The Answer to Blowback

By Larry Seltzer  |  Posted 2006-07-27 Print this article Print

Of course, all the general efforts at SMTP authentication, like DKIM (DomainKeys Identified Mail), would solve this problem along the way to solving even more serious ones, but theyre too revolutionary. We might all agree theyre good things and worth supporting, but they only achieve usefulness when they have been widely adopted.

Thats why BATV (Bounce Address Tag Validation) was invented. BATV allows the targets of blowback attacks to protect themselves with minimal effort and without anyone elses cooperation. Its effective as a unilateral measure. IronPort Systems is announcing this week that BATV will be included for no charge in the regular software updates to its own devices.

Heres how BATV works: The MAIL-FROM address in the messages envelope, which is the opening portion of the SMTP transaction, is digitally signed with a private key known only to the mail server. The normal format for the envelope is:
With BATV, this becomes (for example, in IronPorts implementation):
The "EA6D0F" part is the hex representation of the first three bytes of the SHA-1 HMAC—in other words, a hash—of the other parts of the address and a private key. The "415" is a portion of a time stamp.

The IronPort implementation differs slightly from the current BATV specification. The spec is continually evolving, and if administrators are careful it shouldnt matter if more than one version has to be supported at once.

If a bounce comes in to the system to the same address, the server can look up and confirm that the tag is valid and then deliver the bounce message. If it just comes in to, or to some BATV value that doesnt correspond to a valid one, the server can ditch it at the SMTP envelope conversation. This aborts the transaction at an early stage, before the large majority of the message transfer has been performed. So it doesnt really stop the attack, but it mitigates it to the degree that it is much more manageable.

Its not hard to imagine some problems caused by all the messing with addresses. For instance, I heard one report that the variability in the address from date stamp changes can cause challenge/response systems to complain, but thats hardly the first problem with challenge/response. Also its also somewhat disappointing that the perpetrator of the attack doesnt learn from the process that the attack failed.

BATV can piggyback on top of SMTP authentication efforts. One day in the hypothetical future when DMIM or some such standard is ubiquitous, BATV can be turned off. But, in the meantime, its an unusually low-impact way to eliminate some real threats.

Many thanks to John Levine for help with this and other articles Ive written. John is principal author of "The Internet for Dummies" and co-author, with the famous Dave Crocker and others, of the BATV specification.

Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983. Check out eWEEK.coms for the latest security news, reviews and analysis. And for insights on security coverage around the Web, take a look at Security Center Editor Larry Seltzers Weblog. More from Larry Seltzer

Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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