SMB远程代码执行漏洞（CVE-2020-0796）分析验证及加固

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        这几天有点忙，CVE-2020-0796出来了，没静下心来关注一下，显得太不尊重这个漏洞了，今天周末，关注一下，水一篇。
一、漏洞描述
      漏洞公告显示，SMB 3.1.1协议中处理压缩消息时，对其中数据没有经过安全检查，直接使用会引发内存破坏漏洞，可能被攻击者利用远程执行任意代码。攻击者利用该漏洞无须权限即可实现远程代码执行，受黑客攻击的目标系统只需开机在线即可能被入侵。

二、漏洞危害等级        

高

三、影响版本

Windows 10 Version 1903 for 32-bit Systems
Windows 10 Version 1903 for x64-based Systems
Windows 10 Version 1903 for ARM64-based Systems
Windows Server, Version 1903 (Server Core installation)
Windows 10 Version 1909 for 32-bit Systems
Windows 10 Version 1909 for x64-based Systems
Windows 10 Version 1909 for ARM64-based Systems
Windows Server, Version 1909 (Server Core installation)

四、漏洞原理

    漏洞发生在srv2.sys中,由于SMB没有正确处理压缩的数据包,在解压数据包的时候使用客户端传过来的长度进行解压时,并没有检查长度是否合法.最终导致整数溢出。

SMB v3中支持数据压缩,如果SMB Header中的ProtocolId为0x424D53FC也就是0xFC, ‘S‘, ‘M‘, ‘B‘.那么就说明数据是压缩的,这时smb会调用压缩解压处理的函数.

首先SMB会调用srv2!Srv2ReceiveHandler函数接收smb数据包，并根据ProtocoIId设置对应的处理函数。

__int64 __fastcall Srv2ReceiveHandler(__int64 a1, void *Src, __int64 a3, unsigned int a4, unsigned int *a5, char *Srca, struct _SLIST_ENTRY *a7, _QWORD *a8)
{
    ...
    //
    // 这里判断头部ProtocolId
    //
     if ( **((_DWORD **)&v20[15].Next[1].Next + 1) == ‘BMSxFC‘ )
      {
        if ( KeGetCurrentIrql() > 1u )
        {
          v20[14].Next = (_SLIST_ENTRY *)v11;
          v20[2].Next = (_SLIST_ENTRY *)Srv2DecompressMessageAsync;
          v43 = HIDWORD(v20->Next) == 5;
          *((_DWORD *)&v20[3].Next + 2) = 0;
          if ( v43 )
          {
            LOBYTE(v71) = 1;
            LOBYTE(v35) = 1;
            SRV2_PERF_ENTER_EX(&v20[32].Next + 1, v35, 307i64, "Srv2PostToThreadPool", (_DWORD)v71);
          }
          v44 = *((_QWORD *)&v20[3].Next[8].Next + 1);
          v45 = *(_QWORD *)(v44 + 8i64 * KeGetCurrentNodeNumber() + 8);
          if ( !ExpInterlockedPushEntrySList((PSLIST_HEADER)(v45 + 16), v20 + 1) && *(_WORD *)(v45 + 66) )
            RfspThreadPoolNodeWakeIdleWorker(v45);
          goto LABEL_168;
            }
        }
}
产生整数溢出漏洞的代码如下：
__int64 __fastcall Srv2DecompressData(__int64 pData)
{
  __int64 v2; // rax
  COMPRESSION_TRANSFORM_HEADER Header; // xmm0 MAPDST
  __m128i v4; // xmm0
  unsigned int CompressionAlgorithm; // ebp
  __int64 UnComparessBuffer; // rax MAPDST
  int v9; // eax
  int v11; // [rsp+60h] [rbp+8h]
  v11 = 0;
  v2 = *(_QWORD *)(pData + 0xF0);
  if ( *(_DWORD *)(v2 + 0x24) < 0x10u )         // 这里判断数据包长度的最小值
    return 0xC000090Bi64;
  Header = *(COMPRESSION_TRANSFORM_HEADER *)*(_QWORD *)(v2 + 0x18);// [v2+0x18]中为客户端传进来的Buffer
                                                // [v2+0x24]为数据包长度
  v4 = _mm_srli_si128((__m128i)Header, 8);
  CompressionAlgorithm = *(_DWORD *)(*(_QWORD *)(*(_QWORD *)(pData + 0x50) + 0x1F0i64) + 0x8Ci64);
  if ( CompressionAlgorithm != v4.m128i_u16[0] )
    return 0xC00000BBi64;                      
  UnCompressBuffer = SrvNetAllocateBuffer((unsigned int)(Header.OriginalCompressedSegmentSize + v4.m128i_i32[1]), 0i64);// OriginalCompressedSegmentSize + CompressedSegmentSize，这里没有检查相加的值，导致整数溢出，分配一个较小的UnCompressBuffer
  if ( !UnComparessBuffer )
    return 0xC000009Ai64;
  if ( (int)SmbCompressionDecompress(
              CompressionAlgorithm,             // CompressionAlgorithm
              *(_QWORD *)(*(_QWORD *)(pData + 0xF0) + 0x18i64) + (unsigned int)Header.Length + 0x10i64,// CompressedBuffer
              (unsigned int)(*(_DWORD *)(*(_QWORD *)(pData + 0xF0) + 0x24i64) - Header.Length - 0x10),// CompressedBufferSize
              (unsigned int)Header.Length + *(_QWORD *)(UnComparessBuffer + 0x18),// UncompressedBuffer，会传入SmbCompressionDecompress函数进行Decompress处理。
              Header.OriginalCompressedSegmentSize,
              &v11) < 0
    || (v9 = v11, v11 != Header.OriginalCompressedSegmentSize) )
  {
    SrvNetFreeBuffer(UnComparessBuffer);
    return 0xC000090Bi64;
  }
  if ( Header.Length )
  {
    memmove(
      *(void **)(UnComparessBuffer + 24),
      (const void *)(*(_QWORD *)(*(_QWORD *)(pData + 240) + 24i64) + 16i64),
      (unsigned int)Header.Length);
    v9 = v11;
  }
  *(_DWORD *)(UnComparessBuffer + 36) = Header.Length + v9;
  Srv2ReplaceReceiveBuffer(pData, UnComparessBuffer);
  return 0i64;
}

五、漏洞检测

       已经很多的验证脚本，整体的思路都是验证回包中的特定位置是否包含十六进制的x11x03或x02x00这两个关键字。
在存在漏洞的SMB版本的通信回包如下：
技术图片
python3版POC
import socket
import struct
import sys
from netaddr import IPNetwork

pkt = b‘x00x00x00xc0xfeSMB@x00x00x00x00x00x00x00x00x00x1fx00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00$x00x08x00x01x00x00x00x7fx00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00xx00x00x00x02x00x00x00x02x02x10x02"x02$x02x00x03x02x03x10x03x11x03x00x00x00x00x01x00&x00x00x00x00x00x01x00 x00x01x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x03x00
x00x00x00x00x00x01x00x00x00x01x00x00x00x01x00x00x00x00x00x00x00‘

subnet = sys.argv[1]

for ip in IPNetwork(subnet):

    sock = socket.socket(socket.AF_INET)
    sock.settimeout(3)

    try:
        sock.connect(( str(ip),  445 ))
    except:
        sock.close()
        continue

    sock.send(pkt)

    nb, = struct.unpack(">I", sock.recv(4))
    res = sock.recv(nb)
    if res[68:70] != b"x11x03" or res[70:72] != b"x02x00":
        print(f"{ip} Not vulnerable.")
    else:
        print(f"{ip} Vulnerable")
也可以使用nmap的脚本进行验证，依托nmap的强大框架，更方便。
local smb = require "smb"
local stdnse = require "stdnse"
local nmap = require "nmap"

description = [[
smb-protocols script modified to apply check for CVE-2020-0796 by psc4re. 
Attempts to list the supported protocols and dialects of a SMB server.
Packet check based on https://github.com/ollypwn/SMBGhost/
The script attempts to initiate a connection using the dialects:
* NT LM 0.12 (SMBv1)
* 2.02       (SMBv2)
* 2.10       (SMBv2)
* 3.00       (SMBv3)
* 3.02       (SMBv3)
* 3.11       (SMBv3)
Additionally if SMBv1 is found enabled, it will mark it as insecure. This
script is the successor to the (removed) smbv2-enabled script.
]]

---
-- @usage nmap -p445 --script smb-protocols <target>
-- @usage nmap -p139 --script smb-protocols <target>
--
-- @output
-- | smb-protocols:
-- |   dialects:
-- |     NT LM 0.12 (SMBv1) [dangerous, but default]
-- |     2.02
-- |     2.10
-- |     3.00
-- |     3.02
-- |_    3.11 (SMBv3.11) compression algorithm - Vulnerable to CVE-2020-0796 SMBGhost
--
-- @xmloutput
-- <table key="dialects">
-- <elem>NT LM 0.12 (SMBv1) [dangerous, but default]</elem>
-- <elem>2.02</elem>
-- <elem>2.10</elem>
-- <elem>3.00</elem>
-- <elem>3.02</elem>
-- <elem>3.11 (SMBv3.11) [Potentially Vulnerable to CVE-2020-0796 Coronablue]</elem>
-- </table>
---

author = "Paulino Calderon (Modified by Psc4re)"
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
categories = {"safe", "discovery"}

hostrule = function(host)
  return smb.get_port(host) ~= nil
end

action = function(host,port)
  local status, supported_dialects, overrides
  local output = stdnse.output_table()
  overrides = {}
  status, supported_dialects = smb.list_dialects(host, overrides)
  if status then
    for i, v in pairs(supported_dialects) do -- Mark SMBv1 as insecure
      if v == "NT LM 0.12" then
        supported_dialects[i] = v .. " (SMBv1) [dangerous, but default]"
      end
      if v == "3.11" then
        local msg 
        local response
        local compresionalg
        local comp
        msg = ‘x00x00x00xc0xfeSMB@x00x00x00x00x00x00x00x00x00x1fx00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00$x00x08x00x01x00x00x00x7fx00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00xx00x00x00x02x00x00x00x02x02x10x02"x02$x02x00x03x02x03x10x03x11x03x00x00x00x00x01x00&x00x00x00x00x00x01x00 x00x01x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x00x03x00
x00x00x00x00x00x01x00x00x00x01x00x00x00x01x00x00x00x00x00x00x00‘
        local socket = nmap.new_socket()
        socket:set_timeout(3000)
        socket:connect(host.ip,445)
        socket:send(msg)
        response,data = socket:receive()
        compressionalg=  string.sub(data,-2)    
        if compressionalg == "x01x00" then
          comp = "LZNT1 compression algorithm - Vulnerable to CVE-2020-0796 SMBGhost"
        elseif compressionalg == "x02x00" then
          comp ="LZ77 compression algorithm - Vulnerable to CVE-2020-0796 SMBGhost"
        elseif compressionalg == "x00x00" then
          comp ="No Compression Not Vulnerable"
        elseif compressionalg == "x03x00" then
          comp="LZ77+Huffman compression algorithm - Vulnerable to CVE-2020-0796 SMBGhost"
        end
        supported_dialects[i] = v .." " .. comp
      end
    end
    output.dialects = supported_dialects
  end

  if #output.dialects>0 then
    return output
  else
    stdnse.debug1("No dialects were accepted")
    if nmap.verbosity()>1 then
      return "No dialects accepted. Something may be blocking the responses"
    end
  end
end

六、漏洞加固


1. 更新，完成补丁的安装。

操作步骤：设置->更新和安全->Windows更新，点击“检查更新”。

2.微软给出了临时的应对办法：
运行regedit.exe，打开注册表编辑器，在HKLMSYSTEMCurrentControlSetServicesLanmanServerParameters建立一个名为DisableCompression的DWORD，值为1，禁止SMB的压缩功能。

3.对SMB通信445端口进行封禁。


补丁链接

参考链接

技术图片



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