CWE-95: Eval Injection
Improper Neutralization of Directives in Dynamically Evaluated Code
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 it's found
Eval Injection is a specific, narrowly defined instance of a broader pattern. Testers confirm it with targeted code review and a proof-of-concept input that exercises the exact code path this weakness describes.
Vulnerable vs. safe
numbers = eval(input("Enter a comma-separated list of numbers: "))
print("Error: invalid input")return
sum = sum + numsum = 0try:except SyntaxError:for num in numbers:print(f"Sum of {numbers} = {sum}")def main():main()sum = sum + int(num)for num in numbers:print(f"Sum of {numbers} = {sum}")
print("Error: invalid input")sum = 0numbers = input("Enter a comma-separated list of numbers: ").split(",")try:except ValueError:def main():main()Consequences
- Read Files or Directories, Read Application Data: The injected code could access restricted data / files.
- Bypass Protection Mechanism: In some cases, injectable code controls authentication; this may lead to a remote vulnerability.
- Gain Privileges or Assume Identity: Injected code can access resources that the attacker is directly prevented from accessing.
- 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.
- Hide Activities: Often the actions performed by injected control code are unlogged.
Mitigations
- Architecture and Design/Implementation: 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 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].
How TurboPentest tests for this (white-box)
This weakness (Eval Injection) is caught by white-box static analysis when you connect a GitHub repo: IntegSec's Opengrep SAST rule pack carries 2 rules for it, flagging the issue directly in your source code as part of the pentest.
Frequently asked questions
What is CWE-95?
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 do you find Eval Injection?
Eval Injection is a specific, narrowly defined instance of a broader pattern. Testers confirm it with targeted code review and a proof-of-concept input that exercises the exact code path this weakness describes.
What is the impact of CWE-95?
Read Files or Directories, Read Application Data: The injected code could access restricted data / files. Bypass Protection Mechanism: In some cases, injectable code controls authentication; this may lead to a remote vulnerability. Gain Privileges or Assume Identity: Injected code can access resources that the attacker is directly prevented from accessing. 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. Hide Activities: Often the actions performed by injected control code are unlogged.
Does TurboPentest test for Eval Injection?
This weakness (Eval Injection) is caught by white-box static analysis when you connect a GitHub repo: IntegSec's Opengrep SAST rule pack carries 2 rules for it, flagging the issue directly in your source code as part of the pentest.
Related CWEs
Written and reviewed by
Michel Chamberland - Founder & CEO, IntegSec
CISSP, OSCP, OSCE, CEH, GIAC, CCSK · 20+ years in offensive security
Michel has spent 20+ years on offensive security teams including IBM X-Force Red and Trustwave SpiderLabs, leading penetration tests, red team engagements, and breach response for Fortune 500 customers. He is the founder of IntegSec and the architect of TurboPentest.
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