[DER ADV#8] - Local off by one in CVSD

["david evlis reign" <davidreign () hotmail com>]

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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