Linux Malware (August 2025): Fileless RAR Filename Exploit Delivers VShell Backdoor

In August 2025, researchers uncovered a novel Linux malware campaign that hides malicious code in a RAR archive filename. This fileless Linux attack begins with a phishing email disguised as a survey invitation. The email carries a RAR archive whose single file entry has a specially crafted name containing a hidden Bash command. When a Linux shell automatically processes that filename (for example, by listing directory contents), the Base64-encoded payload is executed. This launches a script that downloads a system-specific ELF loader. The loader connects to a command-and-control (C2) server to retrieve and decrypt the VShell backdoor directly in memorythehackernews.comtrellix.com. Traditional antivirus tools often miss this because they typically scan file contents, not filenamesthehackernews.com.

How the RAR Filename Exploit Works

The attack begins with a phishing email disguised as a beauty product survey offering a small rewardtrellix.com. Crucially, the email includes a RAR archive attachment (e.g. yy.rar), even though it doesn’t explicitly tell the user to open it. Inside that RAR is a file named something like:

ziliao2.pdf`{echo,<Base64>}|{base64,-d}|bash`

This filename looks like a normal PDF document (ziliao2.pdf) followed by backticks with shell commands. Simply extracting the archive does nothing. Instead, when a shell command (such as a loop or eval) processes this filename, the backticks cause the hidden commands to executetrellix.comtrellix.com. That command decodes and runs a small Bash downloader script.

• Phishing Email: A fake survey invitation (with a small reward) delivers the RAR archivetrellix.com.
• Weaponized Filename: The RAR contains a file whose name embeds an encoded Bash payload (using backticks and Base64)trellix.com.
• Automatic Trigger: When the system lists or evaluates filenames in a script (e.g. using eval, echo, or ls), the payload in the filename is decoded and executedtrellix.comtrellix.com.
• Downloader Stage: The executed payload runs a Bash script that detects the CPU architecture and downloads a matching ELF binary from the attacker’s servertrellix.com.
• VShell Backdoor: The downloaded ELF connects to a C2 server, retrieves an encrypted VShell payload, decrypts it in memory, and executes it on the hostthehackernews.comtrellix.com.

Figure: Linux malware infection chain. (1) A phishing email with a RAR archive is received. (2) A hidden Bash script in the archive’s filename triggers when processed by a shell, downloading and running a loader. (3) The loader fetches and executes the VShell backdoor in memorytrellix.comtrellix.com.

Why Fileless Attacks Evade Detection

This incident shows how attackers exploit common scripting habits. Many Linux admins use quick shell loops or automated scripts to process files. If those scripts aren’t careful, even a seemingly safe command can launch malware. As Trellix explains, a simple loop with eval "echo $f" can act as a launchpad for arbitrary code when filenames contain hidden commandstrellix.comtrellix.com. Because the malicious payload is encoded in the filename, traditional defenses often fail:

• No Disk Footprint: The backdoor runs in RAM with no on-disk binary. Traditional antivirus tools (which scan files on disk) typically won’t detect ittrellix.com.
• Masqueraded Process: The VShell process hides in plain sight by using a name like a system thread ([kworker/*]), making it harder to spot in process listingstrellix.com.
• Multi-Architecture Loader: The initial script detects the CPU architecture (x86_64, ARM, etc.) and retrieves a matching binarytrellix.com. This lets the same exploit target servers, IoT devices, and cloud containers alike.
• Filename Obfuscation: Since most antivirus engines do not analyze archive entry names, the Base64-encoded payload in the filename bypasses signature scansthehackernews.comtrellix.com.

In summary, this fileless Linux attack demonstrates that even well-armed Linux systems can be compromised by low-complexity tricks. Attackers are increasingly weaponizing simple behaviors and metadata (like filenames) to bypass traditional security models

Protecting Against Linux Phishing Malware

Preventing such attacks requires both user vigilance and technical controls:

• Treat Filenames as Untrusted: Avoid processing downloaded files with unsafe shell loops or using eval on filenames. Use safe iteration (for example, null-delimited loops) when handling file lists.
• Scan and Filter Attachments: Use email security tools that inspect not only file content but also archive entries. Flag any attachments with unusual characters (backticks, pipes, base64, etc.) in their names.
• Monitor for Anomalies: Log and alert on suspicious system activity. For example, watch for new processes that appear immediately after extracting attachments, especially if they use kernel-like thread names.
• Harden Email Policies: Limit automatic execution of scripts from emails, and educate users to be wary of unexpected attachments even in Linux environments.
• Stay Updated: Follow security advisories and research blogs (like Trellix’s analysis or other threat reportstrellix.comsysdig.com) to keep up with emerging Linux malware techniques.

Conclusion: The August 2025 Linux malware incident highlights a clever new attack tactic: hiding a backdoor entirely in a file name. By weaponizing a RAR filename, attackers can deliver a powerful fileless VShell backdoor that bypasses traditional defenses. System administrators should review their shell scripts and archive-handling routines to ensure they aren’t inadvertently executing filenames as code. Stay informed by following security advisories and expert research. Subscribe to our newsletter or follow our updates for ongoing coverage of Linux security trends.