Thursday, February 7, 2008
Wednesday, February 6, 2008
E-mail spam
E-mail spam, also known as "bulk e-mail" or "junk e-mail," is a subset of spam that involves nearly identical messages sent to numerous recipients by e-mail. A common synonym for spam is unsolicited bulk e-mail (UBE). Definitions of spam usually include the aspects that email is unsolicited and sent in bulk.[1][2][3][4][5] "UCE" refers specifically to "unsolicited commercial e-mail."
E-mail spam has existed since the beginning of the Internet, and has grown to about 90 billion messages a day, although about 80% is sent by fewer than 200 spammers. Botnets, virus infected computers, account for about 80% of spam. Laws against spam have been sporadically implemented, with some being opt-out laws and others being opt-in. The total amount of spam has leveled off slightly in recent years. The cost of spam is borne mostly by the recipient, so it is a form of postage due advertising.
E-mail addresses are collected from chatrooms, websites, newsgroups, and viruses which harvest users address books, and are sold to other spammers. Much of the traffic is sent to invalid e-mail addresses. ISPs have attempted to recover the cost of spam through lawsuits against spammers, although they have been mostly unsuccessful in collecting damages despite winning in court.Overview
From the beginning of the Internet, sending of junk e-mail has been prohibited,[8] enforced by the Terms of Service/Acceptable Use Policy (ToS/AUP) of internet service providers (ISPs) and peer pressure. Even with a thousand users junk e-mail for advertising is not tenable, and with a million users it is not only impractical[9] but also expensive, costing US businesses on the order of $10 billion per year in 2003.
As the Internet has grown, ISPs and the public have turned to government for relief from spam.[10] This relief has failed to materialize, particularly in the U.S. where tough state laws were superseded by a permissive federal law, the CAN-SPAM Act of 2003. Some other countries have passed laws against spam, notably Australia and all the countries of the European Union.
As the recipient directly bears the cost of delivery, storage, and processing, one could regard spam as the electronic equivalent of "postage-due" junk mail. Due to the low cost of sending unsolicited e-mail and the potential profit entailed, some believe that only strict legal enforcement can stop junk e-mail. The Coalition Against Unsolicited Commercial Email (CAUCE) argues "Today, much of the spam volume is sent by career criminals and malicious hackers who won't stop until they're all rounded up and put in jail."[11]
Spam sent by well-known companies is sometimes called mainsleaze.[12] A widely-known instance of spamming by a large corporation was Kraft Foods' marketing of its Gevalia coffee brand.[13] Another more recent offender was the company iDate, which used e-mail harvesting directed at subscribers to the Quechup website to spam their friends and contacts.[14]
Advance fee fraud spam such as the Nigerian "419" scam may be sent by a single individual from a cyber cafe in a developing country. Organized "spam gangs" operating from Russia or eastern Europe share many features in common with other forms of organized crime, including turf battles and revenge killings.[15] As much as 80% of spam received by Internet users in North America and Europe can be traced to fewer than 200 spammers.[16]
Spam is also a medium for fraudsters to scam users to enter personal information on fake Web sites using e-mail forged to look like it is from a bank or other organization such as PayPal. This is known as phishing.
Spammers may engage in deliberate fraud to send out their messages. Spammers often use false names, addresses, phone numbers, and other contact information to set up "disposable" accounts at various Internet service providers. They also often use falsified or stolen credit card numbers to pay for these accounts. This allows them to move quickly from one account to the next as the host ISPs discover and shut down each one.
Senders may go to great lengths to conceal the origin of their messages. Large companies may hire another firm to send their messages so that complaints or blocking of email falls on a third party. Others engage in spoofing of e-mail addresses (much easier than IP address spoofing). The e-mail protocol (SMTP) has no authentication by default, so the spammer can pretend to originate a message apparently from any e-mail address. To prevent this, some ISPs and domains require the use of SMTP-AUTH, allowing positive identification of the specific account from which an e-mail originates.
Senders cannot completely spoof e-mail delivery chains (the 'Received' header), since the receiving mailserver records the actual connection from the last mailserver's IP address. To counter this, some spammers forge additional delivery headers to make it appear as if the e-mail had previously traversed many legitimate servers.
Spoofing can have serious consequences for legitimate e-mail users. Not only can their e-mail inboxes get clogged up with "undeliverable" e-mails in addition to volumes of spam, they can mistakenly be identified as a spammer. Not only may they receive irate e-mail from spam victims, but (if spam victims report the e-mail address owner to the ISP, for example) a naive ISP may terminate their service for spamming.
Spammers frequently seek out and make use of vulnerable third-party systems such as open mail relays and open proxy servers. SMTP forwards mail from one server to another—mail servers that ISPs run commonly require some form of authentication to ensure that the user is a customer of that ISP. Open relays, however, do not properly check who is using the mail server and pass all mail to the destination address, making it harder to track down spammers.
Increasingly, spammers use networks of malware-infected PCs (zombies) to send their spam. Zombie networks are also known as Botnets (such zombifying malware is known as a bot, short for robot). In June 2006, an estimated 80% of e-mail spam was sent by zombie PCs, an increase of 30% from the prior year. An estimated 55 billion e-mail spam were sent each day in June 2006, an increase of 25 billion per day from June 2005.[17]
Anti-spam techniques
The US Department of Energy Computer Incident Advisory Committee (CIAC) has provided specific countermeasures against electronic mail spamming.[40]
Some popular methods for filtering and refusing spam include e-mail filtering based on the content of the e-mail, DNS-based blackhole lists (DNSBL), greylisting, spamtraps, Enforcing technical requirements of e-mail (SMTP, checksumming systems to detect bulk email, and by putting some sort of cost on the sender via a Proof-of-work system or a micropayment. Each method has strengths and weaknesses and each is controversial due to its weaknesses.
Detecting spam based on the content of the e-mail, either by detecting keywords such as "viagra" or by statistical means, is very popular. Such methods can be very accurate when they are correctly tuned to the types of legitimate email that an individual gets, but they can also make mistakes such as detecting the keyword "cialis" in the word "specialist". The content also doesn't determine whether the email was either unsolicited or bulk, the two key features of spam. So, if a friend sends you a joke that mentions "viagra", content filters can easily mark it as being spam even though it is neither unsolicited nor sent in bulk.
The most popular DNSBLs are lists of IP addresses of known spammers, open relays, zombie spammers etc.
Spamtraps are often email addresses that were never valid or have been invalid for a long time that are used to collect spam. An effective spamtrap is not announced and is only found by dictionary attacks or by pulling addresses off hidden webpages. For a spamtrap to remain effective the address must never be given to anyone. Some black lists, such as spamcop, use spamtraps to catch spammers and blacklist them.
Enforcing technical requirements of the Simple Mail Transfer Protocol (SMTP) can be used to block mail coming from systems that are not compliant with the RFC standards. A lot of spammers use poorly written software or are unable to comply with the standards because they do not have legitimate control of the computer sending spam (zombie computer). So by setting restrictions on the mail transfer agent (MTA) a mail administrator can reduce spam significantly. In many situations, simply requiring a valid fully qualified domain name (FQDN) in the SMTP's EHLO (extended hello) statement is enough to block 25% of incoming spam. Similarly, enforcing the correct fall back of Mail eXchange (MX) records in the Domain Name System, or the correct handling of delays (Teergrube) can be effective.
Using other people's computers
Early on, spammers discovered that if they sent large quantities of spam directly from their ISP accounts, recipients would complain and ISPs would shut their accounts down. Thus, one of the basic techniques of sending spam has become to send it from someone else's computer and network connection. By doing this, spammers protect themselves in several ways: they hide their tracks, get others' systems to do most of the work of delivering messages, and direct the efforts of investigators towards the other systems rather than the spammers themselves. The increasing broadband usage gave rise to a great number of computers that are online as long as they are turned on, and whose owners do not always take steps to protect them from malware. A botnet consisting of several hundred compromised machines can effortlessly churn out millions of messages per day. This also complicates the tracing of spammers.
Open relays
In the 1990s, the most common way spammers did this was to use open mail relays. An open relay is an MTA, or mail server, which is configured to pass along messages sent to it from any location, to any recipient. In the original SMTP mail architecture, this was the default behavior: a user could send mail to practically any mail server, which would pass it along towards the intended recipient's mail server.
The standard was written in an era before spamming when there were few hosts on the internet, and those on the internet abided by a certain level of conduct. While this cooperative, open approach was useful in ensuring that mail was delivered, it was vulnerable to abuse by spammers. Spammers could forward batches of spam through open relays, leaving the job of delivering the messages up to the relays.
In response, mail system administrators concerned about spam began to demand that other mail operators configure MTAs to cease being open relays. The first DNSBLs, such as MAPS RBL and the now-defunct ORBS, aimed chiefly at allowing mail sites to refuse mail from known open relays.
Open proxies
Within a few years, open relays became rare and spammers resorted to other tactics, most prominently the use of open proxies. A proxy is a network service for making indirect connections to other network services. The client connects to the proxy and instructs it to connect to a server. The server perceives an incoming connection from the proxy, not the original client. Proxies have many purposes, including Web-page caching, protection of privacy, filtering of Web content, and selectively bypassing firewalls.
An open proxy is one which will create connections for any client to any server, without authentication. Like open relays, open proxies were once relatively common, as many administrators did not see a need to restrict access to them.
A spammer can direct an open proxy to connect to a mail server, and send spam through it. The mail server logs a connection from the proxy -- not the spammer's own computer. This provides an even greater degree of concealment for the spammer than an open relay, since most relays log the client address in the headers of messages they pass. Open proxies have also been used to conceal the sources of attacks against other services besides mail, such as Web sites or IRC servers.
Besides relays and proxies, spammers have used other insecure services to send spam. One example is FormMail.pl, a CGI script to allow Web-site users to send e-mail feedback from an HTML form.[44] Several versions of this program, and others like it, allowed the user to redirect e-mail to arbitrary addresses. Spam sent through open FormMail scripts is frequently marked by the program's characteristic opening line: "Below is the result of your feedback form."
As spam from proxies and other "spammable" resources grew, DNSBL operators started listing their IP addresses, as well as open relays.
Spammer viruses
In 2003, spam investigators saw a radical change in the way spammers sent spam. Rather than searching the global network for exploitable services such as open relays and proxies, spammers began creating "services" of their own. By commissioning computer viruses designed to deploy proxies and other spam-sending tools, spammers could harness hundreds of thousands of end-user computers. The widespread change from Windows 9x to Windows XP for many home computers, which started in early 2002 and was well under way by 2003, greatly accelerated the use of home computers to act as remotely-controlled spam proxies. The original version of Windows XP as well as XP-SP1 had several major vulnerabilities that allowed the machines to be compromised over a network connection without requiring actions on the part of the user or owner. While Windows 2000 had similar vulnerabilities, that operating system was never widely used on home computers.
Most of the major Windows e-mail viruses of 2003, including the Sobig and Mimail virus families, functioned as spammer viruses: viruses designed expressly to make infected computers available as spamming tools.[45][46]
Besides sending spam, spammer viruses serve spammers in other ways. Beginning in July 2003, spammers started using some of these same viruses to perpetrate distributed denial-of-service (DDoS) attacks upon DNSBLs and other anti-spam resources.[47] Although this was by no means the first time that illegal attacks have been used against anti-spam sites, it was perhaps the first wave of effective attacks.
In August of that year, engineering company Osirusoft ceased providing DNSBL mirrors of the SPEWS and other blocklists, after several days of unceasing attack from virus-infected hosts.[48] The very next month, DNSBL operator Monkeys.com succumbed to the attacks as well.[49] Other DNSBL operators, such as Spamhaus, have deployed global mirroring and other anti-DDoS methods to resist these attacks.
Zombie networks are particularly active in North America where about half of the Internet users are on a broadband connection and many leave their computers on all the time. In January, 2008, 8% of all e-mail spam was sent by the Storm botnet, created by the Storm Worm, first released in January, 2007.[50] It is estimated that as many as 1 million or more computers have been infected and their owners are unwilling
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