CWE-94: Failure to Control Generation of Code ('Code Injection')
Failure to Control Generation of Code ('Code Injection')
Weakness ID: 94 (Weakness Class)
Status: Draft
Description
Description Summary
The product does not sufficiently filter code (control-plane)
syntax from user-controlled input (data plane) when that input is used within
code that the product generates.
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
Confidentiality
The injected code could access restricted data / files
Authentication
In some cases, injectable code controls authentication; this may lead
to a remote vulnerability
Access Control
Injected code can access resources that the attacker is directly
prevented from accessing
Integrity
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.
Accountability
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.
Potential Mitigations
Phase
Description
Architecture and Design
Refactor your program so that you do not have to dynamically generate
code.
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.
Implementation
Assume all input is malicious. Use an "accept known good" input
validation strategy (i.e., use a whitelist). Reject any input that does
not strictly conform to specifications, or transform it into something
that does. Use a blacklist to reject any unexpected inputs and detect
potential attacks.
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().
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.
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.
Operation
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 you 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).
Description
