Re: Re: Security advisory for musl libc - stack-based buffer overflow in ipv6 literal parsing [CVE-2015-1817]

[Rich Felker <dalias@libc.org>]

On Fri, Apr 17, 2015 at 01:23:27PM -0400, Rich Felker wrote:
> On Fri, Apr 17, 2015 at 09:10:08PM +0800, Matt Johnston wrote:
> > Hi,
> > 
> > I think Dropbear probably is vulnerable to CVE-2015-1817
> > post-authentication. TCP forwarding requests will call
> > getaddrinfo() 
> > https://secure.ucc.asn.au/hg/dropbear/file/cbd674d63cd4/dbutil.c#l415
> > (moved to netio.c in head, and PF_UNSPEC has been fixed to
> > AF_UNSPEC). Pre-authentication should be OK, only
> > getnameinfo() is called (if that's enabled).
> 
> Yes, I agree dropbear is affected.
> 
> And wow, this is an utter mess. Not only does dropbear fail to drop
> root before processing forwards; it NEVER drops root at all. The
> user's session remains running as root for its full lifetime. Aside
> from being a huge risk, it also allows users to bypass uid-based
> firewall rules via port forwarding; for example, a rule that forbids
> normal users from making outgoing connections on port 25 would not be
> honored.
> 
> Is there any reason for not performing the setgroups/setgid/setuid
> immediately after authentication succeeds? Have you looked at whether
> it would be easy to patch that in?

Simply copying/moving the code from svr-chansession.c's execchild to
svr-auth.c's send_msg_userauth_success seems to fix the entire issue.
I had to disable the (useless on systems with proper pty support)
chown/chmod for the pty, but otherwise it seems to be working fine.

Rich