Public Vulnerability Reports

TCPDUMP Denial of Service Vulnerability in ISAKMP Packet Parsing



TCPDUMP is a widely used network debugging tool that prints out the headers of packets on a network interface that match a boolean expression. More information about the application is available at . 


A vulnerability exists in the parsing of ISAKMP packets (UDP port 500) that allows an attacker to force TCPDUMP into an infinite loop upon receipt of a specially crafted packet.

The following output is from TCPDUMP replaying a previously captured malformed ISAKMP packet:

# tcpdump -vvvr tcpdump_isakmp_inf_loop | head 05:14:57.954719 > isakmp 8.9 msgid 7d380dee
cookie 773b4e8a1618caa8->51efacc0a65e0334: phase 2/others ? #69[C]:

The string "(#237)" will continue to print indefinitely; at this point TCPDUMP is no longer processing other packets. The vulnerable segment of code has been narrowed down to the following loop from print_isakmp.c:isakmp_sub_print(), the while() loop is never broken as the variable 'np' never equates to zero:

while (np) {
    safememcpy(&e, ext, sizeof(e));

    if (ep < (u_char *)ext + ntohs(e.len)) {
        printf(" [|%s]", NPSTR(np));
        cp = ep + 1;
    for (i = 0; i < depth; i++)
        printf("    ");
    cp = isakmp_sub0_print(np, ext, ep, phase, doi, proto);

    np =;
    ext = (struct isakmp_gen *)cp;


Any remote user can generate a specially crafted packet that will cause TCPDUMP to enter an infinite loop thereby rendering the application useless and preventing the administrator from viewing network traffic. Attackers can anonymously trigger this vulnerability by spoofing the source address of the malicious packet, this is possible because it traverses over the User Datagram Protocol (UDP), a stateless protocol.
While the vulnerability exists specifically in the TCPDUMP code base there are some applications that utilize code from TCPDUMP or "wrap" around TCPDUMP, and such applications would also be affected.


iDEFENSE has confirmed the existence of this vulnerability in TCPDUMP versions 3.6, 3.6.3, 3.7.1 built against LIBPCAP versions .6 and .7 on both the Linux and FreeBSD platform.  Many other Linux distributions contain vulnerable TCPDUMP versions


An affected application is rendered useless upon entry into the infinite loop. The application must be restarted to regain normal functionality. 


This vulnerability was already closed by Guy Harris during routine development; users of the CVS version downloaded since September 6, 2002 (revision 1.34 of print-isakmp.c) are not vulerable.  The new 3.7.2 tcpdump release includes this and a couple of additional security fixes; the 0.7.2 libpcap release includes new functionality but no
security fixes.

The following packages are available:

Debian 2.2 (potato) contains tcpdump 3.4a6, which does not appear to be vulnerable (version 3.4a6 does not include an isakmp dissector). Debian 3.0 (woody) contains tcpdump 3.6.2, which is vulnerable.  Updated packages are available from .


The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has assigned the identification number CAN-2003-0108 to this issue.


12/09/2002 Issue disclosed to iDEFENSE
02/25/2003 TCPDUMP maintainers notified:,
02/25/2003 Responses from Guy Harris, Bill Fenner, Michael Richardson
02/25/2003 iDEFENSE clients notified
02/26/2003 OS vendors notified via
02/27/2003 Public Disclosure


Andrew Griffiths ( is credited with discovering this vulnerability.