Full Report
Recently, we came across some firmware samples from D-Link routers that we were unable to unpack properly. Luckily, we got our hands on an older, cheaper but similar device (DIR882) that we could analyze more closely. The goal is to find a way to mitigate the firmware encryption that was
Analysis Summary
# Tool/Technique: D-Link Firmware Encryption/Decryption Mechanism
## Overview
This summary focuses on the observed firmware encryption scheme applied to D-Link router firmware (specifically DIR-3060 and DIR-882 models) which prevents standard static analysis tools like `binwalk` from extracting embedded file systems. The analysis identifies a custom (likely proprietary) encryption method, confirmed by the consistently high entropy across the firmware binaries, and points to a specific utility, `imgdecrypt`, as the tool used for decryption on the target devices.
## Technical Details
- Type: Technique/Custom Utility (Firmware Security Feature)
- Platform: MIPS-based embedded systems (D-Link Routers, e.g., DIR-3060, DIR-882)
- Capabilities: Encrypts firmware images, resulting in high entropy data streams (>0.97) preventing easy extraction of file systems. The decryption routine (`imgdecrypt`) appears capable of handling cryptographic operations like AES and RSA verification.
- First Seen: Firmware v1.02B03 for DIR-3060 (October 2019) and v1.30B10 for DIR-882 (February 2020).
## MITRE ATT&CK Mapping
Since this describes a defensive measure (firmware protection) being bypassed for analysis, direct mapping is challenging, but the goal aligns with intelligence gathering on the system:
- **TA0008 - Lateral Movement**
- T1059.004 - Command and Scripting Interpreter: Unix Shell (Used to analyze the router environment once access to the firmware is achieved).
- **TA0005 - Defense Evasion**
- T1027 - Obfuscated Files or Information (The encryption itself serves as obfuscation).
## Functionality
### Core Capabilities (Firmware Protection)
- Firmware images exhibit constant, high entropy (near 1.0), indicating they are encrypted or compressed data from offset 0x0.
- Initial bytes observed: `SHRS` signature sequence (seen in both DIR-3060 and DIR-882 initial headers).
### Advanced Features (Decryption Utility - `imgdecrypt`)
The identified `imgdecrypt` binary suggests the presence of strong cryptographic primitives used in the decryption routine:
- Supports decryption routines related to **AES** and potentially **RSA** verification.
- Implements cryptographic hashing functions like **SHA512**.
- Compiled as an **ELF 32-bit executable for MIPS architecture**.
## Indicators of Compromise
(Indications related to the firmware file structure itself, not active malware infection)
- File Hashes:
- DIR-3060 v1.02B03 Firmware: `86e3f7baebf4178920c767611ec2ba50`
- DIR-882 Firmware Sample: `89a80526d68842531fe29170cbd596c3`
- `imgdecrypt` utility hash: `a5474af860606f035e4b84bd31fc17a1`
- File Names: `DIR-3060_RevA_Firmware111B01.bin`, `DIR_882_FW120B06.BIN`
- Network Indicators: None directly observed in the analysis phase (focus was on static decryption).
- Behavioral Indicators: Firmware analysis using `binwalk` reports no identifiable file sections and shows rapidly rising entropy edge at offset 0x0.
## Associated Threat Actors
- This is a defensive mechanism deployed by the vendor (D-Link). The analysis is performed by security researchers attempting to reverse-engineer the protection mechanism. No specific threat actor is associated with the *creation* of this defense.
## Detection Methods
- Signature-based detection: Look for the `SHRS` magic sequence in the first 4 bytes of firmware images.
- Behavioral detection: Monitoring for firmware update processes that result in files with sustained high entropy histograms using tools like `binwalk -E`.
## Mitigation Strategies
- For researchers: Focus analysis on the target device's operating environment (UART/serial console) to locate the decryption utility (`imgdecrypt` on MIPS target) if the firmware itself cannot be unpacked statically.
- For devices: Organizations deploying these routers should monitor for unauthorized firmware modification or patching based on identified decryption routines.
- **Note**: The primary mitigation discussed is developing a custom decryption routine based on analyzing the `imgdecrypt` binary's MIPS assembly instructions identified on the target platform.
## Related Tools/Techniques
- **MIPS Assembly/Architecture**: The analysis required understanding MIPS32 instruction sets (XOR, ADD, Jumps/Branches) to reverse-engineer the decryption routine.
- **binwalk**: Used for initial reconnaissance and entropy analysis to confirm encryption.
- **IDA Pro**: Mentioned as the next step for deep disassembly and analysis of the `imgdecrypt` binary.