Challenge-response spam technology intercepts incoming emails and sends a challenge to the sender, such as a request...
By submitting your email address, you agree to receive emails regarding relevant topic offers from TechTarget and its partners. You can withdraw your consent at any time. Contact TechTarget at 275 Grove Street, Newton, MA.
to click on a link or answer a simple question. If the sender correctly responds to the challenge message, then the original email is forwarded on; otherwise it is discarded. The process authenticates the sender by confirming that he or she sent the message.
Requiring human verification may seem like quite an effective approach to stopping spam, but there are various drawbacks that you need to consider. The challenge-response process effectively shifts the work of filtering email from the recipient to the sender, legitimate or otherwise. Publishers, however, may not complete the challenge-response when they have thousands of messages to send. Many people don't reply to challenge emails either because they don't know what they are, don't trust them or simply refuse. There are, therefore, many false positives. Other less knowledgeable users often complete the response even when they didn't actually send the original email, producing a false negative.
There is another problem, too. What if the challenge message doesn't reach the sender? The challenge itself could be blocked by the sender's antispam filter! If antispam filters are set to always allow challenges through, then spammers will create spam that looks like a challenge message. Also if the challenges become too predictable, then the spammers will be able to develop computer programs that spot the challenges and auto-send the required responses.
My personal preference is to deploy antispam tools that use a variety of filtering techniques. Bayesian filtering techniques, for example, compare the contents of incoming messages to those of previous legitimate mail. Heuristic filters look for patterns in the content of an email and match them against a database of known spam characteristics. Whitelists and blacklists are time-consuming to keep up to date, but they do provide flexibility so that users can decide which messages should be treated as spam.
Thankfully, server authentication initiatives can control spam at the mail server. Two of the most popular tools are the Sender Policy Framework (SPF) and Sender ID. These authentication mechanisms can verify whether a mail server is authorized to send on behalf of a given domain. Records are published in the domain name system (DNS), which lists the authorized email servers for a domain. If we are ever going to beat spam, it is this type of control that will help in its defeat -- not challenge and response.
Dig Deeper on Email Security Guidelines, Encryption and Appliances
Related Q&A from Michael Cobb
Amazon disabled native encryption capabilities in the latest Fire OS version. Expert Michael Cobb explains what this means for security, and if ...continue reading
A pirated app called Happy Daily English beat Apple's App Store security review. Expert Michael Cobb explains how it works and what security teams ...continue reading
The Lenovo SHAREit file-sharing app has a hardcoded password vulnerability, among other issues. Expert Michael Cobb explains these flaws and how to ...continue reading
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.