bugtraq - bss-based buffer overflow in l2tpd

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Date: Fri, 4 Jun 2004 18:53:52 +0000
From: Thomas Walpuski <thomas-bugtraq@...roved.org>
To: bugtraq@...urityfocus.com, full-disclosure@...ts.netsys.com
Subject: bss-based buffer overflow in l2tpd

All versions of l2tpd contain a bss-based buffer overflow. After
circumventing some minor obstacles (i.e., faking a L2TP tunnel
establishment) the overflow can be triggered by sending a specially
crafted packet.

The crucial code can be found in write_packet() in control.c:

    static unsigned char wbuf[MAX_RECV_SIZE];
    int pos = 0;
    [..]
    e = PPP_FLAG;
    wbuf[pos++] = e;
    for (x = 0; x < buf->len; x++)
    {
        e = *((char *) buf->start + x);
        if ((e < 0x20) || (e == PPP_ESCAPE) || (e == PPP_FLAG))
        {
            /* Escape this */
            e = e ^ 0x20;
            wbuf[pos++] = PPP_ESCAPE;
        }
        wbuf[pos++] = e;

    }
    wbuf[pos++] = PPP_FLAG;

Nota bene: buf->len can be upto 4080 = 4096 (=: MAX_RECV_SIZE) - 16.

It might be hard or even impossible to exploit this buffer overflow.

Thomas Walpuski

_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.netsys.com/full-disclosure-charter.html