Python中的eval

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缘由:

在Freebuf上看到的两篇文章:Python eval的常见错误封装及利用原理Python安全编码与代码审计,里面涉及到了对Python的eval函数的说明和讲解,之后我又去网上搜了一些资料,整理成了这篇文章。

搜索关键字:

python eval literal_eval

参考链接:
# Python中的eval是用来干嘛的?
# eval是非常危险的
# 用 eval / ast.literal_eval 来操作json数据?
# Python eval的常见错误封装及利用原理
参考解答:

问:Python中的eval是用来干嘛的?

答:eval() 将第一个字符串参数解释为Python代码并执行。

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The eval function lets a python program run python code within itself.eval example (interactive shell):

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eval() interprets a string as code. The reason why so many people have warned you about using this is because a user can use this as an option to run code on the computer. If you have eval(input()) and os imported, a person could type into input() os.system(‘rm -R *’) which would delete all your files in your home directory. (Assuming you have a unix system). Using eval() is a security hole. If you need to convert strings to other formats, try to use things that do that, like int().

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与eval()函数相比,ast.literal_eval()函数仅认为少数(安全的)Python语法操作是合法的:

The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and None.

Passing __import__(‘os’).system(‘rm -rf /’) into ast.literal_eval() will raise an error, but eval() will happily wipe your drive.

Since it looks like you’re only letting the user input a plain dictionary, use ast.literal_eval(). It safely does what you want and nothing more.

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You cannot secure eval with a blacklist approach like this. See Eval really is dangerous for examples of input that will segfault the CPython interpreter, give access to any class you like, and so on.

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Note that this doesn’t protect you from CPU or memory resource attacks (for example, something like:

will most likely cause your program to run out of memory after a while)

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下面这段代码则是退出解释器:

初步理解一下整个过程:

这句Python代码的意思就是找tuple的class,再找它的基类,也就是object,再通过object找他的子类,具体的子类也如代码中的输出一样。从中可以看到了有file模块,zipimporter模块,是不是可以利用下呢?首先从file入手。

假如用户如果构造:

这个restrictected mode简单理解就是Python解释器的沙盒,一些功能被限制了,比如说不能修改系统,不能使用一些系统函数,如file,详情见Restricted Execution Mode,那怎么去绕过呢?这时我们就想到了zipimporter了,假如引入的模块中引用了os模块,我们就可以进行利用了。也就是说刚才的safe_eval其实是不安全的。

如何正确使用
  1. 使用ast.literal_eval
  2. 如果仅仅是将字符转为dict,可以使用json格式

=EOF=

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