Base weakness

Draft

CWE-94: Improper Control of Generation of Code ('Code Injection')

Also known as: Code Injection

The product 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.

2,590

Recorded CVEs

With this primary weakness

KEVs

In the CISA KEV catalog

Medium

Likelihood of exploit

Per MITRE

Base

Abstraction

MITRE classification

Overview

CWE-94 (Improper Control of Generation of Code ('Code Injection')) is a base-level software weakness catalogued by MITRE in the Common Weakness Enumeration (CWE). It describes a recurring type of mistake that can lead to exploitable security vulnerabilities.

How to prevent it

Practical mitigations for CWE-94, grouped by where in the lifecycle they apply.

Architecture and Design

Refactoring

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 product.

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

Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list 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. This 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, denylists 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 allowlists 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 dynamic tools and techniques that interact with the product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Operation

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 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).

Operation

Environment Hardening

Implementation

For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].

Effectiveness: Discouraged Common Practice

Illustrative examples

Real CVEs that MITRE cites as examples of this weakness.

CVE-2021-22204
CISA KEV
— Chain: regex in EXIF processor code does not correctly determine where a string ends (CWE-625), enabling eval injection (CWE-95), as exploited in the wild per CISA KEV.
CVE-2020-8218
CISA KEV
— "Code injection" in VPN product, as exploited in the wild per CISA KEV.
CVE-2023-29374 — Math component in an LLM framework translates user input into a Python expression that is input into the Python exec() method, allowing code execution - one variant of a "prompt injection" attack.
CVE-2024-5565 — Python-based library uses an LLM prompt containing user input to dynamically generate code that is then fed as input into the Python exec() method, allowing code execution - one variant of a "prompt injection" attack.
CVE-2024-4181 — Framework for LLM applications allows eval injection via a crafted response from a hosting provider.
CVE-2022-2054 — Python compiler uses eval() to execute malicious strings as Python code.
CVE-2008-5071 — Eval injection in PHP program.
CVE-2002-1750 — Eval injection in Perl program.
CVE-2008-5305 — Eval injection in Perl program using an ID that should only contain hyphens and numbers.
CVE-2002-1752 — Direct code injection into Perl eval function.
CVE-2002-1753 — Eval injection in Perl program.
CVE-2005-1527 — Direct code injection into Perl eval function.
CVE-2005-2837 — Direct code injection into Perl eval function.
CVE-2005-1921 — MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-2498 — MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-3302 — Code injection into Python eval statement from a field in a formatted file.
CVE-2007-1253 — Eval injection in Python program.
CVE-2001-1471 — chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement.
CVE-2002-0495 — Perl code directly injected into CGI library file from parameters to another CGI program.
CVE-2005-1876 — Direct PHP code injection into supporting template file.
CVE-2005-1894 — Direct code injection into PHP script that can be accessed by attacker.
CVE-2003-0395 — PHP code from User-Agent HTTP header directly inserted into log file implemented as PHP script.

CWE-94: Improper Control of Generation of Code ('Code Injection')

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-94?

What CVEs are caused by CWE-94?

How do you prevent CWE-94?

How is CWE-94 detected?

What are the consequences of CWE-94?

Is CWE-94 actively exploited?

References

Stay ahead of CWE-94

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Related weaknesses

Parent

Child

Related

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-94?

What CVEs are caused by CWE-94?

How do you prevent CWE-94?

How is CWE-94 detected?

What are the consequences of CWE-94?

Is CWE-94 actively exploited?

References

Stay ahead of CWE-94