[BRLY-2021-005] The stack buffer overflow vulnerability leads to arbitrary code execution in UEFI DXE driver on multiple HP devices.

Summary

Binarly REsearch Team has discovered a stack overflow vulnerability that allows a local priviledged user to access UEFI DXE driver and execute arbitrary code.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-005
• HP PSIRT assigned CVE identifier: CVE-2021-39299
• CERT/CC assigned case number: VU#917518
• CVSS v3.1 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected HP firmwares with confirmed impact by Binarly REsearch Team

Potential impact

An attacker with local privileged access can exploit this vulnerability to elevate privileges from ring 3 or ring 0 (depends on the operating system) to a DXE Runtime UEFI application and execute arbitrary code.A malicious code installed as a result of vulnerability exploitation in DXE driver could survive across an operating system (OS) boot process and runtime or modify NVRAM area on the SPI flash storage (to gain persistence on target platform).Additionally, this vulnerability potentially could be used by threat actors to bypass OS security mechanisms (modify privileged memory or runtime variables), influence on OS boot process, and in some cases would allow an attacker to hook or modify EFI Runtime services.

Vulnerability description

The following piece of code contains a stack overflow vulnerability (firmware name: Q92_011500.bin, driver name: 0D24, code address: 0x1490):

...
Attributes = a2;
DataSize = 0i64;
Interface = 0i64;
gBS_23A0->LocateProtocol(&ProprietaryProtocol_2040, 0i64, &Interface);
if ( gRT_23B0->GetVariable((CHAR16 *)L"DptfOptions", &VendorGuid, (UINT32 *)&Attributes, &DataSize, &Buffer) != EFI_BUFFER_TOO_SMALL
  || gRT_23B0->GetVariable((CHAR16 *)L"DptfOptions", &VendorGuid, (UINT32 *)&Attributes, &DataSize, &Buffer) )
{
  goto _GetCoolControlData;
}
...

The first call to GetVariable is used to get the length of NVRAM DptfOptions variable.The second call to GetVariable is used to get the value of NVRAM DptfOptions variable (which will be written to the Buffer stack variable).

Below is a section of the stack for the function under consideration:

-0000000000000030 Buffer          dq ?
-0000000000000028 var_28          dd ?
-0000000000000024                 db ? ; undefined
-0000000000000023                 db ? ; undefined
-0000000000000022                 db ? ; undefined
-0000000000000021                 db ? ; undefined
-0000000000000020                 db ? ; undefined
-000000000000001F                 db ? ; undefined
-000000000000001E                 db ? ; undefined
-000000000000001D                 db ? ; undefined
-000000000000001C                 db ? ; undefined
-000000000000001B                 db ? ; undefined
-000000000000001A                 db ? ; undefined
-0000000000000019                 db ? ; undefined
-0000000000000018 var_18          dq ?
-0000000000000010 var_10          dd ?
-000000000000000C                 db ? ; undefined
-000000000000000B                 db ? ; undefined
-000000000000000A                 db ? ; undefined
-0000000000000009                 db ? ; undefined
-0000000000000008                 db ? ; undefined
-0000000000000007                 db ? ; undefined
-0000000000000006                 db ? ; undefined
-0000000000000005                 db ? ; undefined
-0000000000000004                 db ? ; undefined
-0000000000000003                 db ? ; undefined
-0000000000000002                 db ? ; undefined
-0000000000000001                 db ? ; undefined
+0000000000000000  s              db 16 dup(?)
+0000000000000010  r              db 8 dup(?)
+0000000000000018 Interface       dq ?                    ; offset
+0000000000000020 Attributes      dq ?
+0000000000000028 DataSize        dq ?
+0000000000000030 arg_18          dq ?                    ; offset
+0000000000000038
+0000000000000038 ; end of stack variables

Thus, by assigning a 64-byte value to the DptfOptions variable, an attacker can completely overwrite the return address from the current function and execute arbitrary code.

Disclosure timeline

This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.

Acknowledgements

Binarly REsearch Team

References

• efiXplorer:Hunting for UEFI Firmware Vulnerabilities at Scale with Automated Static Analysis