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？

• https://docs.python.org/2/library/functions.html#eval
• http://stackoverflow.com/questions/9383740/what-does-pythons-eval-do

# eval是非常危险的

• http://stackoverflow.com/questions/15197673/using-pythons-eval-vs-ast-literal-eval
• http://stackoverflow.com/questions/661084/security-of-pythons-eval-on-untrusted-strings
• http://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html
• http://effbot.org/zone/librarybook-core-eval.htm
• https://esdiscuss.org/topic/eval-literal-eval-safe-eval

# 用 eval / ast.literal_eval 来操作json数据？

• http://stackoverflow.com/questions/9949533/python-eval-vs-ast-literal-eval-vs-json-decode
• https://docs.python.org/2/library/ast.html#ast.literal_eval

# Python eval的常见错误封装及利用原理

• http://www.freebuf.com/articles/web/73658.html

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。

==

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

>>> x = 1
>>> eval('x + 1')
2
>>> eval('x')
1

==

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().

==

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.

==

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.

==

In [1]: print eval("__import__('os').getcwd()", {})
D:\

In [2]: print eval("__import__('os').remove('file')", {"__builtins__": {}})
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-2-ca308c631e67> in <module>()
----> 1 print eval("__import__('os').remove('file')", {"__builtins__": {}})

<string> in <module>()

NameError: name '__import__' is not defined

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

>>> eval("'*'*1000000*2*2*2*2*2*2*2*2*2")

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

==

下面这段代码则是退出解释器：

>>> s = """
... [
...     c for c in
...     ().__class__.__bases__[0].__subclasses__()
...     if c.__name__ == "Quitter"
... ][0](0)()
... """
>>> eval(s, {'__builtins__':{}})

D:\>

初步理解一下整个过程：

>>> ().__class__.__bases__[0].__subclasses__()
...
...

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：

>>> s1 = """
... [
... c for c in ().__class__.__bases__[0].__subclasses__()
... if c.__name__ == "file"
... ][0]("/etc/passwd").read()()
... """
>>> eval(s1, {'__builtins__':{}})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 5, in <module>
IOError: file() constructor not accessible in restricted mode

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

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