LinuxSecurity.com
Share your story
The central voice for Linux and Open Source security news
Home News Topics Advisories HOWTOs Features Newsletters About Register

Welcome!
Sign up!
EnGarde Community
Login
Polls
What is the most important Linux security technology?
 
Advisories
Community
Linux Events
Linux User Groups
Link to Us
Security Center
Book Reviews
Security Dictionary
Security Tips
SELinux
White Papers
Featured Blogs
All About Linux
DanWalsh LiveJournal
Securitydistro
Latest Newsletters
Linux Advisory Watch: August 15th, 2014
Linux Advisory Watch: August 8th, 2014
Subscribe
LinuxSecurity Newsletters
E-mail:
Choose Lists:
About our Newsletters
RSS Feeds
Get the LinuxSecurity news you want faster with RSS
Powered By

  
NetBSD-SA1999-001 select(2)/accept(2) race condition in TCP servers Print E-mail
User Rating:      How can I rate this item?
Posted by LinuxSecurity.com Team   
NetBSD select(2)/accept(2) race condition in TCP servers
-----BEGIN PGP SIGNED MESSAGE-----


                 NetBSD Security Advisory 1999-001
                 ---------------------------------

Topic:          select(2)/accept(2) race condition in TCP servers
Version:        All current versions of NetBSD
Severity:       Problem may allow denial of service.


Abstract
========

A problem has been identified which allows remote attackers to wedge
many TCP services running on 4.4BSD-derived systems, including X
servers and all services run from inetd.  Other (non-BSD) systems are
believed to be affected as well.


Technical Details
=================

Many TCP servers open a TCP socket in the default blocking mode, use
select(2) to wait for connections, and then accept(2) connections in
blocking mode.  Under some circumstances, the accept(2) may hang
waiting for another connection, denying service to clients trying to
connect to other ports.

The scenario which causes this is:

* Connection is initiated by client; 3WHS completes.
* Server process is awakened and select(2) succeeds.
* Connection is closed by client (e.g. by sending a RST).  Connection
  is removed from accept(2) queue on server.
* Server process does an accept(2), which hangs waiting for a
  connection.

This scenario is sometimes difficult to reproduce, particularly if the
server is very fast and the network is relatively slow.  It is most
effective if the server is slow and/or must do a lot of work between
the select(2) and accept(2).


Solutions and Workarounds
=========================

Two solutions are possible:

1) Modify all TCP servers to use non-blocking listening sockets.
   Unfortunately, this requires changing a large amount of code, much
   of it maintained by third parties.

2) Modify the kernel to not remove sockets from the accept(2) queue
   when they are closed.  A change that implements this has been added
   to NetBSD-current, and is available at:
       ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/19990120-accept


Thanks To
=========

Thanks go to Fyodor for providing nmap, with which this vulnerability
was discovered.  See http://www.insecure.org/nmap/ for more information.
Thanks to Charles M. Hannum  for providing the solution.


More Information
================

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.


Copyright 1999, The NetBSD Foundation.  All Rights Reserved.


-----BEGIN PGP SIGNATURE-----
Version: 2.6.3i
Charset: noconv

iQCVAwUBNqWj/D5Ru2/4N2IFAQF3AwP7B/sbL1Ar8NCP/vLIaeYq698bSR2SIQRC
21yFSNY7h0qGxpsEtJ0132wIHVYp4Ho3Pbd1308ZOykx22zfZr11IlkgInW8kFKf
7K2yQOc47RAKxyaAZvgR/oqUCQE+FiZ4DYv4WDjkbUluYpcxnHmbhO/tIqbYHJqE
ue/dnlXwvcA=
=GHyB
-----END PGP SIGNATURE-----
 
< Prev   Next >
    
Partner

 

Latest Features
Peter Smith Releases Linux Network Security Online
Securing a Linux Web Server
Password guessing with Medusa 2.0
Password guessing as an attack vector
Squid and Digest Authentication
Squid and Basic Authentication
Demystifying the Chinese Hacking Industry: Earning 6 Million a Night
Free Online security course (LearnSIA) - A Call for Help
What You Need to Know About Linux Rootkits
Review: A Practical Guide to Fedora and Red Hat Enterprise Linux - Fifth Edition
Yesterday's Edition
Moving toward smart and secure continuous software delivery
Stealthy, Razor Thin ATM Insert Skimmers
Partner Sponsor

Community | HOWTOs | Blogs | Features | Book Reviews | Networking
 Security Projects |  Latest News |  Newsletters |  SELinux |  Privacy |  Home
 Hardening |   About Us |   Advertise |   Legal Notice |   RSS |   Guardian Digital
(c)Copyright 2014 Guardian Digital, Inc. All rights reserved.