No. BitLocker does not stop ransomware from encrypting files on an unlocked Windows drive. It encrypts a volume to help prevent someone from reading its data when a device is lost, stolen, or taken out of service. Once you sign in and Windows unlocks the volume, ransomware running with your access can still change or encrypt files. BitLocker helps protect data at rest; protected backups and a recovery plan help you get data back after an attack.
What BitLocker protects—and what it does not
Microsoft describes BitLocker as a Windows feature that encrypts entire volumes to address data theft or exposure from lost, stolen, or improperly decommissioned devices. When a computer is powered off, encryption helps keep someone with physical access to its drive from simply reading the stored files.
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That protection is different from stopping malware. After Windows starts and unlocks the volume, authorized software can work with its files. Ransomware running under your account can therefore encrypt or alter accessible data. BitLocker is not a filter that recognizes ransomware or blocks its file operations.
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| Control | Main purpose | What it does not do |
|---|---|---|
| BitLocker volume encryption | Helps protect data from offline access if a device or drive is lost, stolen, or improperly retired. | Does not prevent ransomware from changing files on an unlocked volume. |
| Protected backup and recovery plan | Provides a way to restore data after ransomware or other data loss. | A backup is not dependable if an attacker can also erase or encrypt it. |
How TPM, a PIN, and a startup key affect protection
BitLocker can use a Trusted Platform Module (TPM) to help check that a device has not been tampered with while offline. Depending on the configuration, startup can also require a preboot PIN or a USB startup key. These controls affect access before Windows unlocks the drive; they do not change what ransomware can do after unlock.
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TPM-only startup
TPM-only startup is more convenient because it avoids an extra preboot prompt. It still provides BitLocker’s volume encryption and TPM-based startup checks.
TPM with a PIN or startup key
Adding a PIN or startup key adds a factor required before Windows starts. The tradeoff is operational: a forgotten PIN or lost key can prevent access until recovery, and preboot prompts can make unattended restarts harder.
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Keep recovery information separate and accessible
A BitLocker recovery password is a 48-digit number. Depending on the device and how it is managed, recovery information may be stored in a Microsoft account, Microsoft Entra ID, Active Directory Domain Services, a file, a printout, or removable media. Microsoft advises keeping it secure and separate from the protected device, because someone who obtains the recovery information may be able to unlock the volume.
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Store the recovery information somewhere you can reach if the PC is unavailable, while protecting it from unauthorized access. A USB drive can be used to store recovery information or serve as a startup key, but it does not prevent ransomware.
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A key package may help recover portions of a severely damaged volume when used with the required recovery information. It is a recovery aid, not a backup. Microsoft warns that BitLocker is designed to make an encrypted drive unrecoverable without the required authentication; losing recovery information can therefore mean losing access to the data.
Choose used-space encryption with the drive’s history in mind
When turning on BitLocker for an existing drive, used-space-only encryption can leave remnants of previously stored, unencrypted data recoverable until those sectors are overwritten. For a brand-new volume, newly written data is encrypted, so that specific concern about old data does not apply. The choice can reduce deployment time, but it should reflect whether the drive has held sensitive data before encryption.
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Build ransomware recovery around protected backups
Microsoft’s ransomware guidance recommends a recovery plan and backups that attackers cannot deliberately erase or encrypt. It identifies immutable storage as an important way to support restoration. The practical test for any backup is whether an attacker who compromises the PC could also modify or destroy the backup.
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- Consider immutable storage for critical data so stored copies cannot be altered during their protected period.
- Make sure recovery information and backup access are available through a plan that does not depend on the affected PC.
An ordinary external drive left connected is not automatically ransomware-safe: if the compromised computer can write to it, malware may be able to damage its contents too.
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Performance impact in context
Microsoft’s BitLocker FAQ characterizes performance overhead as typically small and often in the single-digit percentages relative to storage-operation throughput. That is a general vendor statement, not a benchmark for every PC, workload, or drive.
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