Block E-Mail Bounces with BATV - ' The Answer to Blowback ' (
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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):
MAIL FROM pvrs=sender=415EA6D0F@foo.com
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 sender@foo.com, 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.
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