Title: Linux Sendmail Denial of Service Vulnerability
Severity: MODERATE
Description:
The 2.0.x kernels have a quirk in the TCP implementation that have to do with the accept() call returning after only a syn has been recieved (as opposed to the three way handshake having been completed). Sendmail, which is compiled on many unices, makes the assumption that the three way handshake has been completed and a tcp connection has been fully established. This trust in a standard tcp implementation is seen in the following section of code <src/daemon.c>:
t = accept(DaemonSocket,
(struct sockaddr *)&RealHostAddr, &lotherend);
if (t >= 0 || errno != EINTR)
break;
}
savederrno = errno;
(void) blocksignal(SIGALRM);
if (t < 0)
{ errno = savederrno;
syserr("getrequests: accept");
/* arrange to re-open the socket next time around */
(void) close(DaemonSocket);
DaemonSocket = -1;
refusingconnections = TRUE;
sleep(5);
continue;
}
It's possible to cause a denial of service here if a RST is sent after the initial SYN to the sendmail smtpd on port 25. If that were to be done, the sendmail smtpd would be caught in a loop (above) accepting, testing the socket [yes, the one which accept returned on listening on port 25], sleeping, and closing the socket for as long as the syns and following rsts are sent. It is also completely possible to do this with spoofed packets.
Affected Products:
- Linux kernel 2.0.0
Juniper Networks provides this content via a wide variety of sources and production methods. If notified of errors or omissions in the content of this page, Juniper Networks, at its discretion, will modify or remove the page or leave the content as is, depending on various factors including but not limited to the reputation and authority of the party providing the notification. Please use the contact information displayed elsewhere on this page to report any errors or omissions regarding the content on this page.
