CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

Description

The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes code syntax before using the input in a dynamic evaluation call (e.g. "eval").

Extended Description

N/A

ThreatScore

Threat Mapped score: 0.0

Industry: Finiancial

Threat priority: Unclassified

Observed Examples (CVEs)

CVE: CVE-2024-4181

Framework for LLM applications allows eval injection via a crafted response from a hosting provider.
CVE: CVE-2022-2054

Python compiler uses eval() to execute malicious strings as Python code.
CVE: CVE-2021-22204 — 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: CVE-2021-22205 — KEV

Chain: backslash followed by a newline can bypass a validation step (CWE-20), leading to eval injection (CWE-95), as exploited in the wild per CISA KEV.
CVE: CVE-2008-5071

Eval injection in PHP program.
CVE: CVE-2002-1750

Eval injection in Perl program.
CVE: CVE-2008-5305

Eval injection in Perl program using an ID that should only contain hyphens and numbers.
CVE: CVE-2002-1752

Direct code injection into Perl eval function.
CVE: CVE-2002-1753

Eval injection in Perl program.
CVE: CVE-2005-1527

Direct code injection into Perl eval function.
CVE: CVE-2005-2837

Direct code injection into Perl eval function.
CVE: CVE-2005-1921

MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE: CVE-2005-2498

MFV. code injection into PHP eval statement using nested constructs that should not be nested.
CVE: CVE-2005-3302

Code injection into Python eval statement from a field in a formatted file.
CVE: CVE-2007-1253

Eval injection in Python program.
CVE: 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: CVE-2007-2713

Chain: Execution after redirect triggers eval injection.

Related Attack Patterns (CAPEC)

CAPEC-35

Attack TTPs

T1027.009 — Embedded Payloads (defense-evasion)
T1564.009 — Resource Forking (defense-evasion)
T1027.006 — HTML Smuggling (defense-evasion)

Malware

APTs (Intrusion Sets)

Modes of Introduction

Phase	Note
Implementation	REALIZATION: This weakness is caused during implementation of an architectural security tactic.
Implementation	This weakness is prevalent in handler/dispatch procedures that might want to invoke a large number of functions, or set a large number of variables.

Common Consequences

Impact: Read Files or Directories, Read Application Data — Notes: The injected code could access restricted data / files.
Impact: Bypass Protection Mechanism — Notes: In some cases, injectable code controls authentication; this may lead to a remote vulnerability.
Impact: Gain Privileges or Assume Identity — Notes: Injected code can access resources that the attacker is directly prevented from accessing.
Impact: Execute Unauthorized Code or Commands — Notes: 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.
Impact: Hide Activities — Notes: Often the actions performed by injected control code are unlogged.

Potential Mitigations

Architecture and Design: If possible, refactor your code so that it does not need to use eval() at all. (N/A)
Implementation: 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. (N/A)
Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). 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. (N/A)
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]. (Discouraged Common Practice)

Applicable Platforms

Java (N/A, Undetermined)
JavaScript (N/A, Undetermined)
Python (N/A, Undetermined)
Perl (N/A, Undetermined)
PHP (N/A, Undetermined)
Ruby (N/A, Undetermined)
None (Interpreted, Undetermined)

Demonstrative Examples

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

Body: 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:

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"; }

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

Body: 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:

def main(): sum = 0 numbers = eval(input("Enter a space-separated list of numbers: ")) for num in numbers: sum = sum + num print(f"Sum of {numbers} = {sum}") main()

Notes

Other: Factors: 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.

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