CWE-95 动态执行代码中指令转义处理不恰当（Eval注入）

Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')

结构: Simple

Abstraction: Variant

状态: Incomplete

被利用可能性: Medium

基本描述

The software 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").

扩展描述

This may allow an attacker to execute arbitrary code, or at least modify what code can be executed.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 94 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 94 cwe_View_ID: 699 cwe_Ordinal: Primary

适用平台

Language: [{'cwe_Name': 'Java', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'JavaScript', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'Python', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'Perl', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'PHP', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'Ruby', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Class': 'Interpreted', 'cwe_Prevalence': 'Undetermined'}]

常见的影响

可能的缓解方案

['Architecture and Design', 'Implementation']

策略:

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

MIT-5 Implementation

策略: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist 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 (i.e., do not rely on a blacklist). A blacklist 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, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Implementation

策略:

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 whitelist 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.

示例代码

例

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

bad Perl

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

This would produce the following string in handleConfigAction():

result

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.

分析过的案例

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.
Research Gap
This issue is probably under-reported. Most relevant CVEs have been for Perl and PHP, but eval injection applies to most interpreted languages. Javascript eval injection is likely to be heavily under-reported.

分类映射

相关攻击模式

• CAPEC-35

引用

• REF-62 The Art of Software Security Assessment

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