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Bug 164466 - sys-process/vixie-cron: local DoS (CVE-2007-1856)
Summary: sys-process/vixie-cron: local DoS (CVE-2007-1856)
Alias: None
Product: Gentoo Security
Classification: Unclassified
Component: Vulnerabilities (show other bugs)
Hardware: All Linux
: High normal (vote)
Assignee: Gentoo Security
Whiteboard: A3 [glsa] Falco
Depends on:
Reported: 2007-01-29 21:10 UTC by Raphael Marichez (Falco) (RETIRED)
Modified: 2020-03-02 16:40 UTC (History)
5 users (show)

See Also:
Package list:
Runtime testing required: ---


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Description Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-01-29 21:10:51 UTC

i may have found a problem in some implementations of cron.
vixie-cron and fcron are concerned, dcron is not; i haven't checked other ones.

cron checks if the crontab files have st_nlink==1, and if not, cron doesn't execute those files.

If /home is not on a separate partition, i can :
ln /etc/crontab .
(then wait for a reload, or force a reload with crontab -e)
and the cron service is nearly entirely shut down (/etc/cron.daily|monthy|hourly)

i can also ln /etc/cron.d/* to deny the execution of those files

finally if i am in the crontab group, i can cd /var/tmp (which is usually rwrwrw), and "ln /var/spool/cron/crontabs/user ."
I force a cron reload with a quick "crontab -e"

Jan 29 21:16:01 localhost cron[6014]: (*system*) BAD LINK COUNT (/etc/cron.d/test)
Jan 29 21:11:01 localhost cron[6014]: (*system*) BAD LINK COUNT (/etc/crontab)
Jan 29 21:11:01 localhost cron[6014]: (falco) BAD LINK COUNT (crontabs/falco)

Debian has the same behaviour, but the access to /var/spool/cron/crontabs is more restricted.
Jan 29 22:09:01 djali /usr/sbin/cron[10918]: (x2002marichez) WRONG INODE INFO (crontabs/x2002marichez)

i restrict this bug today, but it may be useful to get some feedback from several developers. This check is probably here for some reason...!
Comment 1 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-01-30 19:53:46 UTC
Aaron, Wolfram, do you have an idea on why does this st_nlink check exist? Do you have any comment?
Comment 2 Wolfram Schlich (RETIRED) gentoo-dev 2007-01-30 22:58:26 UTC
FCron (2 and 3) only seems to check link count for temporary files.
Thus, it does not seem to be affected.
Comment 3 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-02-06 12:13:28 UTC
(In reply to comment #2)
> FCron (2 and 3) only seems to check link count for temporary files.
> Thus, it does not seem to be affected.

Right, thanks.

But vixie-cron doesn't deal with that kind of temporary files. It really checks st_nlinks on the real crontabs.

Vapier do you have any clue here? I'm still looking for a possible reason why does vixie-cron check st_links==1 on its crontabs, allowing for a local DoS throught hardlinks created on /etc/crontab /etc/cron.*/* /var/spool...etc

Comment 4 SpanKY gentoo-dev 2007-02-16 06:40:06 UTC
i cant guess; seems like a dumb check
Comment 5 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-02-27 00:00:36 UTC
so i'd tend to ask to Wolfram if he wants to patch that and fix it. I don't know how to handle that issue without an active upstream.
Comment 6 Wolfram Schlich (RETIRED) gentoo-dev 2007-02-27 10:01:43 UTC
Sorry, but I have nothing to do with vixie-cron  -- only fcron :-)
Comment 7 Matthias Geerdsen (RETIRED) gentoo-dev 2007-03-07 13:15:31 UTC
So what is going to happen here?

Falco, do you want to contact upstream? Might be best.
Comment 8 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-03-13 23:07:27 UTC
vorlon, i wanted to contact upstream but i'm afraid there's no upstream.
Comment 9 Sune Kloppenborg Jeppesen (RETIRED) gentoo-dev 2007-03-25 07:52:22 UTC
Falco, any news on this one?
Comment 10 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-03-26 22:07:53 UTC
(In reply to comment #9)
> Falco, any news on this one?

i wish to talk about it on v-sec but i'm not officially introduced yet.
Comment 11 Sune Kloppenborg Jeppesen (RETIRED) gentoo-dev 2007-04-04 08:01:18 UTC
Falco, any more news on this one?
Comment 12 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-04-09 01:04:21 UTC
Hi dear arches security liaisons,

please test vixie-cron-4.1-r10 which changes vixie-cron behaviour on /var/spool/cron/crontabs. Upgrade should be OK, so should the new installations, but please test it deeply since it's a major package, and don't hesitate to comment.

After upgrading , /var/spool/cron/crontabs should be :
drwx-wx--T 2 root crontab
And inside:
-rw------- 1 apache crontab  417 Mar 11 20:53 apache
-rw------- 1 falco  crontab 1.1K Apr  8 23:36 falco
And /usr/bin/crontab is no longer SUID, but now SGID.

Very few linux distros are concerned by this bug so i think we will disclose it very soon. No need to urge here, you can just report on that bug if the tests are OK. Thanks in advance.
Comment 13 Markus Rothe (RETIRED) gentoo-dev 2007-04-10 06:43:47 UTC
(In reply to comment #12)
> please test vixie-cron-4.1-r10

fine on ppc64. no stable marking yet?
Comment 14 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-04-10 15:50:20 UTC
Hi arches,

you can now mark it stable if it runs fine for you, since it's already partially public via the OWL patch
Comment 15 Raphael Marichez (Falco) (RETIRED) gentoo-dev 2007-04-10 15:53:38 UTC
Goes public now, removing liaisons and adding arch aliases. Please see comment #12.
Comment 16 Raúl Porcel (RETIRED) gentoo-dev 2007-04-10 16:17:37 UTC
x86 + ia64 stable
Comment 17 Gustavo Zacarias (RETIRED) gentoo-dev 2007-04-10 17:21:20 UTC
sparc stable.
would have been good to remove the liasons when adding arches too...
Comment 18 Christoph Mende (RETIRED) gentoo-dev 2007-04-10 17:31:52 UTC
emerges fine and works on amd64

Portage (default-linux/amd64/2006.1/desktop, gcc-4.1.1, glibc-2.5-r0, 2.6.20-beyond2 x86_64)
System uname: 2.6.20-beyond2 x86_64 AMD Athlon(tm) 64 X2 Dual Core Processor 4600+
Gentoo Base System release 1.12.9
Timestamp of tree: Tue, 10 Apr 2007 15:20:01 +0000
ccache version 2.4 [enabled]
dev-java/java-config: 1.3.7, 2.0.31-r5
dev-lang/python:     2.4.3-r4
dev-python/pycrypto: 2.0.1-r5
dev-util/ccache:     2.4-r6
sys-apps/sandbox:    1.2.17
sys-devel/autoconf:  2.13, 2.61
sys-devel/automake:  1.4_p6, 1.5, 1.6.3, 1.7.9-r1, 1.8.5-r3, 1.9.6-r2, 1.10
sys-devel/binutils:  2.16.1-r3
sys-devel/gcc-config: 1.3.14
sys-devel/libtool:   1.5.22
virtual/os-headers:  2.6.17-r2
CFLAGS="-march=k8 -Os -pipe -msse3 -w"
CONFIG_PROTECT="/etc /usr/share/X11/xkb"
CONFIG_PROTECT_MASK="/etc/env.d /etc/env.d/java/ /etc/gconf /etc/java-config/vms/ /etc/php/apache1-php5/ext-active/ /etc/php/apache2-php5/ext-active/ /etc/php/cgi-php5/ext-active/ /etc/php/cli-php5/ext-active/ /etc/revdep-rebuild /etc/splash /etc/terminfo"
CXXFLAGS="-march=k8 -Os -pipe -msse3 -w"
FEATURES="buildsyspkg ccache collision-protect distlocks metadata-transfer multilib-strict parallel-fetch sandbox sfperms strict test"
MAKEOPTS="-j3 -l3 -s"
PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --compress --force --whole-file --delete --delete-after --stats --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages --filter=H_**/files/digest-*"
USE="X a52 aac acpi alsa amd64 amr audiofile bitmap-fonts bzip2 cairo cdinstall cdr cli cracklib crypt cups dbus dri dts dvd dvdr dvdread emboss encode exif fam firefox fortran gdbm gif gstreamer gtk gtk2 hal iconv jpeg libg++ logrotate mad midi mikmod minimal mp3 mpeg ncurses nptl nptlonly offensive ogg opengl pam pcre php png ppds pppd quicktime readline reflection sdl session smp spl ssl svg symlink tcpd test tiff truetype truetype-fonts type1-fonts unicode v4l vim vorbis x264 xinerama xorg xv xvid zlib" ALSA_CARDS="emu10k1" ALSA_PCM_PLUGINS="adpcm alaw asym copy dmix dshare dsnoop empty extplug file hooks iec958 ioplug ladspa lfloat linear meter mulaw multi null plug rate route share shm softvol" ELIBC="glibc" INPUT_DEVICES="evdev keyboard" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIRC_DEVICES="inputlirc" USERLAND="GNU" VIDEO_CARDS="nvidia"
Comment 19 Peter Weller (RETIRED) gentoo-dev 2007-04-10 18:17:11 UTC
amd64 stable!
Comment 20 Markus Rothe (RETIRED) gentoo-dev 2007-04-10 23:16:15 UTC
ppc64 stable
Comment 21 Jeroen Roovers (RETIRED) gentoo-dev 2007-04-10 23:53:30 UTC
Stable for HPPA (killerfox).
Comment 22 Tobias Scherbaum (RETIRED) gentoo-dev 2007-04-11 19:42:30 UTC
ppc stable
Comment 23 Fernando J. Pereda (RETIRED) gentoo-dev 2007-04-13 15:20:02 UTC
Alpha done.
Comment 24 Matthias Geerdsen (RETIRED) gentoo-dev 2007-04-16 19:06:50 UTC
GLSA 200704-11

thanks everyone once again :)