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⇱ CWE - CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection') (4.20)

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Home > CWE List > CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection') (4.20)  
CWE Glossary Definition
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CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

Weakness ID: 95
Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Variant Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.
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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"). 👁 Diagram for CWE-95
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Common Consequences
👁 Section Help
This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Impact Details

Read Files or Directories; Read Application Data

Scope: Confidentiality
The injected code could access restricted data / files.

Bypass Protection Mechanism

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

Gain Privileges or Assume Identity

Scope: Access Control
Injected code can access resources that the attacker is directly prevented from accessing.

Execute Unauthorized Code or Commands

Scope: Integrity, Confidentiality, Availability, Other
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.

Hide Activities

Scope: Non-Repudiation
Often the actions performed by injected control code are unlogged.
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Potential Mitigations
Phase(s) Mitigation

Architecture and Design; Implementation

Strategy: Refactoring

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

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.

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.

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
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 Relationships
👁 Section Help
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
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Relevant to the view "Research Concepts" (View-1000)
Nature Type ID Name
ChildOf 👁 Base
Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 		94 Improper Control of Generation of Code ('Code Injection')
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Relevant to the view "Architectural Concepts" (View-1008)
Nature Type ID Name
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic. 		1019 Validate Inputs
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Modes Of Introduction
👁 Section Help
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
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.
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Applicable Platforms
👁 Section Help
This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages

Java (Undetermined Prevalence)

JavaScript (Undetermined Prevalence)

Python (Undetermined Prevalence)

Perl (Undetermined Prevalence)

PHP (Undetermined Prevalence)

Ruby (Undetermined Prevalence)

Class: Interpreted (Undetermined Prevalence)

Technologies

AI/ML (Often Prevalent)

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Likelihood Of Exploit
Medium
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Demonstrative Examples

Example 1


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

(bad code)
Example Language: Perl 
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 code)
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.



Example 2


This simple python3 script asks a user to supply a comma-separated list of numbers as input and adds them together.

(bad code)
Example Language: Python 
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 code)
__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 code)
Example Language: Python 
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].



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Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Reference Description
Framework for LLM applications allows eval injection via a crafted response from a hosting provider.
Python compiler uses eval() to execute malicious strings as Python code.
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.
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.
Eval injection in PHP program.
Eval injection in Perl program.
Eval injection in Perl program using an ID that should only contain hyphens and numbers.
Direct code injection into Perl eval function.
Eval injection in Perl program.
Direct code injection into Perl eval function.
Direct code injection into Perl eval function.
MFV. code injection into PHP eval statement using nested constructs that should not be nested.
MFV. code injection into PHP eval statement using nested constructs that should not be nested.
Code injection into Python eval statement from a field in a formatted file.
Eval injection in Python program.
chain: Resultant eval injection. An invalid value prevents initialization of variables, which can be modified by attacker and later injected into PHP eval statement.
Chain: Execution after redirect triggers eval injection.
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Weakness Ordinalities
Ordinality Description
Primary
(where the weakness exists independent of other weaknesses)
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Detection Methods
Method Details

Automated Static Analysis
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.)

Effectiveness: High
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Memberships
👁 Section Help
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature Type ID Name
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 714 OWASP Top Ten 2007 Category A3 - Malicious File Execution
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 727 OWASP Top Ten 2004 Category A6 - Injection Flaws
MemberOf 👁 View
View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).		 884 CWE Cross-section
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 990 SFP Secondary Cluster: Tainted Input to Command
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1179 SEI CERT Perl Coding Standard - Guidelines 01. Input Validation and Data Sanitization (IDS)
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1347 OWASP Top Ten 2021 Category A03:2021 - Injection
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1409 Comprehensive Categorization: Injection
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1440 OWASP Top Ten 2025 Category A05:2025 - Injection
MemberOf 👁 Category
Category - a CWE entry that contains a set of other entries that share a common characteristic.		 1447 General Software Weaknesses that Appear in Products that Use or Support AI/ML Technology
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Vulnerability Mapping Notes
Usage ALLOWED
(this CWE ID may be used to map to real-world vulnerabilities)
Reason Acceptable-Use

Rationale

This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

Comments

Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
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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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Taxonomy Mappings
Mapped Taxonomy Name Node ID Fit Mapped Node Name
PLOVER Direct Dynamic Code Evaluation ('Eval Injection')
OWASP Top Ten 2007 A3 CWE More Specific Malicious File Execution
OWASP Top Ten 2004 A6 CWE More Specific Injection Flaws
Software Fault Patterns SFP24 Tainted input to command
SEI CERT Perl Coding Standard IDS35-PL Exact Do not invoke the eval form with a string argument
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References
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 18, "Inline Evaluation", Page 1095. 1st Edition. Addison Wesley. 2006.
[REF-1372] "How ast.literal_eval can cause memory exhaustion". Reddit. 2022-12-14.
<https://www.reddit.com/r/learnpython/comments/zmbhcf/how_astliteral_eval_can_cause_memory_exhaustion/>. (URL validated: 2023-11-03)
[REF-1373] "ast - Abstract Syntax Trees". ast.literal_eval(node_or_string). Python. 2023-11-02.
<https://docs.python.org/3/library/ast.html#ast.literal_eval>. (URL validated: 2023-11-03)
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Content History
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Submissions
Submission Date Submitter Organization
2006-07-19
(CWE Draft 3, 2006-07-19) 		PLOVER
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Modifications
Modification Date Modifier Organization
2026-04-30
(CWE 4.20, 2026-04-30) 		CWE Content Team MITRE
updated Applicable_Platforms, Potential_Mitigations, Relationships
2025-12-11
(CWE 4.19, 2025-12-11) 		CWE Content Team MITRE
updated Demonstrative_Examples, Relationships
2025-04-03
(CWE 4.17, 2025-04-03) 		CWE Content Team MITRE
updated Common_Consequences, Description, Diagram
2024-07-16
(CWE 4.15, 2024-07-16) 		CWE Content Team MITRE
updated Applicable_Platforms, Observed_Examples
2024-02-29
(CWE 4.14, 2024-02-29) 		CWE Content Team MITRE
updated Demonstrative_Examples, Potential_Mitigations, References
2023-06-29
(CWE 4.12, 2023-06-29) 		CWE Content Team MITRE
updated Mapping_Notes
2023-04-27
(CWE 4.11, 2023-04-27) 		CWE Content Team MITRE
updated Demonstrative_Examples, Detection_Factors, Relationships, Time_of_Introduction
2023-01-31
(CWE 4.10, 2023-01-31) 		CWE Content Team MITRE
updated Demonstrative_Examples, Description
2022-10-13
(CWE 4.9, 2022-10-13) 		CWE Content Team MITRE
updated Observed_Examples
2022-06-28
(CWE 4.8, 2022-06-28) 		CWE Content Team MITRE
updated Observed_Examples
2022-04-28
(CWE 4.7, 2022-04-28) 		CWE Content Team MITRE
updated Research_Gaps
2021-10-28
(CWE 4.6, 2021-10-28) 		CWE Content Team MITRE
updated Relationships
2021-03-15
(CWE 4.4, 2021-03-15) 		CWE Content Team MITRE
updated Relationships
2020-06-25
(CWE 4.1, 2020-06-25) 		CWE Content Team MITRE
updated Potential_Mitigations
2020-02-24
(CWE 4.0, 2020-02-24) 		CWE Content Team MITRE
updated Potential_Mitigations, Relationships
2019-06-20
(CWE 3.3, 2019-06-20) 		CWE Content Team MITRE
updated Type
2019-01-03
(CWE 3.2, 2019-01-03) 		CWE Content Team MITRE
updated Taxonomy_Mappings
2017-11-08
(CWE 3.0, 2017-11-08) 		CWE Content Team MITRE
updated Causal_Nature, Modes_of_Introduction, References, Relationships, Taxonomy_Mappings
2014-07-30
(CWE 2.8, 2014-07-31) 		CWE Content Team MITRE
updated Relationships, Taxonomy_Mappings
2013-02-21
(CWE 2.4, 2013-02-21) 		CWE Content Team MITRE
updated Observed_Examples
2012-10-30
(CWE 2.3, 2012-10-30) 		CWE Content Team MITRE
updated Potential_Mitigations
2012-05-11
(CWE 2.2, 2012-05-15) 		CWE Content Team MITRE
updated Common_Consequences, Demonstrative_Examples, References, Relationships
2011-06-01
(CWE 1.13, 2011-06-01) 		CWE Content Team MITRE
updated Common_Consequences
2010-06-21
(CWE 1.9, 2010-06-21) 		CWE Content Team MITRE
updated Description, Name
2010-02-16
(CWE 1.8, 2010-02-16) 		CWE Content Team MITRE
updated Potential_Mitigations
2009-05-27
(CWE 1.4, 2009-05-27) 		CWE Content Team MITRE
updated Alternate_Terms, Applicable_Platforms, Demonstrative_Examples, Description, Name, References
2009-01-12
(CWE 1.2, 2009-01-12) 		CWE Content Team MITRE
updated Description, Observed_Examples, Other_Notes, Research_Gaps
2008-09-08
(CWE 1.0, 2008-09-09) 		CWE Content Team MITRE
updated Applicable_Platforms, Description, Modes_of_Introduction, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008-08-15
(CWE 1.0, 2008-09-09) 		Veracode
Suggested OWASP Top Ten 2004 mapping
2008-07-01
(CWE 1.0, 2008-09-09) 		Eric Dalci Cigital
updated Time_of_Introduction
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Previous Entry Names
Change Date Previous Entry Name
2010-06-21 Improper Sanitization of Directives in Dynamically Evaluated Code ('Eval Injection')
2009-05-27 Insufficient Control of Directives in Dynamically Evaluated Code (aka 'Eval Injection')
2008-04-11 Direct Dynamic Code Evaluation ('Eval Injection')
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Page Last Updated: April 30, 2026

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