Inspect EFI System Partition Images Using 7-Zip

7-Zip can directly open and inspect EFI System Partition (ESP) image payloads without requiring administrative mounting tools or third-party disk virtualization software. Because the Unified Extensible Firmware Interface (UEFI) specification mandates the FAT file system family (FAT12, FAT16, and FAT32) for the ESP, 7-Zip’s built-in file system parsers can read these images out of the box. This article details which payload types are supported, how to inspect them, and the limitations of this approach.

Supported EFI Image Formats

7-Zip treats raw disk and partition images as standard archives. You can inspect an EFI payload if it is packaged in any of the following formats:

  • Raw Partition Dumps: Sector-by-sector copies saved with extensions like .img, .raw, or .bin.
  • El Torito Boot Images: EFI boot payloads extracted from installation media, such as the efisys.bin or efisys_noprompt.bin files found inside Windows and Linux installation ISOs.
  • Virtual Disk Images: Virtual hard disk formats including .vhd, .vhdx, and .vmdk that contain an EFI partition table and payload.
  • Bootable ISO Files: 7-Zip can navigate directly through a full .iso image to locate and nestedly open the embedded EFI boot image.

How to Inspect an ESP Payload

  1. Locate or Extract the Image: Ensure the EFI payload is accessible as a discrete file (for example, efi.img from a Linux distribution or extracted from a firmware update bundle).
  2. Open with 7-Zip: Right-click the image file, select 7-Zip, and choose Open archive. Alternatively, drag the file directly into the 7-Zip File Manager interface.
  3. Navigate the File Tree: 7-Zip presents the FAT file system structure just like a standard archive. You can traverse standard paths such as \EFI\BOOT\, \EFI\Microsoft\, or vendor-specific directories.
  4. Examine Contents: Within the interface, you can directly view text-based configuration files (such as grub.cfg or startup scripts) using the built-in viewer (F3), or extract specific .efi executable binaries (such as bootx64.efi) to your local machine for reverse engineering or hash verification.

Practical Use Cases

  • Bootloader Verification: Verifying the presence, version, and digital signatures of bootloader binaries before deployment.
  • Configuration Auditing: Reading GRUB configurations, loader entries, or systemd-boot settings directly from an offline image.
  • Malware and Rootkit Analysis: Inspecting an offline dump of a suspected compromise to look for unauthorized EFI binaries in the boot chain.

Limitations

While 7-Zip is effective for inspection, it has specific constraints:

  • Read-Only Access: 7-Zip can extract and view files within FAT/ESP image payloads, but it does not support modifying or writing files back into the disk image.
  • Partition Table Traversal: If the image is an entire disk containing multiple partitions (GPT/MBR) rather than a standalone partition dump, you may need to double-click into the specific FAT/ESP volume listed in the primary archive view.
  • Encryption: If the payload is encrypted or resides within an encrypted container (such as BitLocker), 7-Zip cannot read the file system without prior decryption.