Entries Tagged "Internet"

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Combating Spam

Spam is back in the news, and it has a new name. This time it’s voice-over-IP spam, and it has the clever name of “spit” (spam over Internet telephony). Spit has the potential to completely ruin VoIP. No one is going to install the system if they’re going to get dozens of calls a day from audio spammers. Or, at least, they’re only going to accept phone calls from a white list of previously known callers.

VoIP spam joins the ranks of e-mail spam, Usenet newsgroup spam, instant message spam, cell phone text message spam, and blog comment spam. And, if you think broadly enough, these computer-network spam delivery mechanisms join the ranks of computer telemarketing (phone spam), junk mail (paper spam), billboards (visual space spam), and cars driving through town with megaphones (audio spam). It’s all basically the same thing — unsolicited marketing messages — and only by understanding the problem at this level of generality can we discuss solutions.

In general, the goal of advertising is to influence people. Usually it’s to influence people to purchase a product, but it could just as easily be to influence people to support a particular political candidate or position. Advertising does this by implanting a marketing message into the brain of the recipient. The mechanism of implantation is simply a tactic.

Tactics for unsolicited marketing messages rise and fall in popularity based on their cost and benefit. If the benefit is significant, people are willing to spend more. If the benefit is small, people will only do it if it is cheap. A 30-second prime-time television ad costs 1.8 cents per adult viewer, a full-page color magazine ad about 0.9 cents per reader. A highway billboard costs 0.21 cents per car. Direct mail is the most expensive, at over 50 cents per third-class letter mailed. (That’s why targeted mailing lists are so valuable; they increase the per-piece benefit.)

Spam is such a common tactic not because it’s particularly effective; the response rates for spam are very low. It’s common because it’s ridiculously cheap. Typically, spammers charge less than a hundredth of a cent per e-mail. (And that number is just what spamming houses charge their customers to deliver spam; if you’re a clever hacker, you can build your own spam network for much less money.) If it is worth $10 for you to successfully influence one person — to buy your product, vote for your guy, whatever — then you only need a 1 in a 100,000 success rate. You can market really marginal products with spam.

So far, so good. But the cost/benefit calculation is missing a component: the “cost” of annoying people. Everyone who is not influenced by the marketing message is annoyed to some degree. The advertiser pays a partial cost for annoying people; they might boycott his product. But most of the time he does not, and the cost of the advertising is paid by the person: the beauty of the landscape is ruined by the billboard, dinner is disrupted by a telemarketer, spam costs money to ship around the Internet and time to wade through, etc. (Note that I am using “cost” very generally here, and not just monetarily. Time and happiness are both costs.)

This is why spam is so bad. For each e-mail, the spammer pays a cost and receives benefit. But there is an additional cost paid by the e-mail recipient. But because so much spam is unwanted, that additional cost is huge — and it’s a cost that the spammer never sees. If spammers could be made to bear the total cost of spam, then its level would be more along the lines of what society would find acceptable.

This economic analysis is important, because it’s the only way to understand how effective different solutions will be. This is an economic problem, and the solutions need to change the fundamental economics. (The analysis is largely the same for VoIP spam, Usenet newsgroup spam, blog comment spam, and so on.)

The best solutions raise the cost of spam. Spam filters raise the cost by increasing the amount of spam that someone needs to send before someone will read it. If 99% of all spam is filtered into trash, then sending spam becomes 100 times more expensive. This is also the idea behind white lists — lists of senders a user is willing to accept e-mail from — and blacklists: lists of senders a user is not willing to accept e-mail from.

Filtering doesn’t just have to be at the recipient’s e-mail. It can be implemented within the network to clean up spam, or at the sender. Several ISPs are already filtering outgoing e-mail for spam, and the trend will increase.

Anti-spam laws raise the cost of spam to an intolerable level; no one wants to go to jail for spamming. We’ve already seen some convictions in the U.S. Unfortunately, this only works when the spammer is within the reach of the law, and is less effective against criminals who are using spam as a mechanism to commit fraud.

Other proposed solutions try to impose direct costs on e-mail senders. I have seen proposals for e-mail “postage,” either for every e-mail sent or for every e-mail above a reasonable threshold. I have seen proposals where the sender of an e-mail posts a small bond, which the receiver can cash if the e-mail is spam. There are other proposals that involve “computational puzzles”: time-consuming tasks the sender’s computer must perform, unnoticeable to someone who is sending e-mail normally, but too much for someone sending e-mail in bulk. These solutions generally involve re-engineering the Internet, something that is not done lightly, and hence are in the discussion stages only.

All of these solutions work to a degree, and we end up with an arms race. Anti-spam products block a certain type of spam. Spammers invent a tactic that gets around those products. Then the products block that spam. Then the spammers invent yet another type of spam. And so on.

Blacklisting spammer sites forced the spammers to disguise the origin of spam e-mail. People recognizing e-mail from people they knew, and other anti-spam measures, forced spammers to hack into innocent machines and use them as launching pads. Scanning millions of e-mails looking for identical bulk spam forced spammers to individualize each spam message. Semantic spam detection forced spammers to design even more clever spam. And so on. Each defense is met with yet another attack, and each attack is met with yet another defense.

Remember that when you think about host identification, or postage, as an anti-spam measure. Spammers don’t care about tactics; they want to send their e-mail. Techniques like this will simply force spammers to rely more on hacked innocent machines. As long as the underlying computers are insecure, we can’t prevent spammers from sending.

This is the problem with another potential solution: re-engineering the Internet to prohibit the forging of e-mail headers. This would make it easier for spam detection software to detect spamming IP addresses, but spammers would just use hacked machines instead of their own computers.

Honestly, there’s no end in sight for the spam arms race. Even so, spam is one of computer security’s success stories. The current crop of anti-spam products work. I get almost no spam and very few legitimate e-mails end up in my spam trap. I wish they would work better — Crypto-Gram is occasionally classified as spam by one service or another, for example — but they’re working pretty well. It’ll be a long time before spam stops clogging up the Internet, but at least we don’t have to look at it.

Posted on May 13, 2005 at 9:47 AMView Comments

The Potential for an SSH Worm

SSH, or secure shell, is the standard protocol for remotely accessing UNIX systems. It’s used everywhere: universities, laboratories, and corporations (particularly in data-intensive back office services). Thanks to SSH, administrators can stack hundreds of computers close together into air-conditioned rooms and administer them from the comfort of their desks.

When a user’s SSH client first establishes a connection to a remote server, it stores the name of the server and its public key in a known_hosts database. This database of names and keys allows the client to more easily identify the server in the future.

There are risks to this database, though. If an attacker compromises the user’s account, the database can be used as a hit-list of follow-on targets. And if the attacker knows the username, password, and key credentials of the user, these follow-on targets are likely to accept them as well.

A new paper from MIT explores the potential for a worm to use this infection mechanism to propagate across the Internet. Already attackers are exploiting this database after cracking passwords. The paper also warns that a worm that spreads via SSH is likely to evade detection by the bulk of techniques currently coming out of the worm detection community.

While a worm of this type has not been seen since the first Internet worm of 1988, attacks have been growing in sophistication and most of the tools required are already in use by attackers. It’s only a matter of time before someone writes a worm like this.

One of the countermeasures proposed in the paper is to store hashes of host names in the database, rather than the names themselves. This is similar to the way hashes of passwords are stored in password databases, so that security need not rely entirely on the secrecy of the database.

The authors of the paper have worked with the open source community, and version 4.0 of OpenSSH has the option of hashing the known-hosts database. There is also a patch for OpenSSH 3.9 that does the same thing.

The authors are also looking for more data to judge the extent of the problem. Details about the research, the patch, data collection, and whatever else thay have going on can be found here.

Posted on May 10, 2005 at 9:06 AMView Comments

Anonymity and the Internet

From Slate:

Anonymice on Anonymity Wendy.Seltzer.org (“Musings of a techie lawyer”) deflates the New York Times‘ breathless Saturday (March 19) piece about the menace posed by anonymous access to Wi-Fi networks (“Growth of Wireless Internet Opens New Path for Thieves” by Seth Schiesel). Wi-Fi pirates around the nation are using unsecured hotspots to issue anonymous death threats, download child pornography, and commit credit card fraud, Schiesel writes. Then he plays the terrorist card.

But unsecured wireless networks are nonetheless being looked at by the authorities as a potential tool for furtive activities of many sorts, including terrorism. Two federal law enforcement officials said on condition of anonymity that while they were not aware of specific cases, they believed that sophisticated terrorists might also be starting to exploit unsecured Wi-Fi connections.

Never mind the pod of qualifiers swimming through in those two sentences — “being looked at”; “potential tool”; “not aware of specific cases”; “might” — look at the sourcing. “Two federal law enforcement officials said on condition of anonymity. …” Seltzer points out the deep-dish irony of the Times citing anonymous sources about the imagined threats posed by anonymous Wi-Fi networks. Anonymous sources of unsubstantiated information, good. Anonymous Wi-Fi networks, bad.

This is the post from wendy.seltzer.org:

The New York Times runs an article in which law enforcement officials lament, somewhat breathlessly, that open wifi connections can be used, anonymously, by wrongdoers. The piece omits any mention of the benefits of these open wireless connections — no-hassle connectivity anywhere the “default” community network is operating, and anonymous browsing and publication for those doing good, too.

Without a hint of irony, however:

Two federal law enforcement officials said on condition of anonymity that while they were not aware of specific cases, they believed that sophisticated terrorists might also be starting to exploit unsecured Wi-Fi connections.

Yes, even law enforcement needs anonymity sometimes.

Open WiFi networks are a good thing. Yes, they allow bad guys to do bad things. But so do automobiles, telephones, and just about everything else you can think of. I like it when I find an open wireless network that I can use. I like it when my friends keep their home wireless network open so I can use it.

Scare stories like the New York Times one don’t help any.

Posted on March 25, 2005 at 12:49 PMView Comments

Authentication and Expiration

There’s a security problem with many Internet authentication systems that’s never talked about: there’s no way to terminate the authentication.

A couple of months ago, I bought something from an e-commerce site. At the checkout page, I wasn’t able to just type in my credit-card number and make my purchase. Instead, I had to choose a username and password. Usually I don’t like doing that, but in this case I wanted to be able to access my account at a later date. In fact, the password was useful because I needed to return an item I purchased.

Months have passed, and I no longer want an ongoing relationship with the e-commerce site. I don’t want a username and password. I don’t want them to have my credit-card number on file. I’ve received my purchase, I’m happy, and I’m done. But because that username and password have no expiration date associated with them, they never end. It’s not a subscription service, so there’s no mechanism to sever the relationship. I will have access to that e-commerce site for as long as it remembers that username and password.

In other words, I am liable for that account forever.

Traditionally, passwords have indicated an ongoing relationship between a user and some computer service. Sometimes it’s a company employee and the company’s servers. Sometimes it’s an account and an ISP. In both cases, both parties want to continue the relationship, so expiring a password and then forcing the user to choose another is a matter of security.

In cases with this ongoing relationship, the security consideration is damage minimization. Nobody wants some bad guy to learn the password, and everyone wants to minimize the amount of damage he can do if he does. Regularly changing your password is a solution to that problem.

This approach works because both sides want it to; they both want to keep the authentication system working correctly, and minimize attacks.

In the case of the e-commerce site, the interests are much more one-sided. The e-commerce site wants me to live in their database forever. They want to market to me, and entice me to come back. They want to sell my information. (This is the kind of information that might be buried in the privacy policy or terms of service, but no one reads those because they’re unreadable. And all bets are off if the company changes hands.)

There’s nothing I can do about this, but a username and password that never expire is another matter entirely. The e-commerce site wants me to establish an account because it increases the chances that I’ll use them again. But I want a way to terminate the business relationship, a way to say: “I am no longer taking responsibility for items purchased using that username and password.”

Near as I can tell, the username and password I typed into that e-commerce site puts my credit card at risk until it expires. If the e-commerce site uses a system that debits amounts from my checking account whenever I place an order, I could be at risk forever. (The US has legal liability limits, but they’re not that useful. According to Regulation E, the electronic transfers regulation, a fraudulent transaction must be reported within two days to cap liability at US$50; within 60 days, it’s capped at $500. Beyond that, you’re out of luck.)

This is wrong. Every e-commerce site should have a way to purchase items without establishing a username and password. I like sites that allow me to make a purchase as a “guest,” without setting up an account.

But just as importantly, every e-commerce site should have a way for customers to terminate their accounts and should allow them to delete their usernames and passwords from the system. It’s okay to market to previous customers. It’s not okay to needlessly put them at financial risk.

This essay also appeared in the Jan/Feb 05 issue of IEEE Security & Privacy.

Posted on February 10, 2005 at 7:55 AMView Comments

FBI Retires Carnivore

According to SecurityFocus:

FBI surveillance experts have put their once-controversial Carnivore Internet surveillance tool out to pasture, preferring instead to use commercial products to eavesdrop on network traffic, according to documents released Friday.

Of course, they’re not giving up on Internet surveillance. They’ve just realized that commercial tools are better, cheaper, or both.

Posted on January 24, 2005 at 8:00 AMView Comments

Safe Personal Computing

I am regularly asked what average Internet users can do to ensure their security. My first answer is usually, “Nothing–you’re screwed.”

But that’s not true, and the reality is more complicated. You’re screwed if you do nothing to protect yourself, but there are many things you can do to increase your security on the Internet.

Two years ago, I published a list of PC security recommendations. The idea was to give home users concrete actions they could take to improve security. This is an update of that list: a dozen things you can do to improve your security.

General: Turn off the computer when you’re not using it, especially if you have an “always on” Internet connection.

Laptop security: Keep your laptop with you at all times when not at home; treat it as you would a wallet or purse. Regularly purge unneeded data files from your laptop. The same goes for PDAs. People tend to store more personal data–including passwords and PINs–on PDAs than they do on laptops.

Backups: Back up regularly. Back up to disk, tape or CD-ROM. There’s a lot you can’t defend against; a recent backup will at least let you recover from an attack. Store at least one set of backups off-site (a safe-deposit box is a good place) and at least one set on-site. Remember to destroy old backups. The best way to destroy CD-Rs is to microwave them on high for five seconds. You can also break them in half or run them through better shredders.

Operating systems: If possible, don’t use Microsoft Windows. Buy a Macintosh or use Linux. If you must use Windows, set up Automatic Update so that you automatically receive security patches. And delete the files “command.com” and “cmd.exe.”

Applications: Limit the number of applications on your machine. If you don’t need it, don’t install it. If you no longer need it, uninstall it. Look into one of the free office suites as an alternative to Microsoft Office. Regularly check for updates to the applications you use and install them. Keeping your applications patched is important, but don’t lose sleep over it.

Browsing: Don’t use Microsoft Internet Explorer, period. Limit use of cookies and applets to those few sites that provide services you need. Set your browser to regularly delete cookies. Don’t assume a Web site is what it claims to be, unless you’ve typed in the URL yourself. Make sure the address bar shows the exact address, not a near-miss.

Web sites: Secure Sockets Layer (SSL) encryption does not provide any assurance that the vendor is trustworthy or that its database of customer information is secure.

Think before you do business with a Web site. Limit the financial and personal data you send to Web sites–don’t give out information unless you see a value to you. If you don’t want to give out personal information, lie. Opt out of marketing notices. If the Web site gives you the option of not storing your information for later use, take it. Use a credit card for online purchases, not a debit card.

Passwords: You can’t memorize good enough passwords any more, so don’t bother. For high-security Web sites such as banks, create long random passwords and write them down. Guard them as you would your cash: i.e., store them in your wallet, etc.

Never reuse a password for something you care about. (It’s fine to have a single password for low-security sites, such as for newspaper archive access.) Assume that all PINs can be easily broken and plan accordingly.

Never type a password you care about, such as for a bank account, into a non-SSL encrypted page. If your bank makes it possible to do that, complain to them. When they tell you that it is OK, don’t believe them; they’re wrong.

E-mail : Turn off HTML e-mail. Don’t automatically assume that any e-mail is from the “From” address.

Delete spam without reading it. Don’t open messages with file attachments, unless you know what they contain; immediately delete them. Don’t open cartoons, videos and similar “good for a laugh” files forwarded by your well-meaning friends; again, immediately delete them.

Never click links in e-mail unless you’re sure about the e-mail; copy and paste the link into your browser instead. Don’t use Outlook or Outlook Express. If you must use Microsoft Office, enable macro virus protection; in Office 2000, turn the security level to “high” and don’t trust any received files unless you have to. If you’re using Windows, turn off the “hide file extensions for known file types” option; it lets Trojan horses masquerade as other types of files. Uninstall the Windows Scripting Host if you can get along without it. If you can’t, at least change your file associations, so that script files aren’t automatically sent to the Scripting Host if you double-click them.

Antivirus and anti-spyware software : Use it–either a combined program or two separate programs. Download and install the updates, at least weekly and whenever you read about a new virus in the news. Some antivirus products automatically check for updates. Enable that feature and set it to “daily.”

Firewall : Spend $50 for a Network Address Translator firewall device; it’s likely to be good enough in default mode. On your laptop, use personal firewall software. If you can, hide your IP address. There’s no reason to allow any incoming connections from anybody.

Encryption: Install an e-mail and file encryptor (like PGP). Encrypting all your e-mail or your entire hard drive is unrealistic, but some mail is too sensitive to send in the clear. Similarly, some files on your hard drive are too sensitive to leave unencrypted.

None of the measures I’ve described are foolproof. If the secret police wants to target your data or your communications, no countermeasure on this list will stop them. But these precautions are all good network-hygiene measures, and they’ll make you a more difficult target than the computer next door. And even if you only follow a few basic measures, you’re unlikely to have any problems.

I’m stuck using Microsoft Windows and Office, but I use Opera for Web browsing and Eudora for e-mail. I use Windows Update to automatically get patches and install other patches when I hear about them. My antivirus software updates itself regularly. I keep my computer relatively clean and delete applications that I don’t need. I’m diligent about backing up my data and about storing data files that are no longer needed offline.

I’m suspicious to the point of near-paranoia about e-mail attachments and Web sites. I delete cookies and spyware. I watch URLs to make sure I know where I am, and I don’t trust unsolicited e-mails. I don’t care about low-security passwords, but try to have good passwords for accounts that involve money. I still don’t do Internet banking. I have my firewall set to deny all incoming connections. And I turn my computer off when I’m not using it.

That’s basically it. Really, it’s not that hard. The hardest part is developing an intuition about e-mail and Web sites. But that just takes experience.

This essay previously appeared on CNet

Posted on December 13, 2004 at 9:59 AMView Comments

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Sidebar photo of Bruce Schneier by Joe MacInnis.