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

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: High

Vector: Network

CVE: not yet issued

CWE: 78

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

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 authenticated 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

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 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.

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

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

2017-10-16 Changed command execution vulnerability

Author: Niklas Abel
Date: October 13, 2017