Introduction
Modern IoT devices rely on over-the-air firmware updates to patch vulnerabilities and deliver new features. However, the update protocol itself can become an attack surface if not properly secured.
In this research, we analyze the firmware update mechanism used by a popular line of smart home devices, uncovering a critical vulnerability in the signature verification process.
Protocol Structure
The update process follows a three-phase handshake between the device and the update server:
POST /api/v1/update/check
{ "device_id": "...", "fw_version": "2.1.4" }The server responds with an update manifest containing the download URL, expected hash, and signature.
Vulnerability in Signature Verification
During Phase 3, the device downloads the firmware binary and verifies its signature. We discovered that the verification implementation uses a truncated SHA-256 hash, comparing only the first 16 bytes instead of the full 32.
This drastically reduces the collision resistance from 2^128 to 2^64 — well within the range of practical birthday attacks.
Exploitation
By crafting a firmware image with a partial hash collision, we were able to bypass the signature check and install arbitrary code on the target device. The attack requires approximately 2^32 hash computations, achievable in under an hour on commodity hardware.
Mitigation
We recommend vendors implement full-length hash comparison and add certificate pinning to the update channel. The affected vendor has been notified and a patch is in progress.