CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')Weakness ID: 95 Vulnerability Mapping:
ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction: VariantVariant - 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. |
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 This may allow an attacker to execute arbitrary code, or at least modify what code can be executed. Common Consequences 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.Scope | Impact | Likelihood |
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Confidentiality
| Technical Impact: Read Files or Directories; Read Application Data The injected code could access restricted data / files. | | Access Control
| Technical Impact: Bypass Protection Mechanism In some cases, injectable code controls authentication; this may lead to a remote vulnerability. | | Access Control
| Technical Impact: Gain Privileges or Assume Identity Injected code can access resources that the attacker is directly prevented from accessing. | | Integrity Confidentiality Availability Other
| Technical 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. | | Non-Repudiation
| Technical Impact: Hide Activities Often the actions performed by injected control code are unlogged. | |
Potential Mitigations
Phases: Architecture and Design; Implementation 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 ( 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. |
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 |
Relationships 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. Relevant to the view "Research Concepts" (CWE-1000) Nature | Type | ID | Name |
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ChildOf | 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') |
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. Relevant to the view "Architectural Concepts" (CWE-1008) Nature | Type | ID | Name |
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MemberOf | Category - a CWE entry that contains a set of other entries that share a common characteristic. | 1019 | Validate Inputs |
Modes Of Introduction 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 |
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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. |
Likelihood Of Exploit 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:
add_key(",","); system("/bin/ls");
This would produce the following string in handleConfigAction():
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 script asks a user to supply a list of numbers as input and adds them together. (bad code) Example Language: Python
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()
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:
__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 space-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]. Observed Examples 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. |
Weakness Ordinalities Ordinality | Description |
Primary | (where the weakness exists independent of other weaknesses) |
Detection Methods
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.) |
Memberships 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. Vulnerability Mapping Notes Usage: ALLOWED (this CWE ID could 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. |
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. 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 |
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.
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Content History Submissions |
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Submission Date | Submitter | Organization |
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2006-07-19 (CWE Draft 3, 2006-07-19) | PLOVER | | | Modifications |
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Modification Date | Modifier | Organization |
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2008-07-01 | Eric Dalci | Cigital | updated Time_of_Introduction | 2008-08-15 | | Veracode | Suggested OWASP Top Ten 2004 mapping | 2008-09-08 | CWE Content Team | MITRE | updated Applicable_Platforms, Description, Modes_of_Introduction, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities | 2009-01-12 | CWE Content Team | MITRE | updated Description, Observed_Examples, Other_Notes, Research_Gaps | 2009-05-27 | CWE Content Team | MITRE | updated Alternate_Terms, Applicable_Platforms, Demonstrative_Examples, Description, Name, References | 2010-02-16 | CWE Content Team | MITRE | updated Potential_Mitigations | 2010-06-21 | CWE Content Team | MITRE | updated Description, Name | 2011-06-01 | CWE Content Team | MITRE | updated Common_Consequences | 2012-05-11 | CWE Content Team | MITRE | updated Common_Consequences, Demonstrative_Examples, References, Relationships | 2012-10-30 | CWE Content Team | MITRE | updated Potential_Mitigations | 2013-02-21 | CWE Content Team | MITRE | updated Observed_Examples | 2014-07-30 | CWE Content Team | MITRE | updated Relationships, Taxonomy_Mappings | 2017-11-08 | CWE Content Team | MITRE | updated Causal_Nature, Modes_of_Introduction, References, Relationships, Taxonomy_Mappings | 2019-01-03 | CWE Content Team | MITRE | updated Taxonomy_Mappings | 2019-06-20 | CWE Content Team | MITRE | updated Type | 2020-02-24 | CWE Content Team | MITRE | updated Potential_Mitigations, Relationships | 2020-06-25 | CWE Content Team | MITRE | updated Potential_Mitigations | 2021-03-15 | CWE Content Team | MITRE | updated Relationships | 2021-10-28 | CWE Content Team | MITRE | updated Relationships | 2022-04-28 | CWE Content Team | MITRE | updated Research_Gaps | 2022-06-28 | CWE Content Team | MITRE | updated Observed_Examples | 2022-10-13 | CWE Content Team | MITRE | updated Observed_Examples | 2023-01-31 | CWE Content Team | MITRE | updated Demonstrative_Examples, Description | 2023-04-27 | CWE Content Team | MITRE | updated Demonstrative_Examples, Detection_Factors, Relationships, Time_of_Introduction | 2023-06-29 | CWE Content Team | MITRE | updated Mapping_Notes | 2024-02-29 (CWE 4.14, 2024-02-29) | CWE Content Team | MITRE | updated Demonstrative_Examples, Potential_Mitigations, References | 2024-07-16 (CWE 4.15, 2024-07-16) | CWE Content Team | MITRE | updated Applicable_Platforms, Observed_Examples | Previous Entry Names |
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Change Date | Previous Entry Name |
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2008-04-11 | Direct Dynamic Code Evaluation ('Eval Injection') | | 2009-05-27 | Insufficient Control of Directives in Dynamically Evaluated Code (aka 'Eval Injection') | | 2010-06-21 | Improper Sanitization of Directives in Dynamically Evaluated Code ('Eval Injection') | |
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