Subject: NetBSD Security Advisory 1999-001: select(2)/accept(2) race condition in TCP servers
To: None <>
From: Luke Mewburn <>
List: tech-security
Date: 01/20/1999 20:53:52
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From: Luke Mewburn <>
Subject: NetBSD Security Advisory 1999-001: select(2)/accept(2) race condition in TCP servers
Date: Wed, 20 Jan 1999 20:53:52 +1100


                 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.


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

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:

Thanks To

Thanks go to Fyodor for providing nmap, with which this vulnerability
was discovered.  See 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.

Version: 2.6.3i
Charset: noconv


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