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
Confidentiality
Technical Impact: Read files or
directories; Read application
data
The injected code could access restricted data / files.
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
Other
Technical Impact: Other; 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.
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. Do not rely exclusively on looking for malicious or malformed
inputs (i.e., do not rely on a blacklist). However, blacklists can be
useful for detecting potential attacks or determining which inputs are
so malformed that they should be rejected outright.
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 you are
expecting colors such as "red" or "blue."
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
Strategy: Compilation or Build 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 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
