Link spam (also called blog spam or comment spam) is a form of spamming or spamdexing that recently became publicized most often when targeting weblogs (or blogs), but also affects wikis (where it is often called wikispam), guestbooks, and online discussion boards. Any web application that displays hyperlinks submitted by visitors or the referring URLs of web visitors may be a target.

Adding links that point to the spammer's web site increases the page rankings for the site in the search engine Google. An increased page rank means the spammer's commercial site would be listed ahead of other sites for certain Google searches, increasing the number of potential visitors and paying customers².

Link spamming originally appeared in internet guestbooks, where spammers repeatedly fill a guestbook with links to their own site and no relevant comment to increase search engine rankings. If an actual comment is given it is often just "cool page", "nice website", or keywords of the spammed link.

In 2003, spammers began to take advantage of the open nature of comments in the blogging software like Movable Type by repeatedly placing comments to various blog posts that provided nothing more than a link to the spammer's commercial web site. Jay Allen created a free plugin, called MT-BlackList¹, for the Movable Type weblog tool that attempts to alleviate this problem. Many current blog software now have methods of preventing or reducing the effect of blog spam.

Because of prevention improvements in blog software link spam is now increasingly concentrated on wikis around the World Wide Web including Wikipedia, the largest wiki on the Internet (see [1]). Wiki spam sometimes only appears on a wiki's sandbox page, but is often found defacing multiple pages. This page³ lists URLs and IP addresses of offending wiki spammers.

Possible solutions

• Recently Google introduced an HTML attribute to prevent ranking credits. Search engines and weblog software already follow this convention.:

        <a href="http://www.wiki.org/" rel="nofollow">Link</a>

• Instead of displaying a direct hyperlink submitted by a visitor, a web application could display a link to a script on its own website that redirects to the correct URL. This will not prevent all spam since spammers do not always check for link redirection but has proven very effective. Redirecting links prevent Google from factoring the link in its PageRank algorithm for that site making the spam ineffective. An added benefit is that your redirection script can count how many people visit external URLs, but one drawback is the increased load on your website.

• Idea: However, the script could also be client-side JavaScript. For example,

        <a href="javascript:window.location.href='http://www.wiki.org'">Link</a>

would work as a link but not be picked up by Google. Moreover, the javascript could be more complicated to ensure that the link would never be picked up since it was encoded. For example,

        <a href="javascript:redirectFunction('hfksksgjlsll')">Link</a>

where 'hfksksgjlsll' is an encoded URL that is decoded by the javascript function redirectFunction which presumably is stored in the HEAD tag of the page. A downside of this is that visitors who have disabled Javascript in their browser would be unable to follow the links.

• This redirect can also be done via the .htaccess file in Apache, thus saving the load of a script.

• Require the user to solve a captcha, preventing bots from submitting entries.

External links

1. Six Apart Comment Spam Guide, fairly broad overview from Movable Type's authors.
2. CanBlogSpam.com - blog spam solutions, news and trends
3. MT-BlackList, a plugin for Movable Type.
4. Pivot-BlackList, a plugin for Pivot.
5. Wired Magazine article on blog spam.
6. chongqed.org List of Wiki and Blog Spammers.