Software Engineering Institute

The CERT/CC has received a report from Guardent, Inc. concerning an observed statistical weakness in initial sequence number (ISN) generation for TCP connections. Guardent asserts in copyrighted research forwarded to us that incrementing the ISN by some series of pseudo-random amounts is insufficient to protect some TCP implementations from a practical ISN guessing attack in some real-world situations. Such attacks would not rely on data collected (sniffed) from a a victim site. These observations and statistical analyses provide empirical data which draw attention to the protocol analysis documented by Steve Bellovin (building on work pioneered by Robert Morris), culminating in RFC1948.

In RFC1948, Steve noted:

   The initial sequence numbers are intended to be more or less random.
   More precisely, RFC 793 specifies that the 32-bit counter be
   incremented by 1 in the low-order position about every 4
   microseconds.  Instead, Berkeley-derived kernels increment it by a
   constant every second, and by another constant for each new
   connection.  Thus, if you open a connection to a machine, you know to
   a very high degree of confidence what sequence number it will use for
   its next connection.  And therein lies the attack.

Some TCP/IP implementors chose instead to increment the ISNs using constrained random variables instead of constants. Guardent's research demonstrates that the algorithm implemented in some of these TCP/IP stacks is statistically weak and susceptible to attack.

We are currently soliciting feedback from the vendor community to help us understand the scope of this observed statistical weakness. Guardent's work has drawn attention to the fact that not all current TCP/IP stack implementations have implemented RFC1948 or provided an equivalent fix.

As of 2015, predictable TCP ISN generation is still somewhat common, particularly in low-power/low-bandwidth, embedded, and IoT devices that use older operating systems and networking code.

If the ISN of an existing connection can be determined within some practical range, a malicious agent may be able to close or hijack the connection. If the ISNs of future connections are targeted, an agent may be able to "complete" a TCP three-way handshake and spoof TCP packets delivered to a victim.

Deploy IPsec.

Implement the suggestions in RFC1948, namely the segmentation of the ISN space on a per-host, per-connection basis using cryptographic hashed secrets.