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TUCoPS :: Unix :: General :: n-151.txt

OpenSSH Buffer Management error (CIAC N-151)





             __________________________________________________________

                       The U.S. Department of Energy
                   Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

                        OpenSSH Buffer Management Error

September 17, 2003 17:00 GMT                                      Number N-151
[REVISED 22 Sept 2003]
[REVISED 23 Sept 2003]
[REVISED 1 Oct 2003]
[REVISED 2 Oct 2003]
[REVISED 27 Oct 2003]
______________________________________________________________________________
PROBLEM:       OpenSSH has announced an upgrade that fixes a buffer management 
               error. When a function that expands the size of a buffer 
               detects that the new size will be greater then 10Meg it 
               generates a fatal error. When the error is generated, the 
               function incorrectly sets the size of the allocated buffer to a 
               value larger than the actual allocation. The fatal error 
               processing routines will then attempt to deallocate more memory 
               than was allocated. 
PLATFORM:      All systems using versions of OpenSSH earlier than 3.7 
               Hewlett Packard HP-UX B.11.00, B.11.11, B.11.22 only with the
               T1471AA HP-UX Secure Sheel product installed.
               Mac OS X versions prior to 10.2.8
               IRIX 6.5.22
               SPARC Solaris 9
               x86 Solaris 9
               Red Hat Linux products
DAMAGE:        Could possibly cause a system to crash. 
SOLUTION:      Upgrade to OpenSSH version 3.7.1.
               Download and install appropriate files from appropriate vendor.
______________________________________________________________________________
VULNERABILITY  The risk is LOW. There are no known exploits. We believe it is 
ASSESSMENT:    unlikely that this problem can be exploited. If it can be 
               exploited, it might be possible to crash a system that does not 
               use protected memory (Windows 95, 98, ME). On systems with 
               protected memory (UNIX, Linux, Windows NT, 2000, XP) you might 
               be able to crash ssh but it is already shutting down because of 
               the fatal error. 
______________________________________________________________________________
LINKS: 
 CIAC BULLETIN:      http://www.ciac.org/ciac/bulletins/n-151.shtml 
 ORIGINAL BULLETIN:  http://www.openssh.com/txt/buffer.adv 
 ADDITIONAL LINKS:   http://www.cert.org/advisories/CA-2003-24.html
                     RED HAT
                     https://rhn.redhat.com/errata/RHSA-2003-279.html
                     https://rhn.redhat.com/errata/RHSA-2003-280.html
                     CISCO
                     http://www.cisco.com/warp/public/707/cisco-sa-20030917-
                     openssh.shtml
                     Visit HEWLETT PACKARD Subscription Service for:
                     HPSBUX0309-282 (SSRT3629)
                     Apple Security Advisory - Mac OS X 10.2.8 (APPLE-SA-2003-09-22)
                     http://net-security.org/advisory.php?id=2546
                     http://docs.info.apple.com/article.html?artnum=61798
                     SGI Security Advisory 20030904-01-P
                     http://www.sgi.com/support/security/advisories.html
                     Sun Alert ID: 56861
                     http://www.sunsolve.sun.com/pub-cgi/retrieve.pl?doc=fsalert%
                     2F56861&zone_32=category%3Asecurity
______________________________________________________________________________
REVISION HISTORY: 
9/22/03  - Updated PLATFORM section; SOLUTION section; and added link for Hewlett 
           Packard HPSBUX0309-282 (SSRT3629).
                  
9/23/03  - Updated PLATFORM section; SOLUTION section; and added link for Apple 
           Security Advisory - Mac OS X 10.2.8 (APPLE-SA-2003-09-22).
                  
10/1/03  - Updated PLATFORM section; added link for SGI Security Advisory
           20030904-01-P.

10/2/03  - Added link for Sun Alert ID: 56861 and updated PLATFORM section.

10/27/03 - Added link for Red Hat Advisory RHSA-2003:280 for information on patches 
           to their Red Hat Enterprise Linux products.
				
							
The description of this vulnerability exposes the fact that if a buffer in 
sshd can be expanded beyond about 10Meg, that sshd will shut down by design.  
It may be possible to exploit this feature but only to the copy of sshd that 
you are connected to.  That is, you can possibly kill your own process.


[******  Start OpenSSH Bulletin ******]

Subject: OpenSSH Security Advisory: buffer.adv

This is the 2nd revision of the Advisory.

This document can be found at:  http://www.openssh.com/txt/buffer.adv

1. Versions affected:

        All versions of OpenSSH's sshd prior to 3.7.1 contain buffer
        management errors.  It is uncertain whether these errors are
        potentially exploitable, however, we prefer to see bugs
        fixed proactively.

        Other implementations sharing common origin may also have
        these issues.

2. Solution:

	Upgrade to OpenSSH 3.7.1 or apply the following patch.

===================================================================
Appendix A: patch for OpenSSH 3.6.1 and earlier

Index: buffer.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/buffer.c,v
retrieving revision 1.16
retrieving revision 1.18
diff -u -r1.16 -r1.18
--- buffer.c	26 Jun 2002 08:54:18 -0000	1.16
+++ buffer.c	16 Sep 2003 21:02:39 -0000	1.18
@@ -23,8 +23,11 @@
 void
 buffer_init(Buffer *buffer)
 {
-	buffer->alloc = 4096;
-	buffer->buf = xmalloc(buffer->alloc);
+	const u_int len = 4096;
+
+	buffer->alloc = 0;
+	buffer->buf = xmalloc(len);
+	buffer->alloc = len;
 	buffer->offset = 0;
 	buffer->end = 0;
 }
@@ -34,8 +37,10 @@
 void
 buffer_free(Buffer *buffer)
 {
-	memset(buffer->buf, 0, buffer->alloc);
-	xfree(buffer->buf);
+	if (buffer->alloc > 0) {
+		memset(buffer->buf, 0, buffer->alloc);
+		xfree(buffer->buf);
+	}
 }
 
 /*
@@ -69,6 +74,7 @@
 void *
 buffer_append_space(Buffer *buffer, u_int len)
 {
+	u_int newlen;
 	void *p;
 
 	if (len > 0x100000)
@@ -98,11 +104,13 @@
 		goto restart;
 	}
 	/* Increase the size of the buffer and retry. */
-	buffer->alloc += len + 32768;
-	if (buffer->alloc > 0xa00000)
+	
+	newlen = buffer->alloc + len + 32768;
+	if (newlen > 0xa00000)
 		fatal("buffer_append_space: alloc %u not supported",
-		    buffer->alloc);
-	buffer->buf = xrealloc(buffer->buf, buffer->alloc);
+		    newlen);
+	buffer->buf = xrealloc(buffer->buf, newlen);
+	buffer->alloc = newlen;
 	goto restart;
 	/* NOTREACHED */
 }
Index: channels.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/channels.c,v
retrieving revision 1.194
retrieving revision 1.195
diff -u -r1.194 -r1.195
--- channels.c	29 Aug 2003 10:04:36 -0000	1.194
+++ channels.c	16 Sep 2003 21:02:40 -0000	1.195
@@ -228,12 +228,13 @@
 	if (found == -1) {
 		/* There are no free slots.  Take last+1 slot and expand the 
        array.  */
 		found = channels_alloc;
-		channels_alloc += 10;
 		if (channels_alloc > 10000)
 			fatal("channel_new: internal error: channels_alloc %d "
 			    "too big.", channels_alloc);
+		channels = xrealloc(channels,
+		    (channels_alloc + 10) * sizeof(Channel *));
+		channels_alloc += 10;
 		debug2("channel: expanding %d", channels_alloc);
-		channels = xrealloc(channels, channels_alloc * sizeof
        (Channel *));
 		for (i = found; i < channels_alloc; i++)
 			channels[i] = NULL;
 	}


===================================================================
Appendix B: patch for OpenSSH 3.7

Index: buffer.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/buffer.c,v
retrieving revision 1.17
retrieving revision 1.18
diff -u -r1.17 -r1.18
--- buffer.c	16 Sep 2003 03:03:47 -0000	1.17
+++ buffer.c	16 Sep 2003 21:02:39 -0000	1.18
@@ -23,8 +23,11 @@
 void
 buffer_init(Buffer *buffer)
 {
-	buffer->alloc = 4096;
-	buffer->buf = xmalloc(buffer->alloc);
+	const u_int len = 4096;
+
+	buffer->alloc = 0;
+	buffer->buf = xmalloc(len);
+	buffer->alloc = len;
 	buffer->offset = 0;
 	buffer->end = 0;
 }
@@ -34,8 +37,10 @@
 void
 buffer_free(Buffer *buffer)
 {
-	memset(buffer->buf, 0, buffer->alloc);
-	xfree(buffer->buf);
+	if (buffer->alloc > 0) {
+		memset(buffer->buf, 0, buffer->alloc);
+		xfree(buffer->buf);
+	}
 }
 
 /*
Index: channels.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/channels.c,v
retrieving revision 1.194
retrieving revision 1.195
diff -u -r1.194 -r1.195
--- channels.c	29 Aug 2003 10:04:36 -0000	1.194
+++ channels.c	16 Sep 2003 21:02:40 -0000	1.195
@@ -228,12 +228,13 @@
 	if (found == -1) {
 		/* There are no free slots.  Take last+1 slot and expand the 
        array.  */
 		found = channels_alloc;
-		channels_alloc += 10;
 		if (channels_alloc > 10000)
 			fatal("channel_new: internal error: channels_alloc %d "
 			    "too big.", channels_alloc);
+		channels = xrealloc(channels,
+		    (channels_alloc + 10) * sizeof(Channel *));
+		channels_alloc += 10;
 		debug2("channel: expanding %d", channels_alloc);
-		channels = xrealloc(channels, channels_alloc * sizeof
        (Channel *));
 		for (i = found; i < channels_alloc; i++)
 			channels[i] = NULL;
 	}

===================================================================

[******  End OpenSSH Bulletin ******]
_______________________________________________________________________________

CIAC wishes to acknowledge the contributions of OpenSSH for the 
information contained in this bulletin.
_______________________________________________________________________________


CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
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    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604
    E-mail:   ciac@ciac.org

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:      http://www.ciac.org/
   Anonymous FTP:       ftp.ciac.org

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.org/.

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
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