Spam in blogs (also called simply blog spam or comment spam; note blogspam has another, more common meaning, namely the post of a blogger who creates no-value added posts to submit them to other sites) is a form of spamdexing. It is done by automatically posting random comments 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. 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 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-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, although spammers 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.
It is rare on a site that a user would reply to their own comment, yet spammers typically will do. Checking that the user's IP address is not replying to a user of the same IP address will significantly reduce flooding. This however proves problematic in the fairly rare instance when multiple users, behind the same proxy, wish to comment on the same entry.
Blocking specific words from posts is one of the simplest and most effective ways to reduce spam. Much spam can be blocked simply by banning names of popular pharmaceuticals and casino games.
This is a good long-term solution, because it's not beneficial for spammers to change keywords to "vi@gra" or such, because keywords must be readable and indexed by search engine bots to be effective.
Google announced in early 2005 that hyperlinks with
rel="nofollow" attribute would not be crawled or influence the link target's ranking in the search engine's index. The Yahoo and MSN search engines also respect this tag. 
rel="nofollow" is a much easier solution that makes the improvised techniques above irrelevant. Most weblog software now marks reader-submitted links this way by default (with no option to disable it without code modification). A more sophisticated server software could spare the nofollow for links submitted by trusted users like those registered for a long time, on a whitelist, or with a high karma. Some server software adds
rel="nofollow" to pages that have been recently edited but omits it from stable pages, under the theory that stable pages will have had offending links removed by human editors.
Some weblog authors object to the use of
rel="nofollow", arguing, for example, that
rel="nofollow"in order to reduce the effect of heavy inter-blog linking on page ranking.
rel="nofollow"only to minimize its own filtering efforts and to deflect that this actually had better been called
Other websites like Slashdot, with high user participation, use improvised nofollow implementations like adding
rel="nofollow" only for potentially misbehaving users. Potential spammers posting as users can be determined through various heuristics like age of registered account and other factors. Slashdot also uses the poster's karma as a determinant in attaching a nofollow tag to user submitted links.
rel="nofollow" has come to be regarded as a microformat.
A method to block automated spam comments is requiring a validation prior to publishing the contents of the reply form. The goal is to verify that the form is being submitted by a real human being and not by a spam tool and has therefore been described as a reverse Turing test. The test should be of such a nature that a human being can easily pass and an automated tool would most likely fail.
Many forms on websites take advantage of the CAPTCHA technique, displaying a combination of numbers and letters embedded in an image which must be entered literally into the reply form to pass the test. In order to keep out spam tools with built-in text recognition the characters in the images are customarily misaligned, distorted, and noisy. A drawback of many older CAPTCHAs is that passwords are usually case-sensitive while the corresponding images often don't allow a distinction of capital and small letters. This should be taken into account when devising a list of CAPTCHAs. Such systems can also prove problematic to blind people who rely on screen readers. Some more recent systems allow for this by providing an audio version of the characters.
A simple alternative to CAPTCHAs is the validation in the form of a password question, providing a hint to human visitors that the password is the answer to a simple question like "The Earth revolves around the... [Sun]".
One drawback to be taken into consideration is that any validation required in the form of an additional form field may become a nuisance especially to regular posters. Many bloggers and guestbook owners notice a significant decrease in the number of comments once such a validation is in place.
There is negligible gain from spam that does not contain links, so currently all spam posts contain (excessive number of) links. It is safe to require passing Turing tests only if post contains links and letting all other posts through. While this is highly effective, spammers do frequently send gibberish posts (such as "ajliabisadf ljibia aeriqoj") to test the spam filter. These gibberish posts will not be labeled as spam. They do the spammer no good, but they still clog up comments sections.
Garbage submissions might however also result from level 0 spambots, which don't parse the attacked HTML form fields first, but send generic POST requests against pages. So it happens that a "content" or "forum_post" POST variable is set and received by the blog or forum software, but the "uri" or other wrong url field name is not accepted and thus not saved as spamlink.
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 effectively prevents against increasing their PageRank, just as
rel=nofollow. An added benefit is that the redirection script can count how many people visit external URLs, although it will increase the load on the site.
<a href="http://my-own.net/alias_of_target" rel="nofollow" >Link</a>
where 'alias_of_target' is the alias of target address.
Note however that this prevents users from being able to view the target of a link before clicking it, thus interfering with their ability to ignore websites they know to be spam. TinyURL now offers a preview feature to help avoiding this situation.
This approach is very new to addressing link spam. One of the shortcomings of link spam filters is that most sites receive only one link from each domain which is running a spam campaign. If the spammer varies IP addresses, there is little to no distinguishable pattern left on the vandalized site. The pattern, however, is left across the thousands of sites that were hit quickly with the same links.
A distributed approach, like the free LinkSleeve uses XML-RPC to communicate between the various server applications (such as blogs, guestbooks, forums, and wikis) and the filter server, in this case LinkSleeve. The posted data is stripped of urls and each url is checked against recently submitted urls across the web. If a threshold is exceeded, a "reject" response is returned, thus deleting the comment, message, or posting. Otherwise, an "accept" message is sent.
A more robust distributed approach is Akismet, which uses a similar approach to LinkSleeve but uses API keys to assign trust to nodes and also has wider distribution as a result of being bundled with the 2.0 release of WordPress. They claim over 140,000 blogs contributing to their system. Akismet libraries have been implemented for Java, Python, Ruby, and PHP, but its adoption may be hindered by its commercial use restrictions. In 2008, Six Apart therefore released a beta version of their TypePad AntiSpam software, which is compatible with Akismet but free of the latter's commercial use restrictions.
Project Honey Pot has also begun tracking comment spammers. The Project uses its vast network of thousands of traps installed in over one hundred countries around the world in order to watch what comment spamming web robots are posting to blogs and forums. Data is then published on the top countries for comment spamming, as well as the top keywords and URLs being promoted. The Project's data is then made available to block known comment spammers through http:BL. Various plugins have been developed to take advantage of the http:BL API.
Particularly popular software products such as Movable Type and MediaWiki have developed their own custom anti-spam measures, as spammers focus more attention on targeting those platforms. Whitelists and blacklists that prevent certain IPs from posting, or that prevent people from posting content that matches certain filters, are common defenses. More advanced access control lists require various forms of validation before users can contribute anything like linkspam.
The goal in every case is to allow good users to continue to add links to their comments, as that is considered by some to be a valuable aspect of any comments section.
Some wikis allow you to access an RSS feed of recent changes or comments. If you add that to your news reader and set up a smart search for common spam terms (usually viagra and other drug names) you can quickly identify and remove the offending spam.
Another filter available to webmasters is to add a hidden session token or hash function to their comment form. When the comments are submitted, data stored within the posting such as IP address and time of posting can be compared to the data stored with the session token or hash generated when the user loaded the comment form. Postings that use different IP addresses for loading the comment form and posting the comment form, or postings that took unusually short or long periods of time to compose can be filtered out. This method is particularly effective against spammers who spoof their IP Address (or use the distributed anonymous proxy Tor) in an attempt to conceal their identities. Tor provides additional issues over conventional proxies as the IP address changes on each request. Spammers are often aware of this and use them to commit their spam activities. Response tokens make these Tor sessions easier to track (and due to the prevalent abuse of Tor and if desired, block.). Additionally spammers may not actually load the comments form for an entry, having a unique code for each entry inserted into the comment form and verifying it on receipt of the HTTP POST will significantly increase the number of steps required to spam multiple entries.
Some blog software such as Typo allow the blog administrator to allow only comments submitted via Ajax XMLHttpRequests, and discard regular form POST requests. This causes accessibility problems typical to Ajax-only applications.
Although this technique prevents spam so far, it is a form of security by obscurity and will probably be defeated if it becomes popular enough.