Network Associates, Inc. security advisor: Linux Blind TCP Spoofing
Mar 20, 1999, 17:05 (2 Talkback[s])
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Network Associates, Inc.
March 9, 1999
Linux Blind TCP Spoofing
An implementation flaw in the Linux TCP/IP stack allows remote attackers
to forge TCP connections without predicting sequence numbers and pass
data to the application layer before a connection is established.
This problem is present in Linux kernels up to and including 2.0.35.
Any distribution containing a kernel revision less than this is
TCP is a reliable connection-oriented protocol which requires the
completion of a three way handshake to establish a connection. To
implement reliable and unduplicated delivery of data, the TCP
protocol uses a sequence based acknowledgment system. During connection
establishment each host selects an initial sequence number which is
sent in the first packet of the connection. Each subsequent byte
transmitted in the TCP connection is assigned a sequence number.
To prevent duplicate or invalid segments from impacting established
connections TCP utilizes a state based model. In a typical
client-server application, the client initiates a connection by
transmitting a TCP segment to a listening server process. This
causes the state of the process to move from the LISTEN state into
SYN_RECEIVE if a SYN flag is present. During this state the server
acknowledges the clients request setting both the SYN and ACK
flags. To complete the three way handshake the client acknowledges
the servers response, moving the server from SYN_RECEIVE to
To establish a forged TCP session an attacker must have knowledge
of or be able to predict the initial sequence number that is selected
by the server. An implementation flaw in the Linux kernel allows
data to be delivered to the application layer before the handshake
The combination of three flaws in the Linux TCP/IP implementation
contribute to the existence of a security vulnerability. Firstly,
Linux only verifies the acknowledgment number of incoming segments
if the ACK flag has been set. Linux also queues data from TCP
segments without acknowledgment information prior to the
completion of the three way handshake but after the initial SYN
has been acknowledged by the server. Finally, Linux passes data to
the application layer upon the receipt of a packet containing the
FIN flag regardless of whether a connection has been established.
Together, these flaws allow an attacker to spoof an arbitrary
connection and deliver data to an application without the need to
predict the servers initial sequence number.
According to the standard, there is only one case wherein a correct
TCP/IP stack can accept data in a packet that does not have the ACK
flag set --- the initial connection-soliciting SYN packet can
contain data, but must not have the ACK flag set. In any other case,
a data packet not bearing the ACK flag should be discarded.
When a TCP segment carries an ACK flag, it must have a correct
acknowledgement sequence number (which is the sequence number of the
next byte of data expected from the other side of the connection).
TCP packets bearing the ACK flag are verified to ensure that their
acknowledgement numbers are correct.
Vulnerable Linux kernels accept data segments that do not have the
ACK flag set. Because the ACK flag is not set, the acknowledgement
sequence number is not verified. This allows an attacker to send
data over a spoofed connection without knowing the target's current
(or initial) sequence number.
Linux does not deliver data received from a TCP connection when the
connection is in SYN_RECEIVE state. Thus, an attacker cannot
successfully spoof a TCP transaction to a Linux host without somehow
completing the TCP handshake. However, an implementation flaw in
some Linux kernels allows an attacker to bypass the TCP handshake
entirely, by "prematurely" closing it with a FIN packet.
When a FIN packet is received for a connection in SYN_RECEIVE state,
Linux behaves as if the connection was in ESTABLISHED state and moves
the connection to CLOSE_WAIT state. In the process of doing this,
data queued on the connection will be delivered to listening
applications. If the ACK flag is not set on the FIN segment, the
target's sequence number is not verified in the segment.
It is recommended that kernels below version 2.0.36 be upgraded to
eliminate this vulnerability.
Updated kernel packages for Red Hat Linux which are not vulnerable to
problem are available from
Both Debian and Caldera Linux have been contacted regarding this
vulnerability although no official response has been received.
The latest stable versions of the Linux kernel are available from
Analysis and documentation of this problem was conducted by Anthony
Osborne with the Security Labs at Network Associates. This
vulnerability was discovered on the October 5, 1998.
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As posted to BUGTRAQ by Security Research Labs on Mar 9th,
1999. -lt ed