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Multi Vendor fd_set Structure Bitmap Array Index Overflow
------------------------------------------------------------------------
SUMMARY
If a programmer fails to check socket number before using select() or
fd_set macros, it's possible to overwrite memory behind fd_set structure.
Very few select() based application actually check FD_SETSIZE value.
DETAILS
Affected:
* gnugk version 2.2.0 (confirmed, fixed by vendor)
<http://www.gnugk.org/> gnugk is OpenH323 Gatekeeper - The GNU
Gatekeeper
* jabber version 1.4.1 (tested, confirmed)
<http://www.jabber.org/> jabber is "the Linux of instant messaging" --
an open, secure, ad-free alternative to consumer IM services like AIM,
ICQ, MSN, and Yahoo
* <http://www.gotbnc.com> bnc version 2.8.4 (tested, confirmed, DoS
only) BNC is an IRC (Internet Relay Chat) proxying server
* socks5 version 1.0r1 (untested) socks5 is SOCKS v5 application layer
gateway and clients socks5 is unsupported, contact your package
distributor
* <http://uncensored.citadel.org/citadel/> Citadel version 6.27
(untested) Citadel is flexible, powerful, community-oriented groupware
* <http://www.inet.no/dante/> dante version 1.1 (tested, confirmed)
Dante is a circuit-level firewall/proxy (socks implemented)
* rinetd version 0.62 (untested) rinetd is a simple TCP port redirector
is unsupported, contact your package distributor
* <http://www.online.redhate.org/bld/> bld version 0.3 (limited,
untested) bld is a blacklisting daemon
* <http://www.security.nnov.ru/soft/3proxy> 3proxy version 0.4 (limited,
tested, fixed by vendor) 3Proxy is a really tiny cross-platform
(Win32&Unix) proxy servers set
Introduction:
fd_set structure is defined to be used with select() (man 2 select)
function. fd_set is used in select() and few special macros (FD_SET,
FD_CLR, FD_ISSET, FD_CLEAR). For all POSIX compatible operations systems
fd_set is defined as a bitmask array with a socket number as an array
index.
#ifndef FD_SETSIZE
#define FD_SETSIZE 1024
#endif
#define NBBY 8 /* number of bits in a byte */
typedef long fd_mask;
#define NFDBITS (sizeof (fd_mask) * NBBY) /* bits per mask */
#define howmany(x,y) (((x)+((y)-1))/(y))
typedef struct _types_fd_set {
fd_mask fds_bits[howmany(FD_SETSIZE, NFDBITS)];
} _types_fd_set;
#define fd_set _types_fd_set
A call to FD_SET sets a bit to 1 using socket number as an index:
#define FD_SET(n, p) ((p)->fds_bits[(n)/NFDBITS] |= (1L << ((n) %
NFDBITS)))
select() clears all used fd_set bits and sets a bits for sockets with
requested activity type. Select calculates length of the bitmask array
from the first argument, which must be above the largest number of the
socket used.
Neither FD_SET nor select() do not control socket to be below FD_SETSIZE.
For select() it's not possible in current select() implementation.
A control for socket number to be below FD_SETSIZE is left for programmer.
Vulnerability:
A simplest example of vulnerable function may be:
int waitsockdata(SOCKET sock, int timeosec, int timeousec){
fd_set fds;
struct timeval tv;
int res;
tv.tv_sec = timeosec;
tv.tv_usec = timeousec;
FD_ZERO(&fds);
FD_SET(sock, &fds);
if ((res = select (sock+1, &fds, NULL, NULL, &tv))!=1) return EOF;
return(sock);
}
In this example, if waitsockdata() is called with large socket number,
some data on the stack, for example saved EIP may be overwritten, but
attacker can control only one bit of data. In different situation, if
multiple sockets are placed on the fd_set attacker may have more control.
Vulnerable application:
There can be few types of select() based UNIX servers:
1. One process per client model (server fork()s for every client
connection)
2. Single application with single thread (finite state machine
architecture)
3. Threaded server (server creates a thread for every client)
Different models can be mixed. Models 2 and 3 could be vulnerable to this
kind of attacks if number of clients for each process is not limit or
limits allow large number (under FD_SETSIZE) of files or sockets to be
open in a single process. Model 1 is safe from this kind of vulnerability
(very limited in performance though).
Impact:
Depending on vulnerable application it's possible to overwrite portions of
memory. Impact is close to off-by-one overflows, code execution doesn't
seems exploitable.
Mitigating factors:
For all tested Linux distributions default ulimits for open descriptors
(1024) are equal to FD_SETSIZE (1024) preventing exploitation in default
configuration.
For Windows fd_set is a sockets array, not bitmask and FD_SETSIZE defines
maximum number of sockets in this array. So, Windows application may be
vulnerable only if it places a large number of sockets into same fd_set
structure (finite state machine architecture).
In case of BNC it's possible to overwrite variable with addrlen accept()
parameter effectively preventing new connections from being accepted and
almost fully control stack content. But vulnerable function never returns,
so code execution is probably impossible.
Elevating factors:
For FreeBSD and probably many more systems there is no default ulimits.
For Windows default FD_SETSIZE is 64 and select() is only
POSIX-complatible function to wait on socket input (there is no poll(),
but there are Windows specific functions).
For Cygwin FD_SETSIZE erroneously defined as 64, probably for Windows
compatibility, but fd_set is defined as a bitmask. It makes Cygwin very
sensible to this kind of attacks.
Dante probably have some additional bugs, because in tests it crashed long
before reaching FD_SETSIZE of connections.
Exploitation:
In most cases to exploit this vulnerability to DoS all you need is to
establish large number of concurrent connections. In some cases you need
additionally handshake client protocol. Exploit code is trivial and will
not be published.
Workaround:
Set ulimits for all network daemon accounts below FD_SETSIZE. Do not use
network daemons compiled with Cygwin.
Vendors and solutions:
* 3proxy 0.5b fixes the problem, download available from
<http://www.security.nnov.ru/soft/3proxy>
http://www.security.nnov.ru/soft/3proxy
All vendors were contacted with contacts listed in product packages. Only
Jan Willamowius from GNU h.323 Gatekeeper team replied to bug report.
gnugk 2.2.1 fixes the problem, download available from
<http://www.gnugk.org/h323download.html>
http://www.gnugk.org/h323download.html
References:
1. NetBSD Security Advisory 2002-014: fd_set overrun in mbone tools and
pppd <http://www.security.nnov.ru/search/document.asp?docid=3498>
http://www.security.nnov.ru/search/document.asp?docid=3498
Thanks to:
Ilya Anfimov and Kirill Lopuchov for help in identifying this
vulnerability, Greg MacManus from iDefense for historic information on
this issue and help with impact identification, Phiby for here love and
patience.
ADDITIONAL INFORMATION
The information has been provided by <mailto:3APA3A@security.nnov.ru>
3APA3A.
The original article can be found at:
<http://www.security.nnov.ru/advisiories/sockets.asp>
http://www.security.nnov.ru/advisiories/sockets.asp
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