Description:
------------
ext/standard/scanf.c line ~887
  ---
  if (numVars) {
                    current = args[objIndex++];                
  ---
  
  objIndex points past the end of array in other format cases too

Reproduce code:
---------------
sscanf('foo ','$1s',$str);

http://www.plain-text.info/sscanf_bug.txt - full description

Actual result:
--------------
will try to dereference a pointer to pointer which usually causes
segmentation fault

Please check out this patch:
http://tony2001.phpclub.net/dev/tmp/bug38322.diff

the checkformat function checks the invalid numbers
by subtracting one but the scanning function doesnt
so seems to me the only problem here is that someone has forgotten to
subract 1
code from scanf.c line 737

} else if ( isdigit(UCHAR(*ch))) { 
                value = strtoul(format-1, &end, 10); 
                if (*end == '$') {
                    format = end+1;
                    ch = format++;
                    objIndex = varStart + value;
                }
         }

i think just by putting making a objIndex = varStart + value -1;

it would be secure and keep the functionality. though the if-s wont hurt
if you subract one so they can stay for insurance if performance is not
that big of a issue.

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.