If your corporate website’s search engine rankings suddenly drop for no obvious reason, or if clients start complaining that their security software is blocking access or flagging your site as a source of unwanted content, you might be hosting a hidden block of links. These links typically point to shady websites, such as pornography or online casinos. While these links are invisible to regular users, search engines and security solutions scan and factor them in when judging your website’s authority and safety. Today, we explain how these hidden links harm your business, how attackers manage to inject them into legitimate websites, and how to protect your website from this unpleasantness.

Why hidden links are a threat to your business

First and foremost, hidden links to dubious sites can severely damage your site’s reputation and lower its ranking, which will immediately impact your position in search results. This is because search engines regularly scan websites’ HTML code, and are quick to discover any lines of code that attackers may have added. Using hidden blocks is often viewed by search algorithms as a manipulative practice: a hallmark of black hat SEO (also known simply as black SEO). As a result, search engines lower the ranking of any site found hosting such links.

Another reason for a drop in search rankings is that hidden links typically point to websites with a low domain rating, and content irrelevant to your business. Domain rating is a measure of a domain’s authority — reflecting its prestige and the quality of information published on it. If your site links to authoritative industry-specific pages, it tends to rise in search results. If it links to irrelevant, shady websites, it sinks. Furthermore, search engines view hidden blocks as a sign of artificial link building, which, again, penalizes the victim site’s placement in search results.

The most significant technical issue is the manipulation of link equity. Your website has a certain reputation or authority, which influences the ranking of pages you link to. For example, when you post a helpful article on your site, and link to your product page or contacts section, you’re essentially transferring authority from that valuable content to those internal pages. The presence of unauthorized external links siphons off this link equity to external sites. Normally, every internal link helps search engines understand which pages on your site are most important — boosting their position. However, when a significant portion of this equity leaks to dubious external domains, your key pages receive less authority. This ultimately causes them to rank lower than they should — directly impacting your organic traffic and SEO performance.

In the worst cases, the presence of these links can even lead to conflicts with law enforcement, and entail legal liability for distributing illegal content. Depending on local laws, linking to websites with illegal content could result in fines or even the complete blocking of your site by regulatory bodies.

How to check your site for hidden links

The simplest way to check your website for blocks of hidden links is to view its source code. To do this, open the site in browser and press Ctrl+U (in Windows and Linux) or Cmd+Option+U (in macOS). A new tab will open with the page’s source code.

In the source code, look for the following CSS properties that can indicate hidden elements:

• display:none
• visibility:hidden
• opacity:0
• height:0
• width:0
• position:absolute

These elements relate to CSS properties that make blocks on the page invisible — either entirely hidden or reduced to zero size. Theoretically, these properties can be used for legitimate purposes — such as responsive design, hidden menus, or pop-up windows. However, if they’re applied to links or entire blocks of link code, it could be a strong sign of malicious tampering.

Additionally, you can search the code for keywords related to the content that hidden links most often point to, such as “porn”, “sex”, “casino”, “card”, and the like.

For a deep dive into the specific methods attackers use to hide their link blocks on legitimate sites, check out our separate, more technical Securelist post.

How do attackers inject their links into legitimate sites?

To add an invisible block of links to a website, attackers first need the ability to edit your pages. They can achieve this in several ways.

Compromising administrator credentials

The dark web is home to a whole criminal ecosystem dedicated to buying and selling compromised credentials. Initial-access brokers will provide anyone with credentials tied to virtually any company. Attackers obtain these credentials through phishing attacks or stealer Trojans, or simply by scouring publicly available data breaches from other websites in the hope that employees reuse the same login and password across multiple platforms. Additionally, administrators might use overly simple passwords, or fail to change the default CMS credentials. In these cases, attackers can easily bruteforce the login details.

Gaining access to an account with administrator privileges gives criminals broad control over the website. Specifically, they can edit the HTML code, or install their own malicious plugins.

Exploiting CMS vulnerabilities

We frequently discuss various vulnerabilities in CMS platforms and plugins on our blog. Attackers can leverage these security flaws to edit template files (such as header.php, footer.php, or index.php), or directly insert blocks of hidden links into arbitrary pages across the site.

Compromising the hosting provider

In some cases, it’s the hosting company that gets compromised rather than the website itself. If the server hosting your website code is poorly protected, attackers can breach it and gain control over the site. Another common scenario concerns a server that hosts sites for many different clients. If access privileges are configured incorrectly, compromising one client can give criminals the ability to reach other websites hosted on that same server.

Malicious code blocks in free templates

Not all webmasters write their own code. Budget-conscious and unwary web designers might try to find free templates online and simply customize them to fit the corporate style. The code in these templates can also contain covert blocks inserted by malicious actors.

How do you protect your site from hidden links?

To secure your website against the injection of hidden links and its associated consequences, we recommend taking the following steps:

• Avoid using questionable third-party templates, themes, or any other unverified solutions to build your website.
• Promptly update both your CMS engine and all associated themes and plugins to their latest versions.
• Routinely audit your plugins and themes, and immediately delete the ones you don’t use.
• Regularly create backups of both your website and database. This ensures you can quickly restore your website’s operation in the event of compromise.
• Check for unnecessary user accounts and excessive access privileges.
• Promptly delete outdated or unused accounts, and establish only the minimum necessary privileges for active ones.
• Establish a strong password policy and mandatory two-factor authentication for all accounts with admin privileges.
• Conduct regular training for employees on basic cybersecurity principles. The Kaspersky Automated Security Awareness Platform can help you automate this process.

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