Dell BIOS Passwords Weak XOR Encryption Allows Recovery from...

[carlospolop]

🤖 Automated Content Update

This PR was automatically generated by the HackTricks News Bot based on a technical blog post.

📝 Source Information

• Blog URL: https://mdsec.co.uk/2026/07/dell-bios-passwords-weak-xor-encryption-allows-recovery-from-spi-flash-cve-2026-40639
• Blog Title: Dell BIOS Passwords: Weak XOR Encryption Allows Recovery from SPI Flash (CVE-2026-40639)
• Suggested Section: Hardware/Physical Access > Firmware Analysis, with cross-references from Crypto > Symmetric Crypto and Hardware/Physical Access > Physical Attacks

🎯 Content Summary

Overview

CVE-2026-40639, disclosed by Dell in DSA-2026-197 on June 9, 2026, is a weak password-encoding vulnerability in Dell client BIOS firmware. Affected platforms store BIOS administrator and user passwords as reversibly XOR-encrypted plaintext in the proprietary DVAR region of SPI flash instead of using a one-way password hash. Each password occupies a 32-byte field encrypted with a repeating 20-byte key, while the first password...

🔧 Technical Details

Locating undocumented firmware data through differential flash analysis: Take a complete SPI-flash image, change exactly one firmware setting, acquire another image, and diff the results. Appended records instead of in-place updates can reveal a log-structured variable store and preserve historical values. Extract relevant firmware modules with tools such as binwalk and decompile the responsible UEFI or SMM driver with Ghidra to confirm the record layout, transformation routine, and key derivation.

Recovering repeating-XOR keys from encrypted padding: When a fixed-size secret is zero-padded before repeating-key XOR, every padding byte exposes a key byte because 0x00 XOR key = key. If the padding covers every key index at least once, reconstruct the complete key from the ciphertext tail and decrypt populated positions with plaintext[i] =...

🤖 Agent Actions

Plan:

1. Map relevant hardware, firmware, SPI, UEFI, and XOR documentation.
2. Read at least five candidate pages and check for duplication/style.
3. Verify primary references and select one best existing page.
4. Implement a concise, cited technical addition.
5. Validate links, formatting, and the final diff.
   Progress: the firmware-analysis index is the best fit because it already covers SPI acquisition, binary extraction, differential analysis, and reverse engineering. I’m now checking the repository for any existing DVAR, BIOS-password, historical-record, or null-padding XOR coverage before editing only that page.
   Decision: I’ll update only src/hardware-physical-access/firmware-analysis/README.md. The physical-attacks page mentions BIOS-password bypasses, and the symmetric-crypto page covers generic XOR, but splitting this technique across them would duplicate content; the complete workflow belongs in firmware analysis.
   Implementation is complete. I’m validating the single-file diff now: whitespace, citation numbering, reference placement, forbidden-source absence, code fences, and repository status.
   Updated src/hardware-physical-access/firmware-analysis/README.md with:

• Differential SPI-flash analysis workflow.
• Repeating-XOR key recovery from zero padding.
• Dell DVAR record layout and 12-character recovery boundary.
• Historical-record key lifting and weak key derivation.
• Current dellpwn usage.
• False-positive and bit-flip handling guidance.
• Technical mitigation guidance and numbered references.

Validation passed with git diff --check, balanced code fences, resolved citations, and no forbidden sources. mdbook was unavailable, so a full build was not run.

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[carlospolop]

🔗 Additional Context

Original Blog Post: https://mdsec.co.uk/2026/07/dell-bios-passwords-weak-xor-encryption-allows-recovery-from-spi-flash-cve-2026-40639

Content Categories: Based on the analysis, this content was categorized under "Hardware/Physical Access > Firmware Analysis, with cross-references from Crypto > Symmetric Crypto and Hardware/Physical Access > Physical Attacks".

Repository Maintenance:

• MD Files Formatting: 1029 files processed

Review Notes:

• This content was automatically processed and may require human review for accuracy
• Check that the placement within the repository structure is appropriate
• Verify that all technical details are correct and up-to-date
• All .md files have been checked for proper formatting (headers, includes, etc.)

Bot Version: HackTricks News Bot v1.0