ZIP file encrypted by ransomware
1️⃣ Symptoms of a ZIP file encrypted by ransomware
When ransomware targets ZIP files, the most common symptoms include:- Extension modification
- The .zip file is renamed with a new extension specific to the ransomware, for example: .locked, .anubis, .lockbit, .cl0p.
- Files may appear “intact” at first glance but become inaccessible.
- Unreadable or corrupted files
- Attempts to open the file with WinZip, 7-Zip or similar tools fail.
- Typical error messages include:
- “Cannot open file. The archive is corrupted.”
- “Invalid archive or wrong password.”
- Size changes
- Some ransomware overwrites the original file after encryption, sometimes leaving a file of identical or slightly altered size.
- Internal archive metadata (header, central directory) is often corrupted.
- Associated ransom note
- Often placed in the same folder, typically a text or HTML file such as README.txt or RESTORE_FILES.html.
- Provides decryption instructions, sometimes via a TOR URL.
- Possible propagation
- If the ZIP contained other sensitive files, these are also encrypted.
2️⃣ Impact on data recovery
ZIP files encrypted by ransomware present specific challenges for recovery:- Strong encryption
- Most modern ransomware uses AES-256 or ECIES.
- Without the key provided by the ransomware, decryption is practically impossible.
- Archive corruption
- Ransomware may alter the ZIP header or the file table, making standard recovery tools ineffective.
- Even if the original files are still present on the disk, their content is inaccessible.
- Partial wiping or destruction
- Some ransomware strains include a “wiper” module that deletes file contents after encryption.
- In such cases, full recovery is impossible, even with advanced software.
- Recovery via backups
- Intact ZIP files can only be restored if offline or immutable backups exist.
- Corrupted or partially overwritten files require advanced techniques (manual header reconstruction or sector-level reconstruction).
- Increasing complexity for multi-level archives
- ZIP files containing nested ZIPs or complex compressed formats (RAR, 7z) increase the difficulty.
3️⃣ Time required to recover an encrypted ZIP file
Recovery time depends heavily on three factors:- File size and quantity
- Small ZIPs (<500 MB): a few hours with specialized tools.
- Large ZIPs (>10 GB) or multi-ZIP sets: several days.
- Archive condition
- Intact but encrypted archive: impossible to recover without the key.
- Corrupted archive: manual reconstruction or sector-level recovery may take 1 to 2 weeks depending on complexity and disk integrity.
- Recovery method
- Standard tools (WinRAR Repair, 7-Zip, Disk Drill): partial recovery possible if only the header is damaged.
- Professional recovery (ScanX, EnCase, advanced R-Studio):
- Full sector scan → maximum possible extraction.
- Estimated time: 2 to 7 days for a multi-TB NAS or hard drive.
4️⃣ Best practices for handling an encrypted ZIP file
- Never attempt to re-open or modify the archive
- Any write operation on the disk may compromise unencrypted sectors.
- Immediate isolation
- Move the file to an offline external storage device to prevent further spread.
- Preliminary analysis
- Identify the ransomware type and the encryption used.
- Check for immutable or offline backups.
- Specialized professionals
- Contact a post-ransomware data recovery company to avoid total data loss.
- Use advanced software and techniques (BTRFS, Ext4, NTFS sector-based recovery).
- Documentation for insurance
- Take screenshots, note the ransom note, file size, and new extension.
- Useful for cyber-insurance claims.
Summary
| Aspect | Details |
| Symptoms | Modified extension, unable to open, ransom note, unreadable files |
| Impact on recovery | Strong encryption, archive corruption, possible wiper, impossible without key |
| Recovery time | A few hours for small files, 2–7 days for multi-TB archives on damaged storage |
| Possible methods | Immutable backups, manual reconstruction, advanced professional tools |