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__________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Center ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN FreeBSD Signal Handling Flaw [FreeBSD, Inc. Security Advisory 01:42] July 10, 2001 22:00 GMT Number L-111 ______________________________________________________________________________ PROBLEM: A flaw exists in FreeBSD signal handling clearing that allows some signals handlers to remain in effect in child processes PLATFORM: All versions of 4.x prior to correction date including 4.3-RELEASE DAMAGE: An attacker can execute arbitrary code SOLUTION: Apply the appropriate patch or upgrade system as described below ______________________________________________________________________________ VULNERABILITY The risk is MEDIUM: A local attacker could gain root access by ASSESSMENT: executing code in a setuid context ______________________________________________________________________________ [****** Start FreeBSD Bulletin ******] FreeBSD-SA-01:42 Security Advisory FreeBSD, Inc. Topic: signal handling during exec may allow local root compromise Category: core Module: kernel Announced: 2001-07-10 Credits: Georgi Guninski <guninski@guninski.com> Affects: All released versions of FreeBSD 4.x, FreeBSD 4.3-STABLE prior to the correction date. Corrected: 2001-07-09 FreeBSD only: Yes I. Background When a process forks, it inherits the parent's signals. When the process execs, the kernel clears the signal handlers because they are not valid in the new address space. II. Problem Description A flaw exists in FreeBSD signal handler clearing that would allow for some signal handlers to remain in effect after the exec. Most of the signals were cleared, but some signal hanlders were not. This allowed an attacker to execute arbitrary code in the context of a setuid binary. All versions of 4.x prior to the correction date including and 4.3-RELEASE are vulnerable to this problem. The problem has been corrected by copying the inherited signal handlers and resetting the signals instead of sharing the signal handlers. III. Impact Local users may be able to gain increased privileges on the local system. IV. Workaround Do not allow untrusted users to gain access to the local system. V. Solution One of the following: 1) Upgrade your vulnerable FreeBSD system to 4.3-STABLE after the correction date. 2) To patch your present system: download the relevant patch from the below location, and execute the following commands as root: [FreeBSD 4.1, 4.2, and 4.3 base systems] This patch has been verified to apply to FreeBSD 4.1, 4.2, and 4.3 only. It may or may not apply to older releases. # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:42/signal-4.3.patch # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:42/signal-4.3.patch.asc Verify the detached PGP signature using your PGP utility. # cd /usr/src/sys/kern # patch -p < /path/to/patch [ Recompile your kernel as described in http://www.freebsd.org/handbook/kernelconfig.html and reboot the system ] [****** End FreeBSD Bulletin ******] _______________________________________________________________________________ CIAC wishes to acknowledge the contributions of FreeBSD, Inc for the information contained in this bulletin. _______________________________________________________________________________ CIAC, the Computer Incident Advisory Center, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. CIAC services are available to DOE, DOE contractors, and the NIH. CIAC can be contacted at: Voice: +1 925-422-8193 (7x24) FAX: +1 925-423-8002 STU-III: +1 925-423-2604 E-mail: ciac@ciac.org Previous CIAC notices, anti-virus software, and other information are available from the CIAC Computer Security Archive. World Wide Web: http://www.ciac.org/ Anonymous FTP: ftp.ciac.org PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing communities receive CIAC bulletins. If you are not part of these communities, please contact your agency's response team to report incidents. Your agency's team will coordinate with CIAC. The Forum of Incident Response and Security Teams (FIRST) is a world-wide organization. A list of FIRST member organizations and their constituencies can be obtained via WWW at http://www.first.org/. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. 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