bugtraq - Superuser unsanitized environment vulnerability on Android <= 4.2.x

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Date: Wed, 13 Nov 2013 11:24:42 -0800
From: Kevin Cernekee <cernekee@...il.com>
To: bugtraq@...urityfocus.com
Subject: Superuser unsanitized environment vulnerability on Android <= 4.2.x

Vulnerable releases of several common Android Superuser packages may
allow malicious Android applications to execute arbitrary commands as
root without notifying the device owner:

 - ChainsDD Superuser (current releases, including v3.1.3)
 - CyanogenMod/ClockWorkMod/Koush Superuser (current releases,
including v1.0.2.1)
 - Chainfire SuperSU prior to v1.69

The majority of third-party ROMs include one of these packages.

On a rooted Android <= 4.2.x device, /system/xbin/su is a setuid root
binary which performs a number of privilege checks in order to
determine whether the operation requested by the caller should be
allowed.  In the course of its normal duties, and prior to making the
allow/deny decision, /system/xbin/su invokes external programs under a
privileged UID, typically root (0) or system (1000):

 - /system/bin/log, to record activity to logcat
 - /system/bin/am, to send intents to the Superuser Java app
 - /system/bin/sh, to execute the /system/bin/am wrapper script
 - /system/bin/app_process, the Dalvik VM

The user who invokes /system/xbin/su may have the ability to
manipulate the environment variables, file descriptors, signals,
rlimits, tty/stdin/stdout/stderr, and possibly other items belonging
to any of these subprocesses.  At least two vulnerabilities are
readily apparent:

 - On ClockWorkMod Superuser, /system/xbin/su does not set PATH to a
known-good value, so a malicious user could trick /system/bin/am into
using a trojaned app_process binary:

    echo -e '#!/system/bin/sh\nexport PATH=/system/bin:$PATH\ntouch
/data/trojan.out\nexec $0 "$@"' > app_process ; chmod 755 app_process
    PATH=`pwd`:$PATH su -c 'true'

The PATH vulnerability is being tracked under CVE-2013-6768.

 - Other environment variables could be used to affect the behavior of
the (moderately complex) subprocesses.  For instance, manipulation of
BOOTCLASSPATH could cause a malicious .jar file to be loaded into the
privileged Dalvik VM instance.  All three Superuser implementations
allowed Dalvik's BOOTCLASSPATH to be supplied by the attacker.

The BOOTCLASSPATH vulnerability is being tracked under CVE-2013-6774.