Date: Sat, 25 May 2002 08:08:55 +0000
From: david evlis reign <[email protected]>
To: [email protected], [email protected],
Subject: [DER ADV#8] - Local off by one in CVSD
Local off by one overflow in CVSD.
intro:
the family of scanf functions (scanf, sscanf, fscanf) are generally insecure
in usage and steps have been taken to make them more...secure you might say
like adding bounds checking (sscanf(hey, "%.4096s %d", buffer, int)) but the
function still remains quite insecure, to a lesser known bug like an off by
one.
WRONG:
char buf[10];
int i;
sscanf(hey, "%.10s", buf); <-- boundary checks ten bytes...
RIGHT:
char buf[10];
int i;
sscanf(hey, "%.9s" buf); <-- see!
therefore in the first example (WRONG) the last byte into buf will exeed the
allocated space (10 bytes) by one byte. woops.
(http://www.hert.org/papers/klog-1.html <-- nice article)
details:
in cvs-1.11/src/rcs.c:
info = findnode (vers->other_delta, "special");
if (info != NULL)
{
/* If the size of `devtype' changes, fix the sscanf call also */
char devtype[16]; <-- SIXTEEN BYTES
if (sscanf (info->data, "%16s %lu", <-- WOOPS SHOULD BE 15
devtype, &devnum_long) < 2)
error (1, 0, "%s:%s has bad `special' newphrase %s",
workfile, vers->version, info->data);
devnum = devnum_long;
if (STREQ (devtype, "character"))
special_file = S_IFCHR;
else if (STREQ (devtype, "block"))
special_file = S_IFBLK;
else
error (0, 0, "%s is a special file of unsupported type `%s'",
workfile, info->data);
}
}
this is only a locally exploitable hole since the data is read from
info->data which in turn is from a symlinked local file (heh, you know where
to find it);
we at der sys have created the following patch:
__END_OF_PATCH;
##########################
#DER PATCH FOR CVS < 1.11#
##########################
--- rcs_old.c Mon Jan 25 02:05:16 2002
+++ rcs.c Mon Jan 25 02:05:40 2002
--- 4238: if (sscanf (info->data, "%16s %lu",
+++ 4238: if (sscanf (info->data, "%.15s %lu",
devtype, &devnum_long) < 2)
error (1, 0, "%s:%s has bad `special' newphrase %s",
workfile, vers->version, info->data);
__END_OF_PATCH;
vendor notification: nope.
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