How can malware run silently on a WordPress site without appearing in the plugin list? 🔍🛠️

Threat actors are increasingly exploiting WordPress's "mu-plugins" directory to hide persistent malware. These "must-use plugins" auto-load on every page request without needing activation and are not visible in the WordPress admin dashboard, making them convenient for attackers aiming to stay hidden.

Sucuri researchers reported several recent infections where obfuscated PHP scripts were planted in this directory. One example includes code that pulls remote payloads from a disguised text file (`index.txt`) using `eval()`, `file_get_contents()`, or cURL—functions commonly misused to run arbitrary PHP code fetched from external servers.

Three specific malware variants were uncovered:
- **redirect.php**: Redirects users to malicious sites while bypassing bots and logged-in admins. It impersonates WordPress functions to blend in, often as part of fake update schemes.
- **index.php**: A webshell that fetches and runs remote scripts dynamically, enabling attackers to control the site fully without modifying the file itself—ideal for maintaining long-term access.
- **custom-js-loader.php**: Injects JavaScript that replaces images with explicit content and hijacks outbound links with popup ads, targeting traffic manipulation and SEO spam.

These infections typically result from compromised admin accounts, vulnerable themes/plugins, or unsecured hosting environments. Once deployed, they allow attackers to steal data, inject spam, redirect users, or sustain remote access over time. Unusual behavior like unexpected traffic redirects, suspicious files in `mu-plugins`, or high server load without cause are key indicators.

The use of the mu-plugins directory showcases how attackers adapt to evade conventional security tools. Regular file integrity monitoring, strict access controls, and well-configured WAFs remain essential to counter these tactics.

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