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TUCoPS :: Linux :: Apps A-M :: a6067.htm

ircII-based clients buffer overflows



15th Mar 2003 [SBWID-6067]
COMMAND

	ircII-based clients buffer overflows

SYSTEMS AFFECTED

	BitchX 1.0c19 ircii 20020912  EPIC4  1.1.7.20020907  EPIC4  1.0.1  XChat
	2.0.1

PROBLEM

	After seeing the BitchX "DoS" problem mentioned the n'th  time  already,
	I decided to finally audit  ircII  based  clients  to  show  some  worse
	problems they have. I had been pretty  sure  for  years  that  malicious
	servers can exploit them in multiple ways, and I think many others  have
	known it as well. EPIC and  ircII  authors  have  been  working  to  fix
	these, but looks like their job isn't yet finished.
	
	Let's state this clearly  first:  Regular  USERS  CANNOT  EXPLOIT  these
	bugs. This means that these clients are safe when they're  connected  to
	standard IRC servers. Connecting to special servers can  cause  problems
	though. So it requires user to type /SERVER evil.server.org  to  exploit
	these bugs. I don't think it's  too  difficult  with  a  bit  of  social
	engineering though.
	
	Of course, man-in-the-middle can also exploit these.
	
	There may be  more  problems  than  what  I  list  below.  ircII  wasn't
	originally written to be safe against malicious servers,  so  there's  a
	lot of code that needed fixing. My audit was only a quick  look  at  the
	clients, you may well find more.
	
	So, the safest fix is to not connect to untrusted servers, and  not  IRC
	over insecure network links. (Or just  switch  to  some  safe  non-ircII
	based client.)
	
	
	 BitchX 1.0c19
	 -------------
	
	Full of sprintf() calls  and  relying  on  BIG_BUFFER_SIZE  being  large
	enough.   There's   multiple   ways   to   exploit    it    by    giving
	near-BIG_BUFFER_SIZE strings in various places. 1)
	
	---
	#define IRCD_BUFFER_SIZE 512
	#define BIG_BUFFER_SIZE IRCD_BUFFER_SIZE*4
	
	extern	void	send_ctcp (int type, char *to, int datatag, char *format, ...)
	{
		char putbuf [BIG_BUFFER_SIZE + 1],
		     *putbuf2;
		int len;
		len = IRCD_BUFFER_SIZE - (12 + strlen(to));
		putbuf2 = alloca(len);
	...
			snprintf(putbuf2, len, "%c%s %s%c", 
					CTCP_DELIM_CHAR, 
					ctcp_cmd[datatag].name, putbuf, 
					CTCP_DELIM_CHAR);
	...
		putbuf2[len - 2] = CTCP_DELIM_CHAR;
		putbuf2[len - 1] = 0;
	---
	
	The len part looks interesting. Especially if strlen(to) is larger  than
	IRCD_BUFFER_SIZE-12, which gives alloca()  a  negative  value  (actually
	casted into large  size_t).  alloca()  is  pretty  stupid  function  and
	doesn't perform any bounds checking, so what actually  happens  is  that
	it returns a pointer to somewhere inside the stack already in  use.  You
	should be able to overwrite function's  return  address  with  a  little
	testing. I didn't bother enough to verify that, but I got close enough:
	
	Send: ":A*502 PRIVMSG a :^APING X*151^A"
	
	Program received signal SIGSEGV, Segmentation fault.
	0x08079e90 in send_ctcp (type=1, to=0xbfffee91 'A' <repeats 200 times>..., 
	    datatag=312, format=0x58585858 <Address 0x58585858 out of bounds>)
	    at ctcp.c:1513
	1513            putbuf2[len - 2] = CTCP_DELIM_CHAR;
	(gdb) p /x len
	$79 = 0x58585858
	
	Which gives the possibility to write ASCII 1+0 anywhere in memory.  That
	may be enough for exploit.
	
	2) cannot_join_channel() allows writing one of 6 predefined  texts  past
	buffer in stack if channel name is large enough.
	
	3) cluster() is  called  by  several  autokick/userlist/etc.  functions.
	static char result[] can be overflowed by around 1500 bytes by giving  a
	long hostname.
	
	4)  BX_compress_modes()  allows  overflowing  char  nmodes[16]  if  /set
	compress_modes on (default is off)
	
	5) handle_oper_vision(): Looks like feeding invalid "Client  connecting"
	line could overflow sprintf() in it. Didn't verify.
	
	6) ban_it() uses static char  banstr[BIG_BUFFER_SIZE/4+1].  Very  likely
	overflowable.
	
	Above ones were found just by grepping strcat and sprintf,  there  might
	be more. In general, just think what happens if nick name, host name  or
	channel name is near BIG_BUFFER_SIZE.
	
	No official fixes yet, but I've attached a patch below that fixes  these
	(well, untested but compiles). I've limited the  input  buffer  size  to
	only half of BIG_BUFFER_SIZE, that should  fix  most  of  the  potential
	problems.
	
	
	 ircii 20020912
	 --------------
	
	The  code  was  much  better  than  I  expected.  snprintf()  was   used
	everywhere, only problem that I found was a few my_strcat() calls:
	
	1) "PRIVMSG a a*4080^ASED^A" or "^AUTC 1^A" overflows  ctcp_buffer,  but
	not by much and there doesn't seem to be anything important right  after
	it
	
	2) cannot_join_channel() allows writing one of 5 predefined  texts  past
	buffer in stack. I was able to modify %ebp by with it, but couldn't  get
	anything useful out of it. It just crashed while trying to read  memory.
	I didn't try too hard though,  so  exploiting  might  be  possible  with
	other ways.
	
	3) Statusbar drawing has  several  buffer  overflows.  Usually  it  gets
	truncated to screen width, but a few control characters aren't  counted,
	so we can stuff the buffer nearly full of it.
	
	Next thing that happens is that the last character is filled  until  the
	screen width is full. That  alone  can  overflow  the  buffer.  Then  it
	appends \017 + \0 without overflow checks.
	
	status_make_printable() is called next. It has  a  check  for  pos  <
	BIG_BUFFER_SIZE, but that can still overflow buffer by  two  bytes  (our
	control char and \026) if last character is one the control chars.
	
	I got this far with exploiting:
	
	print SOCKET ":i 001 ".("\002"x3880).("X"x40).("\002"x140)." :\n";
	
	Program received signal SIGSEGV, Segmentation fault.
	0x08079f8a in make_status (window=0x58585858)
	    at /home/cras/src/ircii-20020912/source/status.c:803
	803                             if (window->status_line[k] && (SG == -1))
	
	I think you'd get a real exploit out of that  by  properly  setting  the
	local variables.
	
	4) Some of the other my_strcat() calls may overflow buffer as well  (eg.
	create_server_list()),  but  they  can't  be  exploited  by  (a  single)
	server.
	
	ircii-20030313 fixes these.
	
	
	 EPIC4 1.1.7.20020907
	 --------------------
	
	Note that this is an ALPHA version, and the author  strongly  recommends
	not to use it. However at least Debian is still distributing it  instead
	of 1.0.1 (which is why I audited that  -  I  just  did  "apt-get  source
	epic4").
	
	It contained one user (eg.  regular  PRIVMSG)  exploitable  heap  buffer
	overflow  if  mangle_inbound  setting  contained   ANSI.   Because   the
	overflowable bytes were limited to only  few  specific  characters,  I'm
	not sure if it's more than a remote crash.
	
	It also contained the same two problems as 1.0.1 below, and a  few  more
	potential ones.
	
	20030314 snapshot should fix these.
	
	
	 EPIC4 1.0.1
	 -----------
	
	This is the PRODUCTION release which you should be using.
	
	1) EPIC has grown max. input line of server from the old 4096  to  8192,
	but without growing BIG_SERVER_BUFFER from 4096. There's  at  least  one
	place where you can overflow it:
	
	---
	void	userhost_cmd_returned (UserhostItem *stuff, char *nick, char *text)
	{
		char	args[BIG_BUFFER_SIZE + 1];
	
		/* This should be safe, though its playing it fast and loose */
		strcpy(args, stuff->nick ? stuff->nick : empty_string);
		strcat(args, stuff->oper ? " + " : " - ");
		strcat(args, stuff->away ? "+ " : "- ");
		strcat(args, stuff->user ? stuff->user : empty_string);
		strcat(args, space);
		strcat(args, stuff->host ? stuff->host : empty_string);
		parse_line(NULL, text, args, 0, 0);
	}
	---
	
	Exploiting that requires that  the  client  calls  /USERHOST  nick  -CMD
	<command>, but I think that's quite widely used in EPIC scripts.
	
	2) Statusbar writing isn't fully safe:
	
	---
				/*
				 * XXXXX This is a bletcherous hack.
				 * If i knew what was good for me id not do this.
				 */
				else if (*ptr == 9)	/* TAB */
				{
					fillchar[0] = ' ';
					fillchar[1] = 0;
					do
						*cp++ = ' ';
					while (++(*prc) % 8);
					ptr++;
				}
	---
	
	Giving TAB at the end of nearly full buffer would overflow it.  Normally
	EPIC doesn't allow buffer to get wider  than  screen,  but  we  can  use
	several control characters to fill the buffer without  consuming  screen
	width.
	
	Now, the only problem is  how  do  you  get  large  enough  string  into
	statusbar. I didn't find any way to do it automatically by  default.  %C
	is largest at 512 bytes. strip_ansi() can  grow  the  string,  but  we'd
	still need 2700 bytes  or  so.  Some  user-defined  statusbar  variables
	could be used though, again requiring some script that uses them.
	
	NOTE: I did only a quick check to see  if  the  problems  I  found  from
	ALPHA version were still there. There's quite a lot of  changes  between
	these versions, so there may be more and I'm too busy now to go  through
	it.
	
	1.0.2 will not be released to fix server based  exploits.  1.1.x  series
	intends to fully fix it, so in the  mean  time  just  don't  connect  to
	untrusted servers. Growing BIG_BUFFER_SIZE to  8192  (and  maybe  a  few
	hundred bytes larger) should make you pretty safe though.
	
	
	 XChat 2.0.1
	 -----------
	
	This isn't ircII-based, but I thought I'd check it as well.
	
	Nothing usable found. A few minor potential buffer  overflows  here  and
	there, requiring conditions that currently  don't  exist.  One  thing  I
	don't understand is why  does  it  bother  playing  around  with  unsafe
	buffers while it could use GLIB's GStrings just as easily? This  bothers
	me a bit:
	
	---
		char outbuf[4096];
		char pdibuf_static[522]; /* 1 line can potentially be 512*6 in utf8 */
	---
	
	outbuf is commonly treated as "large enough" to contain most of  pdibuf.
	This gives it around 1024 bytes of extra space. Is it enough?  Seems  to
	be currently, but all you need for an overflow is to get  two  different
	non-truncated server given strings written into it.

SOLUTION

	 BitchX Patch
	 ------------
	
	diff -ru BitchX-old/source/banlist.c BitchX/source/banlist.c
	--- BitchX-old/source/banlist.c	2002-02-28 06:22:46.000000000 +0200
	+++ BitchX/source/banlist.c	2003-03-13 20:09:01.000000000 +0200
	@@ -277,30 +277,30 @@
	 		case 7:
	 			if (ip)
	 			{
	-				sprintf(banstr, "*!*@%s", cluster(ip));
	+				snprintf(banstr, sizeof(banstr), "*!*@%s", cluster(ip));
	 				break;
	 			}
	 		case 2: /* Better 	*/
	-			sprintf(banstr, "*!*%s@%s", t1, cluster(host));
	+			snprintf(banstr, sizeof(banstr), "*!*%s@%s", t1, cluster(host));
	 			break;
	 		case 3: /* Host 	*/
	-			sprintf(banstr, "*!*@%s", host);
	+			snprintf(banstr, sizeof(banstr), "*!*@%s", host);
	 			break;
	 		case 4: /* Domain	*/
	-			sprintf(banstr, "*!*@*%s", strrchr(host, '.'));
	+			snprintf(banstr, sizeof(banstr), "*!*@*%s", strrchr(host, '.'));
	 			break;
	 		case 5: /* User		*/
	-			sprintf(banstr, "*!%s@%s", t, cluster(host));
	+			snprintf(banstr, sizeof(banstr), "*!%s@%s", t, cluster(host));
	 			break;
	 		case 6: /* Screw 	*/
	 			malloc_sprintf(&tmpstr, "*!*%s@%s", t1, host);
	-			strcpy(banstr, screw(tmpstr));
	+			strmcpy(banstr, screw(tmpstr), sizeof(banstr)-1);
	 			new_free(&tmpstr);
	 			break;
	 		case 1:	/* Normal 	*/
	 		default:
	 		{
	-			sprintf(banstr, "%s!*%s@%s", nick, t1, host);
	+			snprintf(banstr, sizeof(banstr), "%s!*%s@%s", nick, t1, host);
	 			break;
	 		}
	 	}
	diff -ru BitchX-old/source/ctcp.c BitchX/source/ctcp.c
	--- BitchX-old/source/ctcp.c	2002-02-28 06:22:47.000000000 +0200
	+++ BitchX/source/ctcp.c	2003-03-13 19:59:35.000000000 +0200
	@@ -1482,6 +1482,7 @@
	 	     *putbuf2;
	 	int len;
	 	len = IRCD_BUFFER_SIZE - (12 + strlen(to));
	+	if (len <= 2) return;
	 	putbuf2 = alloca(len);
	 
	 	if (format)
	diff -ru BitchX-old/source/misc.c BitchX/source/misc.c
	--- BitchX-old/source/misc.c	2002-03-24 11:31:07.000000000 +0200
	+++ BitchX/source/misc.c	2003-03-13 20:02:13.000000000 +0200
	@@ -3121,19 +3121,19 @@
	 	{
	 		if (*hostname == '~')
	 			hostname++;
	-		strcpy(result, hostname);
	+		strmcpy(result, hostname, sizeof(result)-1);
	 		*strchr(result, '@') = '\0';
	 		if (strlen(result) > 9)
	 		{
	 			result[8] = '*';
	 			result[9] = '\0';
	 		}
	-		strcat(result, "@");
	+		strmcat(result, "@", sizeof(result)-1);
	 		if (!(hostname = strchr(hostname, '@')))
	 			return NULL;
	 		hostname++;
	 	}
	-	strcpy(host, hostname);
	+	strmcpy(host, hostname, sizeof(host)-1);
	 
	 	if (*host && isdigit(*(host + strlen(host) - 1)))
	 	{
	@@ -3154,8 +3154,8 @@
	                 for (i = 0; i < count; i++)
	                         tmp = strchr(tmp, '.') + 1;
	                 *tmp = '\0';
	-                strcat(result, host);
	-                strcat(result, "*");
	+                strmcat(result, host, sizeof(result)-1);
	+                strmcat(result, "*", sizeof(result)-1);
	 	}
	 	else
	 	{
	@@ -3177,10 +3177,10 @@
	 			else
	 				return (char *) NULL;
	 		}
	-		strcat(result, "*");
	+		strmcat(result, "*", sizeof(result)-1);
	 		if (my_stricmp(host, temphost))
	-			strcat(result, ".");
	-		strcat(result, host);
	+			strmcat(result, ".", sizeof(result)-1);
	+		strmcat(result, host, sizeof(result)-1);
	 	}
	 	return result;
	 }
	diff -ru BitchX-old/source/names.c BitchX/source/names.c
	--- BitchX-old/source/names.c	2002-03-25 22:47:30.000000000 +0200
	+++ BitchX/source/names.c	2003-03-13 20:10:26.000000000 +0200
	@@ -572,7 +572,7 @@
	 
	    	*nmodes = 0;
	    	*nargs = 0;
	-	for (; *modes; modes++) 
	+	for (; *modes && strlen(nmodes) < sizeof(nmodes)-2; modes++)
	 	{
	 		isbanned = isopped = isvoiced = 0;
	 		switch (*modes) 
	@@ -742,7 +742,7 @@
	 
	    /* modes which can be done multiple times are added here */
	 
	-	for (tucm = ucm; tucm; tucm = tucm->next) 
	+	for (tucm = ucm; tucm && strlen(nmodes) < sizeof(nmodes)-2; tucm = tucm->next)
	 	{
	 		if (tucm->o_ed) 
	 		{
	diff -ru BitchX-old/source/notice.c BitchX/source/notice.c
	--- BitchX-old/source/notice.c	2002-02-28 06:22:50.000000000 +0200
	+++ BitchX/source/notice.c	2003-03-13 20:07:39.000000000 +0200
	@@ -422,10 +422,10 @@
	 	{
	 		char *q = strchr(line, ':');
	 		char *port = empty_string;
	-		int conn = !strncmp(line+7, "connect", 7) ? 1 : 0;
	+		int conn = strlen(line) > 7 && !strncmp(line+7, "connect", 7) ? 1 : 0;
	 		int dalnet = 0, ircnet = 0;
	 
	-		if (*(line+18) == ':')
	+		if (strlen(line) > 18 && *(line+18) == ':')
	 			q = NULL;
	 		else
	 			dalnet = (q == NULL);
	@@ -462,7 +462,7 @@
	 		    else sscanf(p, "%s was %s from %s", for_, fr, temp);
	 
	 		    q = p;
	-		    sprintf(q, "%s@%s", fr, temp);
	+		    snprintf(q, strlen(q)+1, "%s@%s", fr, temp);
	 		    if (!conn) 
	 		    {
	 			port = strstr(temp2, "reason:");
	diff -ru BitchX-old/source/server.c BitchX/source/server.c
	--- BitchX-old/source/server.c	2002-03-25 07:21:24.000000000 +0200
	+++ BitchX/source/server.c	2003-03-13 20:10:00.000000000 +0200
	@@ -474,11 +474,11 @@
	 					}
	 					else
	 #endif
	-						junk = dgets(bufptr, des, 1, BIG_BUFFER_SIZE, server_list[i].ssl_fd);
	+						junk = dgets(bufptr, des, 1, BIG_BUFFER_SIZE/2, server_list[i].ssl_fd);
	 				}
	 				else
	 #endif
	-					junk = dgets(bufptr, des, 1, BIG_BUFFER_SIZE, NULL);
	+					junk = dgets(bufptr, des, 1, BIG_BUFFER_SIZE/2, NULL);
	 			}
	 			switch (junk)
	 			{
	@@ -1741,7 +1741,7 @@
	 			default:
	 				if (FD_ISSET(des, &rd))
	 				{
	-					if (!dgets(buffer, des, 0, BIG_BUFFER_SIZE, NULL))
	+					if (!dgets(buffer, des, 0, BIG_BUFFER_SIZE/2, NULL))
	 						flushing = 0;
	 				}
	 				break;
	@@ -1751,7 +1751,7 @@
	 	FD_ZERO(&rd);
	 	FD_SET(des, &rd);
	 	if (new_select(&rd, NULL, &timeout) > 0)
	-		dgets(buffer, des, 1, BIG_BUFFER_SIZE, NULL);
	+		dgets(buffer, des, 1, BIG_BUFFER_SIZE/2, NULL);
	 }
	


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