Bugtraq mailing list archives
Re: gzip TOCTOU file-permissions vulnerability
From: Martin Pitt <martin.pitt () canonical com>
Date: Tue, 12 Apr 2005 13:47:01 +0200
Hi Imran! Imran Ghory [2005-04-04 20:57 +0100]:
Vulnerable software ==================== gzip 1.2.4 and 1.3.3 and previous versions running on unix. Vulnerability ============== If a malicious local user has write access to a directory in which a target user is using gzip to extract or compress a file to then a TOCTOU bug can be exploited to change the permission of any file belonging to that user. On decompressing gzip copies the permissions from the compressed gzip file to the uncompressed file. However there is a gap between the uncompressed file being written (and it's file handler being close) and the permissions of the file being changed. During this gap a malicious user can remove the decompressed file and replace it with a hard-link to another file belonging to the user. gzip will then change the permissions on the hard-linked file to be the same as that of the gzip file.
I tried this with gzip 1.3.5 and cannot reproduce this; when gunzip'ing a file, the output file is _always_ created with 0600 permissions first, and chmod'ed to the original permission after decompression finished. This matches perfectly to the source code in gzip.c: treat_file() first calls create_outfile(), which does: /* Make sure that ofname is not an existing file */ if (check_ofname() != OK) { close(ifd); return ERROR; } /* Create the output file */ remove_ofname = 1; ofd = OPEN(ofname, flags, RW_USER); if (ofd == -1) { progerror(ofname); close(ifd); return ERROR; } and looks very reasonable (RW_USER equals to 0600); then the decompression takes place, and afterwards treat_file() calls copy_stat() to chmod the output file. This code is the same for version 1.3.2. Of course the file can be removed by other users after gunzip has finished, but that is not a gzip bug, but the result of the really dumb idea to have a group/world-writeable directory without the sticky bit. Maybe I understood you wrong, could you please give a small test case which describes the vulnerability exactly? Thanks! Martin -- Martin Pitt http://www.piware.de Ubuntu Developer http://www.ubuntulinux.org Debian Developer http://www.debian.org
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Current thread:
- gzip TOCTOU file-permissions vulnerability Imran Ghory (Apr 05)
- Re: gzip TOCTOU file-permissions vulnerability Martin Pitt (Apr 13)
- Re: gzip TOCTOU file-permissions vulnerability Derek Martin (Apr 13)
- Re: gzip TOCTOU file-permissions vulnerability Peter J. Holzer (Apr 13)
- Re: gzip TOCTOU file-permissions vulnerability Joey Hess (Apr 13)
- Re: gzip TOCTOU file-permissions vulnerability psz (Apr 14)
- Re: gzip TOCTOU file-permissions vulnerability Theodor Milkov (Apr 15)
- Re: gzip TOCTOU file-permissions vulnerability Derek Martin (Apr 14)
- <Possible follow-ups>
- RE: gzip TOCTOU file-permissions vulnerability Mark Senior (Apr 14)
- Re: gzip TOCTOU file-permissions vulnerability Derek Martin (Apr 14)
- Re: gzip TOCTOU file-permissions vulnerability devnull (Apr 15)
- Re: gzip TOCTOU file-permissions vulnerability Dmitry Yu. Bolkhovityanov (Apr 16)
(Thread continues...)
- Re: gzip TOCTOU file-permissions vulnerability Martin Pitt (Apr 13)