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BINARLY efiXplorer team has discovered a stack overflow vulnerability that allows a potential attacker to access UEFI DXE driver and execute arbitrary code.
An attacker with high local access can exploit this vulnerability to elevate privileges from ring 3 or ring 0 (depends on the operating system) to a DXE driver and execute arbitrary code. Malicious code installed as a result of this exploitation could survive operating system (OS) boot process and runtime, or modify NVRAM area on the SPI flash storage (to gain persistence). Additionally, threat actors could use this vulnerability to bypass OS security mechanisms (modify privileged memory or runtime variables), influence OS boot process, and in some cases allow an attacker to hook or modify EFI Runtime services.
Let's take Edge Gateway 5000/5100's firmware (version: 0.1.19.0, module sha256: 4a34ae9ce361a948fe52338b0d346c7407650564c41271322929438f30d9939c) as an example.
The following code in the module actually allows to overflow a stack buffer:
gRT->GetVariable() offset: 0xba1gRT->GetVariable() offset: 0xbe5__int64 __fastcall sub_A68(void *a1)
{
__int64 v1; // rbx
__int64 result; // rax
__int64 v3; // rax
char v4; // cl
__int64 v5; // rax
char v6; // cl
__int64 v7; // rax
void *v8; // rcx
char v9; // cl
char v10[4]; // [rsp+30h] [rbp-848h] BYREF
unsigned int v11; // [rsp+34h] [rbp-844h] BYREF
int v12; // [rsp+38h] [rbp-840h] BYREF
__int16 v13; // [rsp+3Ch] [rbp-83Ch]
__int16 v14; // [rsp+3Eh] [rbp-83Ah]
char v15; // [rsp+40h] [rbp-838h]
char v16; // [rsp+41h] [rbp-837h]
char v17; // [rsp+42h] [rbp-836h]
char v18; // [rsp+43h] [rbp-835h]
char v19; // [rsp+44h] [rbp-834h]
char v20; // [rsp+45h] [rbp-833h]
char v21; // [rsp+46h] [rbp-832h]
char v22; // [rsp+47h] [rbp-831h]
__int64 v23; // [rsp+48h] [rbp-830h] BYREF
__int64 v24; // [rsp+50h] [rbp-828h] BYREF
_QWORD v25[2]; // [rsp+58h] [rbp-820h] BYREF
__int64 v26; // [rsp+68h] [rbp-810h] BYREF
_BYTE v27[7]; // [rsp+70h] [rbp-808h] BYREF
char v28; // [rsp+77h] [rbp-801h]
char v29; // [rsp+78h] [rbp-800h]
char v30; // [rsp+79h] [rbp-7FFh]
char v31; // [rsp+7Ah] [rbp-7FEh]
char v32; // [rsp+7Bh] [rbp-7FDh]
char v33; // [rsp+7Ch] [rbp-7FCh]
char v34; // [rsp+7Dh] [rbp-7FBh]
char v35; // [rsp+7Eh] [rbp-7FAh]
char v36; // [rsp+7Fh] [rbp-7F9h]
char v37; // [rsp+80h] [rbp-7F8h]
char v38; // [rsp+81h] [rbp-7F7h]
char v39; // [rsp+82h] [rbp-7F6h]
char v40; // [rsp+83h] [rbp-7F5h]
char v41; // [rsp+85h] [rbp-7F3h]
char v42; // [rsp+86h] [rbp-7F2h]
char v43; // [rsp+87h] [rbp-7F1h]
char v44; // [rsp+88h] [rbp-7F0h]
char v45; // [rsp+89h] [rbp-7EFh]
char v46; // [rsp+8Ah] [rbp-7EEh]
char v47; // [rsp+35Bh] [rbp-51Dh]
char v48; // [rsp+35Dh] [rbp-51Bh]
char v49; // [rsp+45Ch] [rbp-41Ch]
void *v50; // [rsp+880h] [rbp+8h] BYREF
char v51; // [rsp+890h] [rbp+18h] BYREF
char v52; // [rsp+898h] [rbp+20h] BYREF
v50 = a1;
v13 = -5212;
v14 = 19381;
v12 = -326642109;
v15 = -95;
v16 = -27;
v17 = 63;
v18 = 62;
v19 = 54;
v20 = -78;
v21 = 13;
v22 = -87;
v51 = 0;
v24 = 1i64;
v25[0] = 0i64;
v10[0] = 0;
v26 = 1i64;
v23 = 2029i64;
v1 = ((__int64 (__fastcall *)(__int64, __int64, __int64 *))gBS->AllocatePool)(4i64, 40i64, &qword_1230);
sub_1120(&byte_1210, 0x1Bui64, 0);
if ( v1 < 0 )
return 0x8000000000000009ui64;
if ( ((__int64 (__fastcall *)(EFI_GUID *, _QWORD, _QWORD *))gBS->LocateProtocol)(
&DELL_PROPERTY_DXE_PROTOCOL_GUID,
0i64,
v25) >= 0 )
(*(void (__fastcall **)(_QWORD, __int64, __int64 *, __int64 *, char *))v25[0])(
v25[0],
9346i64,
qword_240,
&v26,
v10);
v3 = ((__int64 (__fastcall *)(const __int16 *, EFI_GUID *, unsigned int *, __int64 *, char *))gRT->GetVariable)(// <= first call (we can rewrite DataSize here)
L"InternalDisallowTpmFlag",
&PEI_TCG_INTERNAL_FLAGS_GUID,
&v11,
&v24,
&v51);
v4 = v51;
if ( v3 < 0 )
v4 = 0;
v51 = v4;
v5 = ((__int64 (__fastcall *)(const __int16 *, EFI_GUID *, unsigned int *, __int64 *, char *))gRT->GetVariable)(// <= second call
L"TcgInternalSyncFlag",
&TCG_PPI_SYNC_FLAG_GUID,
&v11,
&v24,
&v52);
v6 = v52;
if ( v5 < 0 )
v6 = 0;
v52 = v6;
v7 = ((__int64 (__fastcall *)(const __int16 *, int *, unsigned int *, __int64 *, _BYTE *))gRT->GetVariable)(
L"Setup",
&v12,
&v11,
&v23,
v27);
*(_BYTE *)qword_1230 = 2;
v8 = (void *)(qword_1230 + 1);
if ( v7 >= 0 )
{
sub_1120(v8, 0x1Bui64, 0);
*(_BYTE *)(qword_1230 + 1) = v33;
if ( !v33 && !sub_3EC() )
{
v28 = 0;
((void (__fastcall *)(const __int16 *, EFI_GUID *, _QWORD, _QWORD, _QWORD))gRT->SetVariable)(
L"TpmOldvar",
&TC_EFI_GLOBAL_VARIABLE_GUID,
0i64,
0i64,
0i64);
}
*(_BYTE *)(qword_1230 + 2) = v35;
*(_BYTE *)(qword_1230 + 3) = v28;
*(_BYTE *)(qword_1230 + 4) = v38;
if ( v47 || (v9 = v29) != 0 )
{
v9 = sub_3EC() ? 1 : 5;
v29 = v9;
}
*(_BYTE *)(qword_1230 + 5) = v9;
*(_BYTE *)(qword_1230 + 19) = v52;
if ( v41 )
*(_DWORD *)(qword_1230 + 7) = 2;
else
*(_DWORD *)(qword_1230 + 7) = 1;
*(_BYTE *)(qword_1230 + 11) = v46;
*(_BYTE *)(qword_1230 + 6) = v51;
*(_BYTE *)(qword_1230 + 12) = 1;
*(_BYTE *)(qword_1230 + 13) = v30;
*(_BYTE *)(qword_1230 + 14) = v31;
*(_BYTE *)(qword_1230 + 15) = v32;
*(_BYTE *)(qword_1230 + 16) = 0;
*(_BYTE *)(qword_1230 + 17) = 0;
*(_BYTE *)(qword_1230 + 18) = v40;
*(_BYTE *)(qword_1230 + 20) = 0;
*(_BYTE *)(qword_1230 + 21) = 0;
*(_BYTE *)(qword_1230 + 22) = 0;
*(_BYTE *)(qword_1230 + 23) = 0;
*(_BYTE *)(qword_1230 + 26) = v44;
*(_BYTE *)(qword_1230 + 25) = v43;
*(_BYTE *)(qword_1230 + 24) = v42;
*(_BYTE *)(qword_1230 + 27) = v45;
}
else
{
sub_1120(v8, 0x1Bui64, 0);
}
if ( v10[0] )
{
*(_BYTE *)qword_1230 = 2;
*(_QWORD *)(qword_1230 + 32) = sub_544;
((void (__fastcall *)(const __int16 *, int *, unsigned int *, __int64 *, _BYTE *))gRT->GetVariable)(
L"Setup",
&v12,
&v11,
&v23,
v27);
v39 = v51;
v28 = 0;
v33 = 0;
v35 = 0;
v38 = 0;
v29 = 0;
v30 = 0;
v31 = 0;
v37 = 0;
v32 = 0;
v40 = 0;
v34 = 0;
v36 = 0;
v49 = 0;
v48 = 0;
return ((__int64 (__fastcall *)(const __int16 *, int *, _QWORD, __int64, _BYTE *))gRT->SetVariable)(
L"Setup",
&v12,
v11,
v23,
v27);
}
else
{
if ( MEMORY[0xFED40000] == 0xFF )
goto LABEL_28;
if ( !v48 )
{
v48 = 1;
((void (__fastcall *)(const __int16 *, int *, _QWORD, __int64, _BYTE *))gRT->SetVariable)(
L"Setup",
&v12,
v11,
v23,
v27);
}
if ( MEMORY[0xFED40000] == 0xFF )
{
LABEL_28:
if ( v49 || v48 )
{
v49 = 0;
v48 = 0;
((void (__fastcall *)(const __int16 *, int *, _QWORD, __int64, _BYTE *))gRT->SetVariable)(
L"Setup",
&v12,
v11,
v23,
v27);
}
}
*(_QWORD *)(qword_1230 + 32) = sub_544;
byte_1210 = *(_BYTE *)(qword_1230 + 1);
byte_1211 = *(_BYTE *)(qword_1230 + 2);
byte_1212 = *(_BYTE *)(qword_1230 + 3);
byte_1213 = *(_BYTE *)(qword_1230 + 4);
byte_1214 = *(_BYTE *)(qword_1230 + 5);
dword_1216 = *(_DWORD *)(qword_1230 + 7);
byte_121A = *(_BYTE *)(qword_1230 + 11);
byte_121B = *(_BYTE *)(qword_1230 + 12);
byte_121C = *(_BYTE *)(qword_1230 + 13);
byte_121D = *(_BYTE *)(qword_1230 + 14);
byte_121E = *(_BYTE *)(qword_1230 + 15);
byte_121F = *(_BYTE *)(qword_1230 + 16);
byte_1220 = *(_BYTE *)(qword_1230 + 17);
byte_1221 = *(_BYTE *)(qword_1230 + 18);
byte_1222 = *(_BYTE *)(qword_1230 + 19);
byte_1223 = *(_BYTE *)(qword_1230 + 20);
byte_1224 = *(_BYTE *)(qword_1230 + 21);
byte_1225 = *(_BYTE *)(qword_1230 + 22);
byte_1226 = *(_BYTE *)(qword_1230 + 23);
byte_1229 = *(_BYTE *)(qword_1230 + 26);
byte_1228 = *(_BYTE *)(qword_1230 + 25);
byte_1227 = *(_BYTE *)(qword_1230 + 24);
byte_122A = *(_BYTE *)(qword_1230 + 27);
gBS->InstallMultipleProtocolInterfaces(&v50, &TCG_PLATFORM_SETUP_POLICY_GUID, qword_1230, 0i64);
gImageHandle = v50;
result = ((__int64 (__fastcall *)(__int64, __int64, void (__fastcall *)(EFI_EVENT, void *)))gBS->CreateEvent)(
512i64,
8i64,
NotifyFunction);
if ( result >= 0 )
return ((__int64 (__fastcall *)(EFI_GUID *, _QWORD, void **))gBS->RegisterProtocolNotify)(
&BDS_ALL_DRIVERS_CONNECTED_PROTOCOL_GUID,
v25[1],
&Registration);
}
return result;
}
The DataSize is initialized only once (before the first call to gRT->GetVariable() service).
If the length of InternalDisallowTpmFlag NVRAM variable is greater than 1, the second call to gRT->GetVariable() service will overflow a stack buffer, which in turn could lead to arbitrary code execution.
To fix this vulnerability the DataSize must be re-initialized with the size of TcgInternalSyncFlag before calling gRT->GetVariable().
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.
BINARLY efiXplorer team
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