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Vulnerability Note VU#498440
Multiple TCP/IP implementations may use statistically predictable initial sequence numbers
OverviewAttacks against TCP initial sequence number generation have been discussed for some time now. It has long been recognized that the ability to know or predict ISNs can lead to TCP connection hijacking or spoofing. What was not previously illustrated was just how predictable one commonly-used method of randomizing new connection ISNs is in some modern TCP/IP implementations.
I. DescriptionThe 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.
II. ImpactIf 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.
III. SolutionDeploy IPsec;
Implement the suggestions in RFC1948, namely the segmentation of the ISN space on a per-host, per-connection basis using cryptographic hashed secrets.
Systems Affected
References
CA-1995-01
http://www.cert.org/advisories/CA-1995-01.html
ftp://research.att.com/dist/internet_security/117.ps.Z (Morris)
ftp://research.att.com/dist/internet_security/ipext.ps.Z (Bellovin)
ftp://ftp.isi.edu/in-notes/rfc1948.txt
Related URLs:
ftp://ftp.isi.edu/in-notes/rfc793.txt
ftp://ftp.isi.edu/in-notes/rfc1323.txt (obsoletes RFC1185)
ftp://ftp.isi.edu/in-notes/rfc1750.txt
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-1999-0077
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2000-0328
http://xforce.iss.net/static/139.php
http://www.usenix.com/publications/library/proceedings/security95/full_papers/joncheray.txt
http://www.guardent.com/pr2001-03-12-ips.html
Papers:
TCP session spoofing: "A Weakness in the 4.2BSD UNIX TCP/IP Software",
Morris, R., ComputingScience Technical Report No 117, ATT Bell
Laboratories, Murray Hill,New Jersey, 1985.
TCP session hijacking: "Security Problems in the TCP/IP Protocol
Suite", Bellovin, S., Computer Communications Review, April 1989.
TCP session desyncronization: “Simple Active Attack Against
TCP”, Joncheray, L., Proceedings, 5th USENIX UNIX Security
Symposium, June 1995.
Credit
The CERT/CC thanks the following individuals for their contributions to this advisory
Steve Bellovin, AT&T Labs
Tim Newsham, Guardent, Inc.
??? BindView
Niels Provohs (?)
This document was written by Jeffrey S. Havrilla
Other Information
| Date Public: | 2001-03-12 |
| Date First Published: | 2001-03-13 |
| Date Last Updated: | 2007-08-02 |
| CERT Advisory: | CA-2001-09 |
| CVE-ID(s): | CVE-2001-0328 |
| NVD-ID(s): | CVE-2001-0328 |
| US-CERT Technical Alerts: | |
| Metric: | 15.19 |
| Document Revision: | 67 |
If you have feedback, comments, or additional information about this vulnerability, please send us
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