01 November 2007

Security Advisory: Norton AntiVirus for Macintosh


Symantec's Norton AntiVirus for Macintosh (NAV) contains a vulnerability that can lead to local privilege escalation from group admin to root (the super-user) without any of the usual password prompts Mac OS X presents for gaining super-user access. Group admin includes any users with the "Allow this user to administer this computer" box checked, this generally includes the first user created in an OS X install. This vulnerability is caused by a setuid-root binary in NAV which automatically runs another binary in a location where it can be replaced by users with group admin permissions. Since most Mac OS X users are in group admin on their computers, most NAV users will be vulnerable.


The easiest (and most foolproof) mitigation strategy is to uninstall NAV. (I sure don't feel very secure when a vendor allows a local privilege escalation vulnerability to fester in their security software for over 9 months. Your feelings may vary.)

Set the sticky bit on all directories between the vulnerable binary and the filesystem root that are writable by group admin. This can be done in Terminal.app with sudo chmod +t / /Library /Library/Application\ Support /Library/Application\ Support/Symantec /Library/Application\ Support/Symantec/SmallScanner.app etc. Keep in mind that running "Repair Permissions" on your disk will remove this change and leave your NAV install vulnerable once again. Apple has set the sticky bit on / and /Library by default on Mac OS X 10.5, but not /Library/Application Support and obviously not on the directories that NAV is installed into.

What Symantec Had to Say

The last time I heard from Symantec was August 29th:

As you know, Symantec developers reviewed the issue concerning NAV for the Mac that you sent to us, and agreed that there is cause for concern. However, they felt that the same problem could potentially affect other vendor’s[sic] software as well. We contacted Apple Product Security, and suggested some changes that could improve security for everyone.
Symantec does not currently plan to make changes to our existing products to directly address the problem you reported. The changes would require considerable architectural modifications for those products, and the changes could cause other problems for those products if Apple subsequently release an OS update to address the underlying problem.


Just in case people think the vendor didn't have enough time to address this, here's the timeline:

  • 2007-Jan-16 - Reported issue to Symantec

  • 2007-Jan-17 - Symantec will "review as soon as possible"

  • 2007-Jan-31 - Symantec "working on planning a fix", will coordinate release when product update has ETA

  • 2007-Apr-20 - Symantec thinks it would be better if Apple made changes to Mac OS X to fix the problem in NAV

  • 2007-Jun-21 - Contacted Apple for status

  • 2007-Jun-22 - Apple communicates the changes they're going to make for Leopard. These changes are not enough to workaround Symantec's vulnerable software.

  • 2007-Aug-29 - Symantec says they've made suggestions to Apple, but otherwise aren't going to do anything to fix this issue (see the quote in the previous section)

  • 2007-Oct-01 - It turns out that Apple's Leopard builds at the time didn't do enough to cover up the vulnerability in NAV

  • 2007-Oct-11 - Contacted Symantec and Apple to let them know I was going to post this Nov-1

  • 2007-Nov-1 - Today.


SmallScanner is run as root by the setuid-root binary
/Library/Application Support/Symantec/AntiVirus/DiskMountNotify.app/Contents/MacOS/DiskMountN
. E.g. after inserting a data cdrom:
root 8734 28.9 -2.1 616152 43596 ?? U 7:51PM 0:16.13
/Library/Application Support/Symantec/AntiVirus/SmallScanner.app/Contents/MacOS/SmallScanner

Unfortunately /Library/Application Support/ is writable by group admin and by
mv'ing the Symantec subdirectory out of the way and installing a tree of
symlinks in its place except for malware in place of SmallScanner arbitrary
code can be executed as root. That is you can trampoline from group admin to
user root. (There were a variety of other ways to do this that were coming out
of the woodwork back in January.)

Using the symlink method described above to replace SmallScanner with the following shell script...


touch /tmp/uhoh.this.is.very.bad.news
exec /Library/Application\ Support/Norton\ Solutions\ Support/Norton\ AntiVirus/SmallScanner.

And then inserting a disk (or using hdiutil to mount a disk image) results in normal looking behavior along with a new file...
-rw-r--r-- 1 root wheel 0 Jan 15 20:10 /tmp/uhoh.this.is.very.bad.news


I'd like to thank the Apple Product Security team for being more forthright in their communication with me on this issue, it was a lot better than my previous interactions. I'd like to apologize to all the users that have been unknowingly insecure for the past 289 days, I think I like full disclosure better too given the way vendors seem to drag their feet.


Patrick Gallagher said...

Nice catch. Looks like Symantec has posted a kb about this and gave you credit.



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