Does ransomware change file extension?

Ransomware is a type of malware that encrypts files on a victim’s computer and demands a ransom payment in order to decrypt the files. One of the key indicators of ransomware infection is that it will often change or append file extensions as part of the encryption process.

What is ransomware?

Ransomware is a form of malware that employs encryption to hold a victim’s information at ransom. A user’s data is encrypted so that they cannot access files, databases, or applications. The attackers then demand ransom money in cryptocurrency to provide the decryption key to unlock the data. Ransomware attacks have been steadily rising, affecting companies, government agencies, hospitals, and individuals.

Some of the most common ransomware variants include:

  • CryptoLocker
  • CryptoWall
  • Locky
  • WannaCry
  • Petya
  • Cerber
  • SamSam

Ransomware is typically spread through phishing emails containing malicious attachments or links. Once executed on the system, the ransomware encrypts files and displays a ransom message demanding payment. Most ransomware uses advanced encryption algorithms that are practically impossible to break without the decryption key.

Payment is demanded in cryptocurrency, such as Bitcoin, to maintain the attacker’s anonymity. The ransom note usually includes instructions for how to purchase and transfer the cryptocurrency. Some ransomware threats also set a deadline for payment, threatening to delete files forever or raise the ransom price if not paid in time.

How does ransomware encrypt files?

When ransomware infects a system, it uses encryption algorithms to encode files, making them inaccessible. Symmetric encryption is commonly used, where a single key is used to encrypt and decrypt the data. The ransomware generates a robust key and encrypts files using an algorithm like AES or RSA. The key is then concealed using asymmetric encryption, meaning it requires a different key to decrypt it.

The asymmetric private key remains with the attackers. They promise to provide the private key needed to decrypt files if the ransom is paid. Without that key, it is nearly impossible to recover encrypted files.

The ransomware encrypts files with certain extensions like documents, images, videos, databases, and archives. It traverses the file system, encrypting files on the hard drive and any connected storage devices. The ransomware usually deletes volume shadow copies and disables Windows System Restore to prevent recovering previous versions of files.

How does ransomware change file extensions?

A common indicator of ransomware infection is that it renames encrypted files by appending or altering the file extension. This helps the malware keep track of files it has encrypted. The new extension is usually something random generated by the ransomware.

For example, the Locky ransomware renames encrypted files by appending extensions like .locky, .zepto, .odin, .shit, .thor, .aesir, and more. Other ransomware families also use a technique called “double extension” where the original file extension is preserved, but a new extension is added at the end.

Here are some examples of renamed file extensions after encryption by various ransomware variants:

Original File Renamed by Ransomware
document.docx document.docx.locky
sample.jpg sample.jpg.xtbl
image.png image.png.crypt
database.sql database.sql.encrypted

Appending a new extension helps ransomware keep track of files it has processed. It also has the psychological impact of making it instantly clear that files have been tampered with. When victims see unfamiliar file extensions, they recognize the ransomware attack.

Why does ransomware change file extensions?

There are a few key reasons ransomware attackers change or append file extensions as part of the encryption process:

  • To identify encrypted files: The new extension allows the ransomware to identify which files it has already encrypted. This prevents re-encrypting the same files, which would waste CPU cycles.
  • To make encryption obvious: Appending a new extension makes it immediately clear that files have been tampered with. This sends the message that files are locked up until the ransom is paid.
  • To render files inaccessible: Changing the file extension can prevent programs from opening encrypted files, ensuring they remain locked until decrypted.
  • To prevent recovery: By renaming original files, ransomware makes it harder to recover previous versions or to recognize which encryption key decrypts which files.

Overall, the change in file extension is an integral part of how ransomware encrypts and locks access to files. It aims to maximize damage and fear while demanding ransom payment from victims.

What types of files does ransomware target?

Ransomware aims to encrypt files that are valuable to victims and likely to elicit payment. Files with the following extensions are commonly targeted:

  • Office files: .docx, .xlsx, .pptx
  • Images: .jpg, .jpeg, .png, .bmp
  • SQL databases: .sql
  • Archives: .zip, .rar
  • Email databases: .pst, .ost
  • Multimedia: .mp3, .mp4, .wav
  • Web files: .html, .php
  • Programs: .exe
  • Other: .pdf, .txt, .csv

Files critical to a business or individual are targeted, like documents, databases, archives, and media files. Encrypting these file types allows attackers to cause maximum disruption and damage in order to extort higher ransom payments.

Can files be recovered after ransomware encryption?

Recovering encrypted files without paying the ransom is often very difficult or impossible with modern ransomware. However, there are some recovery methods that may work in certain cases:

  • File backups: Backups allow you to wipe the infected system and restore files from before the attack. However, backups must be offline so they aren’t encrypted too.
  • Shadow volume copies: Volume shadow copies store previous Windows file versions that may be restored. But many ransomware strains now delete volume shadow copies.
  • Ransomware decryption tools: For some older strains, security researchers have broken the encryption and released free decryption tools.
  • Ransomware decryption keys: In rare cases, keys are recovered through bugs in the malware code or law enforcement operations.
  • File recovery software: Software like Photorec can sometimes recover remnants of files. But this depends on disk structure, file fragmentation, and other factors.

Prevention is truly the best defense against ransomware campaigns infecting systems and encrypting files. But in the event of infection, having offline backups offers the best chance of recovering encrypted files without paying the ransom.

Should ransom be paid to decrypt files?

There is ongoing debate about whether ransom payments should ever be made to ransomware attackers. Here are some key considerations:

  • Paying the ransom provides funds for criminals and incentives for more attacks.
  • There is no guarantee files will be recovered after payment. Attackers sometimes just take the money.
  • However, for businesses or entities where downtime is costly, payment may be the quickest way to resume operations.
  • Individuals are less likely to pay ransoms, but business insurance policies sometimes cover ransomware payments.
  • Governments do not recommend payment, but some victims pay as a last resort if data is invaluable.
  • Payment should only be considered along with other recovery efforts like backups.

There are reasonable arguments on both sides of this issue. Each victim needs to evaluate their own situation. Having reliable backups in place provides more options if ransomware strikes.

How can file extensions help identify ransomware?

The altering of file extensions is a tell-tale sign of ransomware infection. Some ways file extensions help identify ransomware attacks include:

  • Unknown extensions like .locky or .crypt appended to files indicate ransomware encryption.
  • Double extensions like .jpg.xtbl show original and new ransomware extensions.
  • Missing file extensions may mean ransomware stripped them during encryption.
  • File icons changed to generic icons signify ransomware encrypted those file types.
  • Certain extensions like .crypt, .lock, .encrypted, .xyz, .xxx, .ttt, .zzz, and .aaa are high-risk ransomware indicators.

IT staff should scan for these file extension warning signs to detect ransomware outbreaks early before significant damage occurs. Anti-malware software can also leverage these ransomware behaviors to halt encryption processes.

What are the best practices for preventing ransomware infections?

These are some best practices organizations and individuals can follow to prevent, detect, and respond to ransomware threats:

  • Back up critical data regularly and keep backups offline.
  • Be cautious of suspicious emails and do not open attachments or enable macros from senders you don’t know.
  • Install software patches and updates as soon as available.
  • Use anti-malware and anti-ransomware tools to scan systems.
  • Disable Remote Desktop Protocol (RDP) if not required.
  • Configure firewalls to block access to known ransomware command and control servers.
  • Restrict application installations to prevent risky apps.
  • Develop an incident response plan for ransomware and cyberattacks.
  • Educate employees on cybersecurity best practices and risks.

Ransomware adapts quickly, so locking down defenses and planning recovery options are key to managing risk. Understanding typical behaviors like file extension changes can also help spot ransomware activity.

Should file extensions guide whether ransom is paid?

The appended file extension is more a characteristic of how ransomware encrypts than an indicator of how readily files can be recovered. Some points on file extensions and paying ransom demands:

  • Unknown extensions like .xyz don’t inherently mean files are less recoverable than .locky extensions.
  • Multiple extensions like .jpg.crypt still mean standard encryption algorithms were used.
  • Paying based on extension alone is not recommended, as criminals can easily change tactics.
  • Backups, security software, and decryption tools should guide paying, not extensions.
  • However, unknown extensions may suggest new ransomware variants warranting more caution.
  • File extensions provide insight into ransomware traits but not necessarily decryptability.

While appended file extensions are an indicator of encryption, they should not solely dictate whether ransom is paid. The overall infection, impact, and recovery options guide ransom decisions more than file extensions alone.


Ransomware frequently changes or appends file extensions as part of the encryption process to identify locked files, render them inaccessible, and communicate damage to victims. Commonly targeted files include documents, databases, images, archives, and backups. While encrypted files can rarely be recovered without the decryption key, methods like file backups provide recovery options in case of ransomware infection. Understanding the typical file extension manipulation behavior can help detect and respond to ransomware more effectively across an organization.