do not rely on a blacklist). A blacklist 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, blacklists can be useful for detecting potential
attacks or determining which inputs are so malformed that they should be
rejected outright.
To reduce the likelihood of code injection, use stringent whitelists
that limit which constructs are allowed. If you are dynamically
constructing code that invokes a function, then verifying that the input
is alphanumeric might be insufficient. An attacker might still be able
to reference a dangerous function that you did not intend to allow, such
as system(), exec(), or exit().
Phase: Testing
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.
Phase: 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.
Phase: Operation
Strategies: Compilation or Build Hardening; Environment Hardening
Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
[REF-17] Michael Howard, David LeBlanc
and John Viega. "24 Deadly Sins of Software Security". "Sin 3: Web-Client Related Vulnerabilities (XSS)." Page
63. McGraw-Hill. 2010.
Description
