On Mon, Oct 7, 2013 at 5:55 PM, Pedro Ribeiro <[email protected]> wrote: > ---------- Forwarded message ---------- > From: Colin Childs via RT <[email protected]> > Date: 7 October 2013 14:25 > Subject: [rt.torproject.org #14731] Off by one buffer overflow in tor stable > To: [email protected] > > > On Mon Oct 07 12:13:21 2013, [email protected] wrote: >> Hi, >> >> I think there is a small buffer overflow (off by one) in the current stable >> version of tor. >> The function in question is format_helper_exit_status, which returns a >> formatted hex string. It is in common/util.c, starting at line 3270. >> The function has a comment header that explains how it works. It >> specifically says it returns a string that is not null terminated, but >> instead terminates with a newline. >> >> The code checks periodically throughout the function whether it has written >> more bytes than it should. If it does, it errors out and writes a null >> character in the current character positions. This by itself can lead to a >> buffer overflow, but I believe the checks ensure that it almost never >> writes over the buffer size - except in one case. >> >> After it has finished everything, it then checks again if there are more >> than 0 bytes left in the buffer. If there are, it writes two characters - a >> newline and a null terminator (lines 3342 to 3347). >> >> The problem here is if the buffer only has one byte left, an off by one >> overflow occurs. These usually are very hard to exploit, but can be a >> security issue nonetheless. >> >> However given that I am not familiar with the tor codebase I might be >> wrong? I also did a quick security audit on the rest of the tor code and >> couldn't find anything else. I was inspire because of the recent events... >>
Thanks, Pedro! Thanks for looking at Tor! I agree that this probably isn't exploitable, but let's not take any chances. (I'm thinking "Not exploitable" not because off-by-one buffer overflows are safe, but because in order to get the overflow to happen at all, you would need to have errno be a high-magnitude negative number, which is not usually something that happens on unixy platforms. Moreover, you'd need to arrange for this to happen as a result of launching a pluggable transport proxy, which is not usually something under the attacker's control.) Nonetheless , this should get fixed. I've opened https://trac.torproject.org/projects/tor/ticket/9928 to track it, and written a possible fix. best wishes, -- Nick _______________________________________________ tor-dev mailing list [email protected] https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
