BENTECH

Excel Security Engineering: Bypassing and Managing Internal Protections

Master the architecture of OpenXML Excel files, understanding how sheet, workbook, read-only, and VBA project protections operate under the hood, and how to safely inspect and…

This hands-on course explores the internal structure of modern Microsoft Excel workbooks (.xlsx and .xlsm). Learners will gain an understanding of how OpenXML archives package data and how feature-level locks (worksheets, workbook structure, modify restrictions, and compiled VBA projects) are stored. Through practical exercises using text and hex editors, students will learn ethical recovery methods, file unblocking, and the clear distinction between container restrictions and full cryptographic file encryption.

Section 1: OpenXML Container Architecture & Sheet Security

Understand the ZIP packaging of Office OpenXML files, Windows Mark-of-the-Web mechanics, and sheet-level protection removal.

Lesson 1.1: Understanding the OpenXML Container and Unblocking Files

  • Mark of the Web (MOTW)
  • Zip package architecture
  • File backups

Modern Microsoft Excel files (.xlsx, .xlsm) are not monolithic binary blobs; they are standard PKZIP archives containing XML structures, relationship definitions, and media elements. Because Windows Defender and Microsoft 365 enforce strict security on downloaded files, workbooks originating from external sources often exhibit blocked macros or locked controls. To safely inspect and modify these workbooks, you must first clear the Mark of the Web (MOTW) attribute. Right-click the workbook in Windows Explorer, open Properties, and tick 'Unblock' under Security options. Always make an exact backup copy of the target file prior to modifying container extensions.

Practical work

  1. Create a test Excel workbook and configure a protected range.
  2. Inspect file Properties in Windows Explorer and verify if the Security Unblock option is present.
  3. Create an identical duplicate of the workbook to serve as a rollback backup.

Lesson 1.2: Removing Sheet Protection via XML Deletion

  • xl/worksheets directory
  • sheetProtection element
  • Archive repacking

Worksheet-level protection enforces cell locking rules defined in the UI. When protection is applied, Excel writes a `<sheetProtection>` tag inside the corresponding worksheet XML file located at `xl/worksheets/sheet{N}.xml`. By converting the file extension to `.zip`, extracting the package, and viewing the target sheet in a text editor like VS Code or Notepad, you can search for `sheetProtection`. Removing this element in its entirety—from `<sheetProtection` through `/>`—completely eliminates the sheet lock. Alternatively, setting attributes like `sheet="0"`, `objects="0"`, and `scenarios="0"` deactivates protection. When repacking the file, you must select the four root-level items (`_rels`, `docProps`, `xl`, `[Content_Types].xml`) and compress them directly into a zip file, then rename the file back to `.xlsx` or `.xlsm`.

Practical work

  1. Rename your protected test workbook to .zip and extract the files.
  2. Locate sheet1.xml in 'xl/worksheets/' and open it in a text editor.
  3. Search for 'sheetProtection', delete the entire tag, and save the file.
  4. Re-zip the four root items and restore the original extension to confirm that cells can now be edited.

Section 2: Workbook Structure & Read-Only Modify Restrictions

Learn how workbook-level constraints are defined in workbook.xml and how to modify structure locks and file-sharing tags.

Lesson 2.1: Unlocking Workbook Structure Protection

  • workbook.xml
  • lockStructure attribute
  • Sheet manipulation locks

Workbook protection prevents users from adding, deleting, hiding, or renaming worksheets. This setting is stored centrally in `xl/workbook.xml` within the `<workbookProtection>` element. Inside this element, Excel sets the attribute `lockStructure="1"` along with hash and salt values. Rather than deleting the entire element, you can safely set `lockStructure="0"` or remove the `<workbookProtection ... />` node entirely. Once re-zipped and converted back to an Excel extension, the workbook's sheets can be fully managed without prompting for a password.

Practical work

  1. Protect a workbook structure with a password via Review > Protect Workbook.
  2. Convert to .zip, open 'xl/workbook.xml', and locate 'workbookProtection'.
  3. Change lockStructure to '0', re-pack the archive, and verify that sheets can now be inserted or deleted.

Lesson 2.2: Bypassing Password to Modify (Read-Only)

  • fileSharing tag
  • General Options
  • Write-access restrictions

When an author saves a workbook via Tools > General Options with a 'Password to modify', Excel appends a `<fileSharing>` tag to `xl/workbook.xml`. This tag specifies reservation parameters and password hashes that trigger the 'Read Only or Enter Password' prompt when the workbook is opened. To permanently eliminate this prompt, extract the archive, open `xl/workbook.xml`, find `<fileSharing`, and delete the node up to its closing tag `/>`. Once repacked, Excel treats the file as standard read-write and opens it directly without modal authentication dialogues.

Practical work

  1. Save a test workbook with a modification password via File > Save As > Tools > General Options.
  2. Open the workbook archive and locate the 'fileSharing' element in 'xl/workbook.xml'.
  3. Remove the '<fileSharing ... />' node, rebuild the file, and open it to verify that the modify dialog no longer appears.

Section 3: VBA Binary Stream Hex-Editing and Encryption Limits

Master the binary manipulation of vbaProject.bin using hex editors, and understand the difference between feature locks and full encryption.

Lesson 3.1: Resetting VBA Project Locks via HxD Hex Editor

  • vbaProject.bin
  • Hex editing mechanics
  • DPB key invalidation
  • Project recompilation

Unlike worksheets and workbook settings, Visual Basic for Applications (VBA) project code is stored in a compiled binary stream named `xl/vbaProject.bin`. You cannot edit this file using standard XML or text editors without corrupting the file stream. To bypass a locked VBA project, open `vbaProject.bin` in a dedicated hex editor such as HxD. Search for the text string `DPB`. This key stores the compiled project password verification parameters. Overwrite `DPB` in-place with an arbitrary string such as `DPG` or `DBx`. Crucially, you must NEVER delete characters or press backspace, as doing so alters stream offsets and destroys the binary layout. Save the binary file, repack the workbook archive, and launch the file. Excel will present error warnings indicating that key 'DPG' is invalid—click Yes/OK through all prompts. Then open Developer > Visual Basic > Tools > VBAProject Properties > Protection, uncheck 'Lock project for viewing', clear all password fields, and save. Finally, use 'Save As' to overwrite or save the workbook under a new name; this forces Excel to compile a completely clean binary header with no residual errors.

Practical work

  1. Lock a sample VBA project with a password and close Excel.
  2. Extract the workbook and open 'xl/vbaProject.bin' in HxD Hex Editor.
  3. Locate text string 'DPB' and overwrite it with 'DPG' without altering the byte count.
  4. Repack the archive, open in Excel, bypass error dialogs, and clear protection in VBAProject Properties.
  5. Execute a 'Save As' and re-open to confirm error-free, unrestricted VBA access.

Lesson 3.2: Cryptographic Container Encryption vs. Feature Locks

  • Password to Open encryption
  • AES container encryption
  • Decryption workflow

Throughout this course, all bypassed protections have been feature-level constraints operating within an unencrypted OpenXML container. In contrast, configuring an 'Encrypt with Password' / 'Password to Open' under File > Info encrypts the entire workbook stream using strong symmetric cryptography (typically AES-128 or AES-256). When a workbook is encrypted with a file-open password, renaming the file to `.zip` will not expose the internal directory tree. The archive will be unreadable because the container itself is encrypted into an OLE Compound File. These files cannot be bypassed through XML or hex editing; they require the correct password to decrypt before protection can be cleared via File > Info > Protect Workbook.

Practical work

  1. Apply a 'Password to Open' to a test file via File > Info > Protect Workbook > Encrypt with Password.
  2. Attempt to rename the file to .zip and observe that extraction fails or produces unreadable encrypted streams.
  3. Decrypt the workbook legitimately using the password, then navigate to File > Info to remove the open password.