EDK II - NULL Pointer Dereference in XhcInitializeDeviceSlot

Executive Summary

An additional unchecked allocation within XhcInitializeDeviceSlot, next to the unchecked InputContext (line 2246) and EndpointTransferRing (line 2307) allocations in the same function. At line 2349, OutputContext = UsbHcAllocateMem(...) is guarded only by ASSERT(OutputContext != NULL) at line 2350, then ZeroMem(OutputContext, sizeof(DEVICE_CONTEXT)) at line 2352 dereferences NULL in RELEASE.

This advisory is one of a set reported to VulnCheck on 2026-06-25 for coordinated disclosure to the EDK II maintainers. After several follow-ups over the following months no response was received. 126 days after the initial report, ByteRay is publishing the findings in good faith so that affected integrators can assess and remediate them. All findings were identified against EDK II commit 70741a15d9a714323a5598eac47727e60507e22f (master, 2026-06-21).

Affected Products and Versions

• Vendor: TianoCore
• Product: EDK II (MdeModulePkg)
• Affected commit: 70741a15d9a714323a5598eac47727e60507e22f (master, 2026-06-21). The code is present on that commit and later revisions that have not changed it.
• Location: MdeModulePkg/Bus/Pci/XhciDxe/XhciSched.c:2349
• Function: XhcInitializeDeviceSlot
• Category: error_handling

Technical Details

Controllability: SEMI_TRUSTED — confidence: high. Physical USB device triggers enumeration.

Data Flow

At line 2349, OutputContext = UsbHcAllocateMem(...) returns NULL on failure. At line 2350, ASSERT(OutputContext != NULL) is a no-op in RELEASE. At line 2352, ZeroMem(OutputContext, sizeof(DEVICE_CONTEXT)) dereferences NULL.

Affected Callers

• XhcPollPortStatusChange

Sanitization / Security Controls

• ASSERT — DEBUG-only.

Reachability and Preconditions

Attacker position and trust level for this issue are stated in the Controllability line and Data Flow above, and the concrete trigger is in Reproduction below.

Exploitability: Low
Realism: Low

Reproduction

1. Physical access to USB port; RELEASE build; memory pressure.
2. Attacker connects malicious USB device.
3. XhcPollPortStatusChange detects the USB connect and calls XhcInitializeDeviceSlot.
4. At line 2349, UsbHcAllocateMem for OutputContext returns NULL on failure.
5. ASSERT(OutputContext != NULL) at line 2350 compiles to no-op in RELEASE; ZeroMem(OutputContext, sizeof(DEVICE_CONTEXT)) at line 2352 dereferences NULL

Impact

Denial of service — firmware crash during USB enumeration. The allocation size is fixed (sizeof (DEVICE_CONTEXT)), so a USB device only reaches this path; the failure needs the xHCI memory pool to be already exhausted.

Remediation and Mitigation

As of commit 70741a15d9a714323a5598eac47727e60507e22f this issue is unfixed upstream and no CVE has been assigned. Check the allocation result for NULL before dereferencing it and fail the operation cleanly when allocation fails. The specific control that is absent or ineffective is identified in the Sanitization / Security Controls list under Technical Details.

Timeline

• 2026-06-25 - Reported to VulnCheck for coordinated disclosure to the TianoCore / EDK II security team.
• 2026-07 to 2026-10 - Several follow-ups sent; no response received.
• 2026-10-29 - Public disclosure in good faith, 126 days after the initial report.

Credits

Discovered and reported by Argus-Systems.

References

• EDK II source (TianoCore)
• Affected file at the referenced commit