X41 D-Sec GmbH Security Advisory: X41-2017-008
Confirmed Affected Versions: Latest commit 2ab8c6b on Sep 6
Confirmed Patched Versions: N/A
Credit: X41 D-Sec GmbH, Niklas Abel
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.
Shadowsocks is a fast tunnel proxy that helps you bypass firewalls.
Severity Rating: Medium
CVE: not yet issued
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
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".
There is no workaround available, do not trust the logfiles until a patch is released.
Severity Rating: High
CVE: not yet issued
CVSS Score: 8.5
CVSS Vector: CVSS:3.0/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
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 authenticated packets.
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.
No workaround available, do not use autoban.py.
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.
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 IPs. 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.
Use python "autoban.py < /var/log/shadowsocks.log" in a cronjob. Do not use autoban.py until the command execution issue gets fixed.
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.
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.
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.
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
2017-10-16 Changed command execution vulnerability