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Home » Security Advisories» FreeBSD » Unauthorized access via buffer overrun in talkd

Unauthorized access via buffer overrun in talkd

This attack requires an intruder to be able to make a network connection to a vulnerable talkd program and provide corrupt DNS information to that host.

  • Vendor: FreeBSD
  • Vendor ID: SA-96:21
  • Date: January 18, 1996


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=====================================================================
FreeBSD-SA-96:21 Security Advisory
FreeBSD, Inc.

Topic: unauthorized access via buffer overrun in talkd

Category: core
Module: talkd
Announced: 1997-01-18
Affects: 1.0, 1.1, 2.1.0, 2.1.5, 2.1.6, 2.1.6.1
Corrected: 2.2-current as of 1997-01-18
2.1-stable as of 1197-01-18
FreeBSD only: no

Patches: ftp://freebsd.org/pub/CERT/patches/SA-96:21/
References: AUSCERT AA-97.01 (Australian CERT organization),
SEI CERT VU#5942 (internal tracking reference only)

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

I. Background

Buffer overrun (aka stack overflow) exploits in system
supplied and locally installed utilities are commonly
used by individuals wishing to obtain unauthorized access to
computer systems. The FreeBSD team has been reviewing and
fixing the source code pool to eliminate potential exploits
based on this technique.

Recently, the Australian CERT organization received information
of a buffer-overrun vulnerability in the talkd daemon shipped in
most modern BSD based systems.


II. Problem Description

To quote AUSCERT:

talk is a communication program which copies text from one
users terminal to that of another, possibly remote, user.
talkd is the daemon that notifies a user that someone else wishes
to initiate a conversation.

As part of the talk connection, talkd does a DNS lookup
for the hostname of the host where the connection is being
initiating from. Due to insufficient bounds checking on
the buffer where the hostname is stored, it is possible to
overwrite the internal stack space of talkd. By carefully
manipulating the hostname information, it is possible to
force talkd to execute arbitrary commands. As talkd runs
with root privileges, this may allow intruders to remotely
execute arbitrary commands with these privileges.

This attack requires an intruder to be able to make a
network connection to a vulnerable talkd program and provide
corrupt DNS information to that host.

This type of attack is a particular instance of the problem
described in CERT advisory CA-96.04 "Corrupt Information
from Network Servers". This advisory is available from:

ftp://info.cert.org/pub/cert_advisories/

Recent versions of FreeBSD 2.2 -current may not be affected
with this vulnerability due to improved security in
new versions of BIND, which sanity-check the results of
reverse name lookups performed by the DNS system.


III. Impact


Intruders may be able to remotely execute arbitrary commands
with root privileges.

Access to a valid user account on the local system is not
required.


IV. Workaround

Disable the ntalkd program found in /etc/inetd.conf by
commenting the appropriate line out and reconfiguring inetd.

# grep -i ntalk /etc/inetd.conf
ntalk dgram udp wait root /usr/libexec/ntalkd ntalkd

After editing /etc/inetd.conf, reconfigure inetd by sending
it a HUP signal.

# kill -HUP `cat /var/run/inetd.pid`

V. Solution

The patches found at the following URL fix this vulnerability.
Patches are available for FreeBSD 2.1.x (-stable) and -current.

Acknowledgment:

These patches were based off of published work provided by
BSDI, Inc.

After applying these patches, recompile and re-install the
affected utilities.

For FreeBSD -current (2.2 prerelease and 3.0 prerelease)
systems:

Index: announce.c
===================================================================
RCS file: /cvs/freebsd/src/libexec/talkd/announce.c,v
retrieving revision 1.6
diff -u -r1.6 announce.c
--- announce.c 1997/01/14 06:20:58 1.6
+++ announce.c 1997/01/18 08:27:04
@@ -34,7 +34,7 @@
*/

#ifndef lint
-static char sccsid[] = "@(#)announce.c 8.2 (Berkeley) 1/7/94";
+static char sccsid[] = "@(#)announce.c 8.3 (Berkeley) 4/28/95";
#endif /* not lint */

#include
@@ -43,13 +43,17 @@
#include
#include
#include
+
#include
+
#include
-#include
-#include
+#include
#include
+#include
#include
-#include
+#include
+#include
+#include

extern char hostname[];

@@ -78,7 +82,7 @@

#define max(a,b) ( (a) > (b) ? (a) : (b) )
#define N_LINES 5
-#define N_CHARS 120
+#define N_CHARS 256

/*
* Build a block of characters containing the message.
@@ -100,33 +104,37 @@
char line_buf[N_LINES][N_CHARS];
int sizes[N_LINES];
char big_buf[N_LINES*N_CHARS];
- char *bptr, *lptr, *ttymsg();
+ char *bptr, *lptr, *vis_user;
int i, j, max_size;

i = 0;
max_size = 0;
gettimeofday(&clock, &zone);
localclock = localtime( &clock.tv_sec );
- (void)sprintf(line_buf[i], " ");
+ (void)snprintf(line_buf[i], N_CHARS, " ");
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "Message from [email protected]%s at %d:%02d ...",
- hostname, localclock->tm_hour , localclock->tm_min );
+ (void)snprintf(line_buf[i], N_CHARS,
+ "Message from [email protected]%s at %d:%02d ...",
+ hostname, localclock->tm_hour , localclock->tm_min );
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "talk: connection requested by %[email protected]%s",
- request->l_name, remote_machine);
+
+ vis_user = malloc(strlen(request->l_name) * 4 + 1);
+ strvis(vis_user, request->l_name, VIS_CSTYLE);
+ (void)snprintf(line_buf[i], N_CHARS,
+ "talk: connection requested by %[email protected]%s", vis_user, remote_machine);
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "talk: respond with: talk %[email protected]%s",
- request->l_name, remote_machine);
+ (void)snprintf(line_buf[i], N_CHARS, "talk: respond with: talk %[email protected]%s",
+ vis_user, remote_machine);
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], " ");
+ (void)snprintf(line_buf[i], N_CHARS, " ");
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
Index: talkd.c
===================================================================
RCS file: /cvs/freebsd/src/libexec/talkd/talkd.c,v
retrieving revision 1.5
diff -u -r1.5 talkd.c
--- talkd.c 1997/01/14 06:21:01 1.5
+++ talkd.c 1997/01/18 08:26:44
@@ -71,7 +71,7 @@
void timeout();
long lastmsgtime;

-char hostname[MAXHOSTNAMELEN];
+char hostname[MAXHOSTNAMELEN + 1];

#define TIMEOUT 30
#define MAXIDLE 120

For FreeBSD 2.1 based systems:

--- announce.c 1995/05/30 05:46:38 1.3
+++ announce.c 1997/01/18 08:33:55 1.3.4.1
@@ -32,7 +32,7 @@
*/

#ifndef lint
-static char sccsid[] = "@(#)announce.c 8.2 (Berkeley) 1/7/94";
+static char sccsid[] = "@(#)announce.c 8.3 (Berkeley) 4/28/95";
#endif /* not lint */

#include
@@ -41,15 +41,18 @@
#include
#include
#include
+
#include
-#include
+
#include
-#include
-#include
+#include
#include
+#include
#include
-#include
-
+#include
+#include
+#include
+
extern char hostname[];

/*
@@ -77,7 +80,7 @@

#define max(a,b) ( (a) > (b) ? (a) : (b) )
#define N_LINES 5
-#define N_CHARS 120
+#define N_CHARS 256

/*
* Build a block of characters containing the message.
@@ -99,33 +102,37 @@
char line_buf[N_LINES][N_CHARS];
int sizes[N_LINES];
char big_buf[N_LINES*N_CHARS];
- char *bptr, *lptr, *ttymsg();
+ char *bptr, *lptr, *vis_user;
int i, j, max_size;

i = 0;
max_size = 0;
gettimeofday(&clock, &zone);
localclock = localtime( &clock.tv_sec );
- (void)sprintf(line_buf[i], " ");
+ (void)snprintf(line_buf[i], N_CHARS, " ");
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "Message from [email protected]%s at %d:%02d ...",
- hostname, localclock->tm_hour , localclock->tm_min );
+ (void)snprintf(line_buf[i], N_CHARS,
+ "Message from [email protected]%s at %d:%02d ...",
+ hostname, localclock->tm_hour , localclock->tm_min );
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "talk: connection requested by %[email protected]%s",
- request->l_name, remote_machine);
+
+ vis_user = malloc(strlen(request->l_name) * 4 + 1);
+ strvis(vis_user, request->l_name, VIS_CSTYLE);
+ (void)snprintf(line_buf[i], N_CHARS,
+ "talk: connection requested by %[email protected]%s", vis_user, remote_machine);
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], "talk: respond with: talk %[email protected]%s",
- request->l_name, remote_machine);
+ (void)snprintf(line_buf[i], N_CHARS, "talk: respond with: talk %[email protected]%s",
+ vis_user, remote_machine);
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
- (void)sprintf(line_buf[i], " ");
+ (void)snprintf(line_buf[i], N_CHARS, " ");
sizes[i] = strlen(line_buf[i]);
max_size = max(max_size, sizes[i]);
i++;
Index: talkd.c
===================================================================
RCS file: /home/ncvs/src/libexec/talkd/talkd.c,v
retrieving revision 1.3
retrieving revision 1.3.4.1
diff -u -r1.3 -r1.3.4.1
--- talkd.c 1995/05/30 05:46:44 1.3
+++ talkd.c 1997/01/18 08:33:56 1.3.4.1
@@ -69,7 +69,7 @@
void timeout();
long lastmsgtime;

-char hostname[MAXHOSTNAMELEN];
+char hostname[MAXHOSTNAMELEN + 1];

#define TIMEOUT 30
#define MAXIDLE 120


=====================================================================
FreeBSD, Inc.

Web Site: http://www.freebsd.org/
Confidential contacts: [email protected]
PGP Key: ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications: [email protected]
Security public discussion: [email protected]

Notice: Any patches in this document may not apply cleanly due to
modifications caused by digital signature or mailer software.
Please reference the URL listed at the top of this document
for original copies of all patches if necessary.
=====================================================================

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