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Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

Incomplete Variant
Structure: Simple
Description

This vulnerability occurs when an application takes user input and passes it directly into a dynamic code execution function, like eval(), without properly sanitizing it. This allows an attacker to inject and execute arbitrary code within the application's context.

This vulnerability occurs when an application takes user input and passes it directly into a dynamic code execution function, like eval(), without properly sanitizing it. This allows an attacker to inject and execute arbitrary code within the application's context.
Extended Description

Eval injection is a critical flaw where untrusted data, such as a URL parameter or form field, is fed directly into a function that interprets a string as code. Functions like eval(), setTimeout() with strings, or new Function() are common culprits. When an attacker can control this input, they can break out of the intended data context and inject malicious commands, potentially taking over the application's process, accessing sensitive data, or compromising the server. To prevent this, developers must avoid using dynamic code evaluation entirely whenever possible. If it's unavoidable, the only robust defense is strict input validation using a whitelist of permitted characters and patterns. Never rely on blacklisting or simple escaping, as these methods are error-prone and often bypassed. Instead, use safe language features or APIs designed for the task, such as parameterized queries for databases or JSON parsers for data structures.
Common Consequences 5
Scope: Confidentiality

Impact: Read Files or DirectoriesRead Application Data

The injected code could access restricted data / files.

Scope: Access Control

Impact: Bypass Protection Mechanism

In some cases, injectable code controls authentication; this may lead to a remote vulnerability.

Scope: Access Control

Impact: Gain Privileges or Assume Identity

Injected code can access resources that the attacker is directly prevented from accessing.

Scope: IntegrityConfidentialityAvailabilityOther

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 or at least modify what code can be executed.

Scope: Non-Repudiation

Impact: Hide Activities

Often the actions performed by injected control code are unlogged.
Detection Methods 1
Automated Static AnalysisHigh
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Potential Mitigations 4
Phase: Architecture and DesignImplementation
If possible, refactor your code so that it does not need to use eval() at all.
Phase: Implementation

Strategy: 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.
Phase: Implementation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize, Incorrect Behavior Order: Validate Before Filter). Make sure that your application does not inadvertently decode the same input twice (Double Decoding of the Same Data). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.
Phase: 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
Demonstrative Examples 2

ID : DX-31

edit-config.pl: This CGI script is used to modify settings in a configuration file.

Code Example:

Bad
Perl
perl

code to add a field/key to a file goes here*

perl
perl

code to delete key from a particular file goes here*

perl
perl
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:

Code Example:

Attack
bash
This would produce the following string in handleConfigAction():

Code Example:

Result
bash
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.

ID : DX-156

This simple script asks a user to supply a list of numbers as input and adds them together.

Code Example:

Bad
Python
python
The eval() function can take the user-supplied list and convert it into a Python list object, therefore allowing the programmer to use list comprehension methods to work with the data. However, if code is supplied to the eval() function, it will execute that code. For example, a malicious user could supply the following string:

Code Example:

Attack
bash
This would delete all the files in the current directory. For this reason, it is not recommended to use eval() with untrusted input.
A way to accomplish this without the use of eval() is to apply an integer conversion on the input within a try/except block. If the user-supplied input is not numeric, this will raise a ValueError. By avoiding eval(), there is no opportunity for the input string to be executed as code.

Code Example:

Good
Python
python
An alternative, commonly-cited mitigation for this kind of weakness is to use the ast.literal_eval() function, 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].
Observed Examples 17
CVE-2024-4181Framework for LLM applications allows eval injection via a crafted response from a hosting provider.
CVE-2022-2054Python compiler uses eval() to execute malicious strings as Python code.
CVE-2021-22204Chain: regex in EXIF processor code does not correctly determine where a string ends (Permissive Regular Expression), enabling eval injection (Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')), as exploited in the wild per CISA KEV.
CVE-2021-22205Chain: backslash followed by a newline can bypass a validation step (Improper Input Validation), leading to eval injection (Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')), as exploited in the wild per CISA KEV.
CVE-2008-5071Eval injection in PHP program.
CVE-2002-1750Eval injection in Perl program.
CVE-2008-5305Eval injection in Perl program using an ID that should only contain hyphens and numbers.
CVE-2002-1752Direct code injection into Perl eval function.
CVE-2002-1753Eval injection in Perl program.
CVE-2005-1527Direct code injection into Perl eval function.
CVE-2005-2837Direct code injection into Perl eval function.
CVE-2005-1921MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-2498MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE-2005-3302Code injection into Python eval statement from a field in a formatted file.
CVE-2007-1253Eval injection in Python program.
CVE-2001-1471chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement.
CVE-2007-2713Chain: Execution after redirect triggers eval injection.
References 3
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
How ast.literal_eval can cause memory exhaustion
Reddit
14-12-2022
https://www.reddit.com/r/learnpython/comments/zmbhcf/how_astliteral_eval_can_cause_memory_exhaustion/(2023-11-03)
ID: REF-1372
ast - Abstract Syntax Trees
Python
02-11-2023
https://docs.python.org/3/library/ast.html#ast.literal_eval(2023-11-03)
ID: REF-1373
Likelihood of Exploit

Medium

Applicable Platforms
Languages:
Java : UndeterminedJavaScript : UndeterminedPython : UndeterminedPerl : UndeterminedPHP : UndeterminedRuby : UndeterminedInterpreted : Undetermined
Technologies:
AI/ML : Undetermined
Modes of Introduction
Implementation
Implementation
Related Attack Patterns
Related Weaknesses
ChildOf: Improper Control of Generation of Code ('Code Injection') (CWE-94)
Taxonomy Mapping
  • PLOVER
  • OWASP Top Ten 2007
  • OWASP Top Ten 2004
  • Software Fault Patterns
  • SEI CERT Perl Coding Standard
Notes
OtherFactors: special character errors can play a role in increasing the variety of code that can be injected, although some vulnerabilities do not require special characters at all, e.g. when a single function without arguments can be referenced and a terminator character is not necessary.