How Unrar Handles Locked RAR Archives

This article explains how the unrar utility detects, interprets, and processes archives marked with a locked status. While the locked attribute permanently prevents changes to an archive’s contents, unrar is specifically designed for decompression rather than creation or modification. Understanding how the utility reads header flags, executes extraction commands, and displays archive metadata clarifies why extraction succeeds even when alteration is blocked.

The Purpose of the Locked Flag in RAR Archives

The locked attribute is a security measure applied during or after archive creation using the -k switch in the full rar utility or WinRAR. Its sole purpose is to prevent accidental or intentional changes, such as adding, deleting, updating, or renaming files within the archive.

At the file structure level, this lock is stored as a specific bit flag inside the main archive header:

  • In RAR 4.x formats, the archive header contains a flags field where the bit mask 0x0004 designates a locked archive.
  • In RAR 5.x formats, the lock property is defined within the main archive header block metadata.

How unrar Processes the Lock During Decompression

The unrar command-line program is a standalone extraction tool compiled strictly with read and unpack routines; it contains no code to write, modify, or append archive data. Because of this architectural separation, unrar treats locked archives with the following behavior:

  1. Header Parsing: When opening an archive, unrar reads the main archive block to verify its integrity, check for encryption, and identify global flags. It reads the lock bit to determine the archive's state.
  2. Standard Extraction Execution: Commands such as unrar x (extract with full paths) and unrar e (extract to current directory) execute without restriction. The locked flag does not restrict read access, decompression algorithms, or checksum verification (CRC32 or BLAKE2sp). As long as the archive is not corrupted and any required decryption passwords are provided, unpacking proceeds normally.
  3. Information and Listing: When using commands such as unrar l (list) or unrar v (verbose list), the utility parses the lock flag and prints the status to the terminal output (typically displayed as Lock or Archive is locked in the summary block).

Difference Between unrar and the Full rar Tool

The impact of the locked state is only observable when attempting write operations. Because unrar inherently lacks commands like a (add), d (delete), u (update), or c (comment), it cannot trigger lock-related errors.

If a user attempts modification using the standard rar binary rather than unrar, the process aborts immediately:

# Attempting to delete a file using the standard rar utility
rar d locked_archive.rar file.txt

Output:

Cannot modify locked archive

In contrast, running extraction via unrar:

# Extracting files remains unaffected
unrar x locked_archive.rar

The operation completes successfully because the locked flag restricts write operations on the archive container, not read or unpacking operations on the compressed streams inside it.