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Spam in blogs (also called simply blog spam, comment spam, or social spam) is a form of spamdexing. (Note that blogspam also has another meaning, namely the post of a blogger who creates posts that have no added value to them in order to submit them to other sites.) It is done by posting (usually automatically) random comments, copying material from elsewhere that is not original, or promoting commercial services to blogs, wikis, guestbooks, or other publicly accessible online discussion boards. Any web application that accepts and displays hyperlinks submitted by visitors may be a target.
Adding links that point to the spammer's web site artificially increases the site's search engine ranking on those where the popularity of the URL contributes to its implied value, an example algorithm would be the PageRank algorithm as used by Google Search. An increased ranking often results in the spammer's commercial site being listed ahead of other sites for certain searches, increasing the number of potential visitors and paying customers.
This type of spam originally appeared in Internet guestbooks, where spammers repeatedly filled a guestbook with links to their own site and with 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-BlackList, for the Movable Type weblog tool (versions prior to 3.2) that attempted to alleviate this problem. Many blogging packages now have methods of preventing or reducing the effect of blog spam built in due to its prevalence, although spammers too have developed tools to circumvent them. Many spammers use special blog spamming tools like trackback submitter to bypass comment spam protection on popular blogging systems like Movable Type, Wordpress, and others.
Other phrases typically used in the comment content can be stolen comments from other websites, "nice article", something about their imaginary friends, plagiarised parts from books, unfinished sentences, nonsense words (usually to defeat a minimum comment length restriction) or the same link repeated.
Particularly popular software products such as Movable Type and MediaWiki have developed or included anti-spam measures, as spammers focus more attention on targeting those platforms due to their prevalence on the Internet. Whitelists and blacklists that prevent certain IPs from posting, or that prevent people from posting content that matches certain filters, are common defences although most software tends to use a combination of the variety of different techniques documented below.
The goal in every potential solution is to allow legitimate users to continue to comment (and often even add links to their comments, as that is considered by some to be a valuable aspect of any comments section when the links are relevant or related to the article or content) whilst preventing all link spam or irrelevant comments from ever being viewable to the site's owner and visitors.
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