## Revised Article

Revised Article

Gootloader's Malformed ZIP Archives Evade Detection While Enabling Targeted Execution

Look, threat intel analysts just exposed something sneaky: hackers are using deliberately broken ZIP archives to bypass security tools. At the center? Gootloader—malware we've seen before helping ransomware gangs get that crucial first foothold. These thieves specialize in breaking in, then selling system access to other criminals. Their secret? Staying completely hidden until the malware activates.

Now, Huntress Labs and independent researchers spotted Gootloader's return in November 2025 after years offline. Turns out, its creators are teaming up again with the ransomware group Vanilla Tempest (yep, the Rhysida ransomware crew). While later infection steps grab headlines, here's the real story: that messy ZIP container itself is their primary evasion trick.

Operational Mechanics Subvert Analysis Tools

So how does this hit your network? Victims get a ZIP packed with scrambled JScript files. Sure, PowerShell persistence hooks kick in upon opening—but the real wizardry lies in the archive's physical quirks. We saw this fingerprint back in 2021–2023, and it’s back: the archives are deliberately mangled to sabotage standard unpacking tools.

Think WinRAR, 7zip, even specialized malware tools choke trying to extract these. But guess what doesn't? Windows’ own decompressor—it handles the mess flawlessly. That genius move lets attackers distribute while ensuring smooth execution. Perfect for access brokers avoiding detections!

Structural Anomalies Enable Detection Evasion

Peeking into the HEX code reveals two clever dodges:

1. Multiple Archive Concatenation: Each "ZIP" is actually 500–1000 mini-ZIPs glued together randomly during creation. Why doesn’t Windows care? Its ZIP checker only focuses on end-of-file signatures. The random count guarantees every archive's crypto hash (SHA256/SHA1/MD5) stays unique.

2. Truncated Core Structures: Hackers shave off exactly two bytes in the "End of Central Directory" section. Even better—they fill "Disk Number" and "Number of Disks" fields with junk values, tricking tools expecting multi-volume archives.

The result? Killer hashbusting. Minor changes create globally unique IDs for every archive. Static-hash threat intel just rolls over dead. Scarier yet: our scans show this pushed Gootloader to 11% of all malware slipping past endpoint security lately.

File Format Integrity Breakdown

Standard ZIPS? They’ve got three clean layers. Gootloader’s? Malcat and ImHex tools expose chaos:

• Central Directory metadata gets random timestamps, OS flags, and size tweaks
• Payload CRC checksums mismatch between headers/directory listings—corrupting verification math during unpacking
• Comment fields are snipped precisely where parsers expect standard byte positions

These "flaws" alone crash forensic tools, but Windows shrugs them off. Oh, and manually fixing the missing bytes? Still useless—they've layered randomization traps beneath.

Defensive Countermeasures

Fighting this demands smarter shields:

• YARA Signatures: Hit them where it hurts—target Local Header duplication and End Directory truncation:

rule gootloader_zip_archive_2025_11_17 : malware {  
meta: [Metadata Data Fields Preserved Exactly]  
strings: [Original Hexadecimal Definitions Unmodified]  
}  

• Application-Script Blocklisting: Block all Wscript calls launching from %LOCALAPPDATA%\Temp\*—fraudsters love that dumpster.
• File Association Reconfiguration: Force .JS/.JSE files to open in Notepad instead of Wscript.exe via Group Policy.

Operational Reality Assessment

With every twist, Gootloader’s creators prove they’re pouring cash into undermining defenses. That ZIP-generation trick? Solid technical circus act—Randy McEoin spotted browser-side XOR decoding/stiching tricks that up the complexity. But don’t panic: doubling down on archive-structure scanners while clamping script execution? That kills scams early, slashing breach risks cold.