EDK II - NULL Pointer Dereference in XhcPeiInitializeDeviceSlot

Executive Summary

The PEI-phase counterpart of XhcInitializeDeviceSlot. Three allocations lack proper NULL checks: (1) InputContext at line 1122 with ASSERT-only, then ZeroMem at 1125; (2) EndpointTransferRing at line 1183 with NO NULL CHECK AT ALL, passed to XhcPeiCreateTransferRing at 1185; (3) OutputContext at line 1225 with ASSERT-only, then ZeroMem at 1228.

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/XhciPei/XhciSched.c:1122
• Function: XhcPeiInitializeDeviceSlot
• Category: error_handling

Technical Details

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

Data Flow

At line 1122, InputContext = UsbHcAllocateMem(...) with ASSERT-only at 1123, then ZeroMem at 1125. At line 1183, EndpointTransferRing = AllocateZeroPool(...) with NO CHECK AT ALL, passed to XhcPeiCreateTransferRing at 1185 which dereferences it. At line 1225, OutputContext = UsbHcAllocateMem(...) with ASSERT-only at 1226, then ZeroMem at 1228.

Affected Callers

• XhcPeiPollPortStatusChange

Sanitization / Security Controls

• DEBUG-only ASSERT (partial).

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; PEI-phase boot; RELEASE build; memory pressure.
2. Attacker connects physical USB device during PEI-phase boot.
3. XhcPeiPollPortStatusChange detects the USB connect and calls XhcPeiInitializeDeviceSlot.
4. At line 1122, UsbHcAllocateMem for InputContext returns NULL; ASSERT at 1123 is no-op; ZeroMem at 1125 dereferences NULL.
5. At line 1183, AllocateZeroPool for EndpointTransferRing returns NULL with NO CHECK; XhcPeiCreateTransferRing at 1185 dereferences NULL

Impact

Denial of service — firmware crash during PEI USB enumeration.

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