Shadowsocks - Log File Command Execution

X41 D-Sec GmbH Security Advisory: X41-2017-008

Multiple Vulnerabilities in Shadowsocks
=======================================

Overview
--------
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A
Vendor: Shadowsocks
Vendor URL: https://github.com/shadowsocks/shadowsocks/tree/master
Vector: Network
Credit: X41 D-Sec GmbH, Niklas Abel
Status: Public
Advisory-URL:
https://www.x41-dsec.de/lab/advisories/x41-2017-008-shadowsocks/


Summary and Impact
------------------
Several issues have been identified, which allow attackers to manipulate
log files, execute commands and to brute force Shadowsocks with enabled
autoban.py brute force detection. Brute force detection from autoban.py
does not work with suggested tail command. The key of captured
Shadowsocks traffic can be brute forced.


Product Description
-------------------
Shadowsocks is a fast tunnel proxy that helps you bypass firewalls.



Log file manipulation
=====================
Severity Rating: Medium
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A
Vector: Network
CVE: not yet issued
CWE: 117
CVSS Score: 4.3
CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

Summary and Impact
------------------
Log file manipulation is possible with a manipulated hostname, sent to
the server from a client, even if Shadowsocks is as quiet as possible
with "-qq".

Therefore a string like "\nI could be any log entry\n" could be sent as
hostname to Shadowsocks. The server would log an additional line with
"I could be any log entry".


Workarounds
-----------
There is no workaround available, do not trust the logfiles until a
patch is released.



Command Execution
=================
Severity Rating: Critical
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A
Vector: Network
CVE: not yet issued
CWE: 78
CVSS Score: 9.0
CVSS Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H


Summary and Impact
------------------
When the brute force detection with autoban.py is enabled, remote
attackers are able to execute arbitrary commands.

Command execution is possible because of because of line 53 "os.system(cmd)"
in autoban.py, which executes "cmd = 'iptables -A INPUT -s %s -j DROP' %
ip". The "ip" parameter gets parsed from the log file, whose contents
can be controlled by a third party sending unauthenticated packets.


Proof of Concept
----------------
When, a string like "can not parse header when ||ls&:\n" is sent as host
name to Shadowsocks, it would end up in the logfile and lead to the
execution of "ls".
Autoban.py does not execute commands with spaces due to internal
sanitization. A requested hostname like:

" can not parse header when ||ls&:\ntouch /etc/evil.txt\nexit\ncan not
parse header when ||/bin/bash</var/log/shadowsocks.log&:\n" could be
used to work around this limitation. It writes the command "touch
/etc/evil.txt" into the logfile and executes it with
"/bin/bash</var/log/shadowsocks.log".
The exit; command is an important factor, without it an unbounded
recursion would occur leading to a DoS.


Workarounds
-----------
No workaround available, do not use autoban.py.



Lack of Bruteforce detection through autoban.py
===============================================
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A


Summary and Impact
------------------
The brute force detection autoban.py does not work at all with the suggested
tail command, suggested at
https://github.com/shadowsocks/shadowsocks/wiki/Ban-Brute-Force-Crackers.

The command "python autoban.py < /var/log/shadowsocks.log" does work,
but the suggested "nohup tail -F /var/log/shadowsocks.log | python
autoban.py > log 2>log &" does not block IP's.
The "for line in sys.stdin:" from autoban.py parses the input until
there is an end of file (EOF). As "tail -F" will never pipe an EOF into
the pyhon script, the sys.stdin will block the script forever. So the
"tail -F /var/log/shodowsocks | autoban.py" will never block anything
except itself.

Workarounds
-----------
Use python "autoban.py < /var/log/shadowsocks.log" in a cronjob. Do not
use autoban.py until the command execution issue gets fixed.



Bruteforcable Shadowsocks traffic because of MD5
================================================
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A

Summary and Impact
------------------
Shadowsocks uses no brute force prevention for it's key derivation function.

The key for Shadowsocks traffic encryption is static and derived from
the password, using MD5. The password derivation is in encrypt.py in
line 56 to 63: "

while len(b''.join(m)) < (key_len + iv_len):
        md5 = hashlib.md5()
        data = password
        if i > 0:
            data = m[i - 1] + password
        md5.update(data)
        m.append(md5.digest())
        i += 1
"

MD5 should not be used to generate keys, since it is a hash function.
A proper key derivation function increases the costs for this operation,
which is a small burden for a user, but a big one for an attacker,
which performs this operation many more times. As passwords usually have
low-entropy, a good password derivation function has to be slow.


Workarounds
-----------
Use a secure password generated by a cryptographically secure random
generator. Wait for a patch that uses a password based key derivation
function like "Argon2" instead of a hash.



About X41 D-Sec GmbH
--------------------
X41 D-Sec is a provider of application security services. We focus on
application code reviews, design review and security testing. X41 D-Sec
GmbH was founded in 2015 by Markus Vervier. We support customers in
various industries such as finance, software development and public
institutions.


Timeline
--------
2017-09-28  Issues found
2017-10-05  Vendor contacted
2017-10-09  Vendor contacted, replied to use GitHub for a full disclosure
2017-10-11  Vendor contacted, asked if the vendor is sure to want a full
disclosure
2017-10-12  Vendor contacted, replied to create a public issue on GitHub
2017-10-13  Created public issues on GitHub
2017-10-13  Advisory release