Case Study: IOMobileFramebuffer NULL Pointer Dereference

How broken access and Null Pointer Dereference vulnerabilities were discovered in the macOS IOMobileFramebuffer (AppleCLCD2) service.

INTRO

In my latest research, I discovered two vulnerabilities in the AppleCLCD2 service on macOS in IOMobileFramebufferUserClient::s_swap_submit external method (selector 5). This issue can be triggered by sending a specially crafted 1300-byte buffer to the driver.

Key technical details:

• Setting a zero byte at offset 0x3F0 in the input buffer bypasses the com.apple.private.gain-map-access entitlement check.
• Placing four NULL bytes at offset 0x430 confuses the driver’s memory validation logic, resulting in a NULL pointer dereference and a kernel panic.
• The bug is reliably triggered using a minimal payload, which can be constructed with standard user-space tools and sent via IOConnectCallMethod.

This vulnerability demonstrates how careful fuzzing and reverse engineering of driver external methods can reveal subtle kernel flaws. However, due to modern macOS and Apple Silicon mitigations, such bugs now only result in Denial of Service (system crash), not code execution.

For the full technical walkthrough, including proof-of-concept code and root cause analysis, read the complete article here: https://afine.com/case-study-iomobileframebuffer-null-pointer-dereference/