From djb@cr.yp.to Wed Dec 15 14:23:07 2004
Date: 15 Dec 2004 08:30:45 -0000
From: D. J. Bernstein <djb@cr.yp.to>
To: securesoftware@list.cr.yp.to, cups@easysw.com
Subject: [local] [kill] CUPS 1.1.22 lppasswd ignores write errors, etc.

Bartlomiej Sieka, a student in my Fall 2004 UNIX Security Holes course,
has discovered several security problems in how lppasswd, version 1.1.22
(current), edits /usr/local/etc/cups/passwd. I'm publishing this notice,
but all the discovery credits should be assigned to Sieka.

First, lppasswd blithely ignores write errors in fputs(line,outfile) at
lines 311 and 315 of lppasswd.c, and in fprintf(...) at line 346. An
attacker who fills up the disk at the right moment can arrange for
/usr/local/etc/cups/passwd to be truncated.

Second, if lppasswd bumps into a file-size resource limit while writing
passwd.new, it leaves passwd.new in place, disabling all subsequent
invocations of lppasswd. Any local user can thus disable lppasswd by
running the attached program 63.c.

Third, line 306 of lppasswd.c prints an error message to stderr but
does not exit. This is not a problem on systems that ensure that file
descriptors 0, 1, and 2 are open for setuid programs, but it is a
problem on other systems; lppasswd does not check that passwd.new is
different from stderr, so it ends up writing a user-controlled error
message to passwd if the user closes file descriptor 2.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

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