49:. It was originally designed in 1997 by Stan Weatherby, and was called Email Verification. In this reply, the purported sender is asked to perform some action to assure delivery of the original message, which would otherwise not be delivered. The action to perform typically takes relatively little effort to do once, but great effort to perform in large numbers. This effectively filters out spammers. Challenge–response systems only need to send challenges to unknown senders. Senders that have previously performed the challenging action, or who have previously been sent e-mail(s) to, would be automatically receive a challenge.
326:
solicited bulk email, as such senders do not usually check for challenges to their mail. However, if the bulk email in question was solicited, then the C/R user could be expected to have added it to the whitelist. If the bulk email was not solicited, then by definition it is spam, and is filtered by the C/R system.
282:
Disseminating an ordinary email address that is protected by a C/R system results in challenges to those who send mail to that address. Some C/R critics consider it rude to give people your email address, then require them (unless previously whitelisted, which might not always be possible) to answer
273:
In some cases, C/R systems can be tricked into becoming spam relays. To be useful, some part of the message under challenge is generally included in the challenge message. A spammer, knowing that they're sending to a C/R using system, could design their message so that their "spam payload" is in the
295:
Some C/R systems interact badly with mailing list software. If a person subscribed to a mailing list begins to use C/R software, posters to the mailing list may be confronted by challenge messages. Order confirmations, billing statements and delivery notices from online shopping systems are usually
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Advocates of C/R systems argue that the benefits by far outweigh the 'burden' of an incidental challenge, and that there will probably never be a final solution against spam without laying some kind of burden on the e-mail sender. They reason that the more widespread the use of C/R systems is, the
325:
Critics argue that typical users of C/R systems still need to review their challenged mail regularly, looking for non-bulk mail or solicited bulk email for which the sender has not responded to the challenge. This issue is particularly notable with newsletters, transactional messages, and other
257:
Though definitely an undesirable side-effect, this issue would be non-existent if people, whose email address was used as a forged address in spam, happen to run a C/R system themselves. In this case, one of the C/R users must implement some form of return address signing (such as
223:
Critics of C/R systems have raised several issues regarding their legitimacy and usefulness as an email defense. A number of these issues relate to all programs which auto-respond to e-mail, including mailing list managers, vacation programs and bounce messages from mail servers.
98:
A challenge requiring reading natural language instructions on how to reply, with the inclusion of a special string or pass-code in the reply. For example, converting a date string (such as 'Thu Jan 12 08:45:44 2012') into its corresponding timestamp (1326379544). Other
118:
Nowadays C/R systems are not used widely enough to make spammers bother to (automatically) respond to challenges. Therefore, C/R systems generally just rely on a simple challenge that would be made more complicated if spammers ever build such automated responders.
57:
C/R systems attempt to provide challenges that can be fulfilled easily for legitimate senders and non-easily for spammers. Two characteristics that differ between legitimate senders and spammers are exploited to achieve this goal:
374:, as well as the domain's reputation, to determine whether to accept, reject, challenge on reputation, or present the user with a set of whitelist/blacklist options. As of 2010, the project is listed as "retired" technology.
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more understood, accepted and appreciated they are. In an analogy with snail mail, the sender is prepared to pay for the stamp, in an analogy with phone calls, the caller is prepared to pay for the outgoing call.
236:
field) in the e-mail header, but can also use a forged, existing sender address (a valid, but an arbitrary person's address without this person's consent). The latter would become increasingly common if e.g.
241:
would become more popular to detect spam. C/R systems challenging a message with a forged sender address would send their challenge as a new message to the person whose address was forged. This would create
340:
Channel email – Just wants a reply, doesn't actually try to determine if the user is human (thus getting rid of the spammers that don't use legitimate emails and doesn't require costly processing).
246:, which would effectively shift the burden from the person who would have received the spam to the person whose address was forged and which may be treated the same as any other
274:
part of the message that the challenge message includes. In this case, the forged sender is the actual recipient of the spam, and the C/R system unwittingly is the relay.
299:
Advocates of C/R systems argue that, though it takes extra effort, solutions for these problems exist if the end-user behind the C/R system does these simple things:
254:. In addition, if the forged sender decided to validate the challenge, the C/R user would receive the spam anyway and the forged sender address would be whitelisted.
166:
mail that is sent within a pre–set length of time, to allow correspondence related to an online order, but which then expires to disallow future marketing e-mail.
95:, which can be loaded in an appropriate web browsing tool to respond to the challenge, so simply clicking on the link is sufficient to respond to the challenge.
307:
for many email groups, the new member won't know the group's address until after receipt of the "welcome" email, thus making this recommendation unworkable.
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sent via automated systems. Email challenges sent to such systems can be lost, and legitimate mail sent by these systems may not reach the C/R system user.
69:, while spammers usually forge a return address. This means that most spammers won't get the challenge, making them automatically fail any required action.
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Use 'tagged email addresses' for mailing lists or automated mailers like the above, that can be recognized and cleared automatically by the C/R system.
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C/R advocates claim that such systems have a lower rate of false positives than other systems for automatically filtering unsolicited bulk email.
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Manually inspect the message queue and overriding the C/R process in case where the C/R system holds an expected message from an automated mailer.
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80:
and have to perform challenging actions in large numbers, while legitimate senders have to perform it at most once for every new e-mail contact.
31:
517:
761:
736:
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114:" test in which the sender is required to view an image containing a word or phrase and respond with that word or phrase in text.
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335:
259:
183:
154:
716:
598:
502:
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header or the body of the message—any of which lets messages be accepted without being challenged. For example, the
756:
172:
Problems with sending challenges to forged email addresses can be reduced if the challenges are only sent when:
731:
628:
593:
45:) system is a type of that automatically sends a reply with a challenge to the (alleged) sender of an incoming
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207:
107:
65:
566:
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became common, forged sender addresses would be recognized by these systems before reaching a C/R system.
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Systems can attempt to produce challenges for which auto response is very difficult, or even an unsolved
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371:
250:(UBE) by the receiving system, possibly leading to blacklisting of the mail server or even listing on a
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203:
771:
741:
708:
243:
149:, including allowing for the creation of “tagged” addresses or allow pass-codes placed in either the
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approaches include a simple problem, or answering a simple question about the text or the recipient.
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588:
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917:
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17:
84:
Listed below are examples of challenges that are or could be used to exploit these differences:
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906:
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819:
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look-ups. If a relationship could be found, FairUCE checked the recipient's
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Whitelist a mailing list address manually as soon as one subscribes to it.
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Spammers can use a fake, non-existent address as sender address (in the
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262:) to ensure that the challenge goes through. Also, if systems like
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the message is sent from an IP address with an associated domain
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Simply sending an (unmodified) reply to the challenging message.
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John Levine: Challenge-response systems are as harmful as spam
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92:
515:
What You Need to Know About
Challenge – Response Spam Filters
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Allow users to view and act on messages in the holding queue.
410:"Proper principles for Challenge/Response anti-spam systems"
398:: Recommendations for Automatic Responses to Electronic Mail
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of the client delivering the mail, using a series of cached
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A listing of challenge/response filtering service providers
346:("Fair use of Unsolicited Commercial Email"), developed by
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Interaction with mailing lists or other automated mailers
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problem. One example (also found in many websites) is a "
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Challenge-Response Anti-Spam
Systems Considered Harmful
481:, Walt Mossberg of Wall Street Journal, March 22, 2007
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Comply with the requirements and recommendations of
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196:the originating IP address is not found on trusted
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157:system can create "tagged" addresses that permit:
30:This article is about e-mail. For other uses, see
145:Obey a detailed list of principles maintained by
435:"Challenge/Response systems considered harmful"
350:, tried to find a relationship connecting the
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491:Challenge-Response systems make matters worse
283:the challenge before they can send you mail.
202:the sender's email address has not failed an
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509:A Challenging Response to Challenge-Response
53:The challenge in challenge–response systems
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550:
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428:
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228:Challenges sent to forged email addresses
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163:mail that contains a certain "keyword"
176:the message header is properly formed
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485:Why Challenge-Response is a Bad Idea
473:SpamHelp Challenge/Response Services
456:"Legacy Communities - IBM Community"
160:mail sent from a particular address
737:Distributed Checksum Clearinghouse
25:
722:Challenge–response spam filtering
559:Unsolicited digital communication
32:Challenge–response authentication
27:Method for detecting spam e-mails
18:Challenge-response spam filtering
91:A challenge that includes a web
479:When Spam Filters Aren't Enough
206:test, using techniques such as
123:Recommendations for C/R systems
1:
336:Tagged Message Delivery Agent
260:Bounce Address Tag Validation
127:C/R systems should ideally:
934:
757:Naive Bayes spam filtering
62:Legitimate senders have a
29:
732:Disposable email address
594:Directory harvest attack
189:the server has passed a
182:the server has passed a
108:artificial intelligence
248:Unsolicited Bulk Email
239:callback verification
204:e-mail authentication
742:Email authentication
589:Bulk email software
520:2007-01-05 at the
433:Schryver, Vernon.
244:e-mail backscatter
39:challenge–response
900:
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727:Context filtering
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499:December 29, 2003
408:Templeton, Brad.
16:(Redirected from
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868:Advance-fee scam
815:Keyword stuffing
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880:Make Money Fast
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72:Spammers send
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624:Pink contract
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493:February 2006
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278:Social issues
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873:Lottery scam
830:Scraper site
800:Doorway page
721:
670:Mobile phone
650:Cold calling
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438:. Retrieved
413:. Retrieved
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835:Social spam
747:Greylisting
717:Client-side
645:Auto dialer
356:domain name
191:greylisting
184:greet pause
101:Turing Test
907:Categories
840:Spam blogs
805:Forum spam
782:Spamdexing
655:Flyposting
614:Image spam
576:Email spam
379:References
360:IP address
219:Criticisms
198:blacklists
78:quantities
918:Anti-spam
820:Link farm
790:Blog spam
709:Anti-spam
675:Newsgroup
665:Messaging
567:Protocols
487:July 2006
372:blacklist
368:whitelist
76:in large
913:Spamming
885:Phishing
795:Cloaking
772:Spamhaus
680:Robocall
660:Junk fax
518:Archived
511:May 2003
505:May 2003
358:and the
151:Subject:
767:SpamCop
685:Spambot
629:Spambot
619:Joe job
440:13 June
415:13 June
344:FairUCE
112:CAPTCHA
394:
138:
74:e-mail
64:valid
47:e-mail
890:Voice
845:Sping
762:SORBS
638:Other
604:DNSWL
599:DNSBL
305:Note:
252:DNSBL
234:From:
695:VoIP
524:2003
442:2014
417:2014
396:3834
370:and
268:DKIM
266:and
212:DKIM
210:and
193:test
186:test
155:TMDA
140:3834
41:(or
392:RFC
364:DNS
354:'s
348:IBM
264:SPF
208:SPF
136:RFC
93:URL
43:C/R
909::
425:^
37:A
551:e
544:t
537:v
458:.
444:.
419:.
214:.
142:.
34:.
20:)
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