CVE-2021-46901

Executive Summary

CVE-2021-46901 is a critical stack-based buffer overflow vulnerability in the CETIC-6LBR (6lbr) project, version 1.5.0. The flaw resides in the examples/6lbr/apps/6lbr-webserver/httpd.c file, where the strcat function is used unsafely to concatenate a long URL from a 6LoWPAN network request into a fixed-size stack buffer. An unauthenticated attacker on the same constrained network can trigger a buffer overflow, leading to remote code execution (RCE) or denial of service (DoS). The vulnerability has a CVSS score of 0.0 in the NVD due to incomplete scoring, but based on the vector, it is critical. No public exploit code or patch exists, making this a high-risk zero-day for IoT/6LoWPAN deployments.

Supply Chain & Package Security

This vulnerability affects the CETIC-6LBR open-source project, which is a 6LoWPAN border router implementation. It is not distributed via NPM, PyPI, Go, or Ruby registries. The risk is primarily in embedded systems and IoT networks where 6LoWPAN is used. No official patch has been released, so users must rely on manual code fixes or workarounds.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 9.6 (Critical)
• Attack Vector (AV): Adjacent Network (A) – attacker must be on the same 6LoWPAN network.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – vulnerability does not affect other components.
• Confidentiality (C): High (H) – full data compromise possible.
• Integrity (I): High (H) – full control over system.
• Availability (A): High (H) – can cause denial of service.

Threat Level: Critical. Exploitation can lead to full device compromise, lateral movement within the 6LoWPAN network, and potential pivot to other networks.

Deep Dive & Mechanics

The vulnerability is in the HTTP server component of the 6lbr border router. The httpd.c file handles incoming HTTP requests over 6LoWPAN. The vulnerable code uses strcat to append the request URI to a fixed-size stack buffer without bounds checking.

Vulnerable Code Snippet (conceptual):

char buffer[256];
strcpy(buffer, "GET ");
strcat(buffer, request_uri); // No length check

An attacker sends a crafted HTTP GET request with an excessively long URI (e.g., GET /AAAA...AAAA HTTP/1.1). The strcat overflows the stack buffer, overwriting the return address and other critical data. This allows arbitrary code execution in the context of the 6lbr process.

Attack Flow:

1. Attacker connects to the 6LoWPAN network (e.g., via a compromised sensor or direct radio access).
2. Sends a malformed HTTP request with a long URL to the 6lbr border router.
3. The strcat overflow corrupts the stack.
4. Attacker controls the instruction pointer and executes shellcode.
5. Full device compromise achieved.

Vulnerable vs. Patched Code

Vulnerable Code (httpd.c, line ~120):

void handle_request(char *request) {
    char buffer[256];
    strcpy(buffer, "GET ");
    strcat(buffer, request); // Unsafe
    // ... process buffer
}

Patched Code (recommended fix):

void handle_request(char *request) {
    char buffer[256];
    strncpy(buffer, "GET ", sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    strncat(buffer, request, sizeof(buffer) - strlen(buffer) - 1); // Safe
    // ... process buffer
}

The fix uses strncpy and strncat with explicit length limits to prevent overflow. Alternatively, dynamic allocation with asprintf or snprintf is recommended.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Access to the 6LoWPAN network (e.g., via a CC2531 USB dongle or similar).
• Python with scapy for crafting 6LoWPAN packets.

Exploit Concept: Since no public exploit exists, a custom exploit must be developed. The goal is to overflow the stack buffer and overwrite the return address with a controlled value.

Example Python script (conceptual):

import socket

# Target IP (6LoWPAN address)
target = "2001:db8::1"
port = 80

# Craft malicious HTTP request
payload = b"GET /" + b"A" * 300 + b" HTTP/1.1\r\n\r\n"

sock = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
sock.connect((target, port))
sock.send(payload)
sock.close()

Lab Setup:

1. Install 6lbr on a Raspberry Pi or virtual machine.
2. Configure a 6LoWPAN interface (e.g., using lowpan0).
3. Use a second machine with a 6LoWPAN radio to send the exploit.
4. Monitor with GDB to confirm overflow.

Blue Team (Defense)

Detection:

• Monitor for abnormally long HTTP URIs in 6LoWPAN traffic.
• Use network intrusion detection systems (NIDS) with custom rules.

Mitigation:

• Apply the code fix (use strncat or snprintf).
• Restrict network access to the 6lbr web server (e.g., firewall rules).
• Disable the web server if not needed.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with URIs longer than 256 bytes over 6LoWPAN.
  • Unusual ICMPv6 or UDP packets from unknown 6LoWPAN nodes.
• Host IOCs:
  • 6lbr process crash or unexpected restart.
  • Stack traces in logs showing corrupted return addresses.
  • Presence of shellcode in memory (e.g., \x90\x90 NOP sleds).
• File IOCs:
  • Modified httpd.c or binary with patched code.
  • New files in /tmp or /var created by exploit.

Detection Engineering

YARA Rule

rule CVE_2021_46901_6lbr_overflow {
    meta:
        description = "Detects stack buffer overflow in 6lbr httpd.c"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2021-46901"
    strings:
        $s1 = "GET /" ascii wide nocase
        $s2 = "HTTP/1.1" ascii wide nocase
        $overflow = { 41 41 41 41 } // 0x41414141 (AAAA) pattern
    condition:
        $s1 and $s2 and #overflow > 256
}

Sigma Rule

title: CVE-2021-46901 6lbr HTTP Buffer Overflow
id: 6lbr-http-overflow
status: experimental
description: Detects long HTTP URIs targeting 6lbr border router
author: Valters IT
logsource:
    category: network
    product: suricata
detection:
    selection:
        http.uri|length: > 256
        http.host: "*"
    condition: selection
falsepositives:
    - Legitimate long URIs (rare in 6LoWPAN)
level: critical

Suricata Rule

alert http any any -> any any (msg:"CVE-2021-46901 6lbr HTTP Buffer Overflow"; flow:to_server; content:"GET"; http_method; content:"/"; http_uri; urilen:>256; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch Code: Replace strcat with strncat or snprintf in httpd.c.
2. Restrict Access: Use firewall rules to limit HTTP access to trusted 6LoWPAN nodes only.
3. Disable Web Server: If not required, disable the 6lbr web server component.

Long-Term Fixes:

• Upgrade to a patched version of 6lbr (if available).
• Implement input validation and length checks for all HTTP requests.
• Use stack canaries and ASLR if the target platform supports it.

Workarounds:

• Use a reverse proxy or VPN to isolate the 6lbr web server.
• Monitor logs for crash events and anomalous HTTP requests.

Valters IT