This is a read-only archive. Find the latest Linux articles, documentation, and answers at the new!

Feature: Security

Linux Advisory Watch - January 6, 2006

By Benjamin D. Thomas on January 06, 2006 (8:00:00 AM)

Share    Print    Comments   

This week, advisories were released for tkdiff, scponly, XnView, pineentry, KPdf, libgphoto, printer-filters-utils, nss_ldap, mdkonline, tkcvs, and ethereal. The distributors include Debian, Gentoo, and Mandriva.
IPv6 approach for TCP SYN Flood attack over VoIP, Part III
By: Suhas Desai

5. The SYN Flooding Attack

When a normal TCP connection starts, a destination host receives a SYN (synchronize/start) packet from a source host and sends back a SYN ACK (synchronize acknowledge). The destination host must then hear an ACK (acknowledge) of the SYN ACK before the connection is established. This is referred to as the "TCP three-way handshake."

While waiting for the ACK to the SYN ACK, a connection queue of finite size on the destination host keeps track of connections waiting to be completed. This queue typically empties quickly since the ACK is expected to arrive a few milliseconds after the SYN ACK.

The TCP SYN flood attack exploits this design by having an attacking source host generate TCP SYN packets with random source addresses toward a victim host. The victim destination host sends a SYN ACK back to the random source address and adds an entry to the connection queue. Since the SYN ACK is destined for an incorrect or nonexistent host, the last part of the "three-way handshake" is never completed and the entry remains in the connection queue until a timer expires, typically for about one minute.

By generating phony TCP SYN packets from random IP addresses at a rapid rate, it is possible to fill up the connection queue and deny TCP services such as e-mail, file Transfer or WWW to legitimate users.

There is no easy way to trace the originator of the attack because the IP address of the source is forged.

5.1 TCP SYN flood

A TCP SYN flood is an attack based on bogus TCP connection requests, created with a spoofed source IP address, sent to the attacked system. Connections are not completed, thus soon it will fill up the connection request table of the attacked system, preventing it from accepting any further valid connection request.

The source host for the attack sends a SYN packet to the target host. The target hosts replies with a SYN/ACK back to the legitimate user of the forged IP source address.

Since the spoofed source IP address is unreachable, the attacked system will never receive the corresponding ACK packets in return, and the connection request table on the

Attacked system will soon be filled up.The attack works if the spoofed source IP address is not reachable by the attacked system. If the spoofed source IP address where reachable by the attacked system, then the legitimate owner of the source IP address would respond with a RST packet back to the target host, closing the connection and defeating the attack.

TCP SYN flood is a denial of service attack that sends a host more TCP SYN packets than the protocol implementation can handle.

This is a resource starvation DoS attack because once the connection table is full; the server is unable to service legitimate requests.

5.2 TCP SYN flood protection

5.2.1 Apply Operating System fixes

Systems periodically check incomplete connection requests, and randomly clear connections that have not completed a three-way handshake. This will reduce the likelihood of a complete block due to a successful SYN attack, and allow legitimate client connections to proceed.

  • Configure TCP SYN traffic rate limiting
  • Install IDS (Intrusion Detection Systems) capable of detecting TCP SYN flood attacks
5.2.2 Filter network traffic

Use circuit level firewalls (stateful inspection) to monitor the handshake of each new connection and maintain the state of established TCP connections. The filtering system must be able to distinguish harmful uses of a network service from legitimate uses.

Static packet filtering (stateless) does not protect from TCP SYN flood attacks.

Read Article:

  Debian: New tkdiff packages fix insecure temporary file creation
  29th, December, 2005

Updated package.

  Gentoo: scponly Multiple privilege escalation issues
  29th, December, 2005

Local users can exploit an scponly flaw to gain root privileges, and scponly restricted users can use another vulnerability to evade shell restrictions.

  Gentoo: XnView Privilege escalation
  30th, December, 2005

XnView may search for shared libraries in an untrusted location, potentially allowing local users to execute arbitrary code with the privileges of another user.

  Gentoo: pinentry Local privilege escalation
  3rd, January, 2006

pinentry is vulnerable to privilege escalation.

  Gentoo: KPdf, KWord Multiple overflows in included Xpdf code
  4th, January, 2006

KPdf and KWord both include vulnerable Xpdf code to handle PDF files, making them vulnerable to the execution of arbitrary code.

  Mandriva: Updated libgphoto packages fixes issue with some cameras
  29th, December, 2005

The hotplug usermap has been restored for this package because it is used by HAL to correctly detect digital cameras which are not using USB Mass storage (for instance, all Canon digital cameras, as well as some Nikon ones and all PTP cameras). This should allow gnome-volume-manager to automatically popup a "Do you want to import photos?" dialog when the camera is plugged in.

  Mandriva: Updated printer-filters-utils packages fix local vulnerability
  30th, December, 2005

"newbug" discovered a local root vulnerability in the mtink binary, which has a buffer overflow in its handling of the HOME environment variable, allowing the possibility for a local user to gain root privileges.

  Mandriva: Updated nss_ldap packages fixes bug with groups with large number
  2nd, January, 2006

A bug was discovered in nss_ldap when a group has a large number of members. This was resulting in SIGABRT of 'ls -la' when attempting to add additional users to the group. Updated packages have been patched to correct the issue.

  Mandriva: Updated mdkonline packages fixes various bugs
  2nd, January, 2006

A bug in the mdkupdate cron job (when automatic update mode is enabled) prevents $ENV{USER} from being defined, which kills mdkupdate before performing the updates.

  Mandriva: Updated tkcvs packages fix insecure temporary file vulnerability
  4th, January, 2006

Javier Fernandez-Sanguino Pena discovered that tkdiff created temporary files in an insecure manner. The updated packages have been patched to correct these problems.

  Mandriva: Updated ethereal packages fix vulnerabilities
  4th, January, 2006

Three vulnerabilities were discovered in Ethereal 0.10.13: The IRC and GTP dissectors could go into an infinite loop. A buffer overflow was discovered by iDefense in the OSPF dissector. Ethereal has been upgraded to 0.10.14 which does not suffer from these problems.


Share    Print    Comments   


on Linux Advisory Watch - January 6, 2006

Note: Comments are owned by the poster. We are not responsible for their content.

What about SYN cookies?

Posted by: Anonymous Coward on January 06, 2006 11:00 PM
Linux has solution on SYN flooding called SYN cookies (<a href="" title=""></a>) for some time now although not enabled by default.

To enable SYN cookies (provided that already activated on kernel compilation) simply apply this command at boot time after<nobr> <wbr></nobr>/proc has been mounted :

echo 1 >/proc/sys/net/ipv4/tcp_syncookies


This story has been archived. Comments can no longer be posted.

Tableless layout Validate XHTML 1.0 Strict Validate CSS Powered by Xaraya