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Variant · Low-Medium

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

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").

CWE-95 · Variant Level ·17 CVEs ·4 Mitigations

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").

Potential Impact

Confidentiality

Read Files or Directories, Read Application Data

Access Control

Bypass Protection Mechanism

Access Control

Gain Privileges or Assume Identity

Integrity, Confidentiality, Availability, Other

Execute Unauthorized Code or Commands

Non-Repudiation

Hide Activities

Demonstrative Examples

edit-config.pl: This CGI script is used to modify settings in a configuration file.
Bad
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";}
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:
Attack
add_key(",","); system("/bin/ls");
This would produce the following string in handleConfigAction():
Result
config_file_add_key(",","); system("/bin/ls");
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.
This simple python3 script asks a user to supply a comma-separated list of numbers as input and adds them together.
Bad
def main():
                  
                    sum = 0
		    try:
		    
                      numbers = eval(input("Enter a comma-separated list of numbers: "))
		    
		    except SyntaxError:
		    
		      print("Error: invalid input")
		      return
		    
                    for num in numbers:
                    
                      sum = sum + num
                    
                    print(f"Sum of {numbers} = {sum}")
                  
                  main()
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:
Attack
__import__('subprocess').getoutput('rm -r *')
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.
Good
def main():
                  
                    sum = 0
                    numbers = input("Enter a comma-separated list of numbers: ").split(",")
                    try:
                    
                      for num in numbers:
                      
                        sum = sum + int(num)
                      
                      print(f"Sum of {numbers} = {sum}")
                    
                    except ValueError:
                    
                      print("Error: invalid input")
                    
                  
                  main()
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].

Mitigations & Prevention

Architecture and DesignImplementation

If possible, refactor your code so that it does not need to use eval() at all.

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 relat

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

Implementation Discouraged Common Practice

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].

Detection Methods

  • Automated Static Analysis High — 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 sea

Real-World CVE Examples

CVE IDDescription
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 (CWE-625), enabling eval injection (CWE-95), as exploited in the wild per CISA KEV.
CVE-2021-22205Chain: 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-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.

Showing 15 of 17 observed examples.

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

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but i...

Taxonomy Mappings

  • PLOVER: — Direct Dynamic Code Evaluation ('Eval Injection')
  • OWASP Top Ten 2007: A3 — Malicious File Execution
  • OWASP Top Ten 2004: A6 — Injection Flaws
  • Software Fault Patterns: SFP24 — Tainted input to command
  • SEI CERT Perl Coding Standard: IDS35-PL — Do not invoke the eval form with a string argument

Frequently Asked Questions

What is CWE-95?

CWE-95 (Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. 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").

How can CWE-95 be exploited?

Attackers can exploit CWE-95 (Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')) to read files or directories, read application data. This weakness is typically introduced during the Implementation, Implementation phase of software development.

How do I prevent CWE-95?

Key mitigations include: If possible, refactor your code so that it does not need to use eval() at all.

What is the severity of CWE-95?

CWE-95 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 17 real-world CVEs.