On Sat, Dec 8, 2018 at 7:57 PM <jf...@ms4.hinet.net> wrote:
>
> Grant Edwards於 2018年12月9日星期日 UTC+8上午12時52分04秒寫道：
> > Just to be clear: you do _not_ want to use eval on the string.
> >
> > If you're not the one who created the string, it might wipe your hard
> > drive or empty your bank account. If you _are_ the one who created
> > the string, then generate the desired result instead.
> >
> > --
> > Grant
>
> I didn't evaluate the input string directly. It's the translated "digit"
> string been evaluated, so shouldn't have any danger on using eval().
Replacing the first five letters of the alphabet is not sufficient to
sanitize untrusted input for eval. Here's a simple example that avoids
using any of those letters:
py> eval(re.sub(r'[a-e]', '0',
"__import__('su\\x62pro\\x63\\x65ss').run('\\x65\\x63ho rm -rf /',
**{'sh\\x65ll': 1})"))
rm -rf /
CompletedProcess(args='echo rm -rf /', returncode=0)
Now, if you remove *all* the characters that could possibly start
identifiers
(https://docs.python.org/3/reference/lexical_analysis.html#identifiers)
then you might be safe. Possibly just removing all the ones in ASCII
(A-Z + a-z + _) would suffice. I make no guarantees either way.
I wish I could say you should just use ast.literal_eval instead.
Unfortunately it doesn't seem to support ==:
py> ast.literal_eval('10 + 20 == 30')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.5/ast.py", line 84, in literal_eval
return _convert(node_or_string)
File "/usr/lib/python3.5/ast.py", line 83, in _convert
raise ValueError('malformed node or string: ' + repr(node))
ValueError: malformed node or string: <_ast.Compare object at 0x78172bee5358>
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