Executive Summary

CVE-2026-6985 is a vulnerability in the Mongoose Embedded Web Server's built-in TCP/IP stack, specifically in the function handle_opt located in /src/net_builtin.c. The issue arises because the function does not properly validate the length of TCP options (optlen). If a TCP option with a zero-length field is received, the function enters an infinite loop because the pointer and length counters do not advance, causing the loop condition to never be false.

This infinite loop occurs during the processing of the initial TCP SYN packet, before any connection is established or authentication happens. Since Mongoose uses a single-threaded event loop, this infinite loop freezes the entire event loop, blocking all network processing, protocol handling, timers, callbacks, and connection management.

Additionally, there is an out-of-bounds read vulnerability when the length is 1, which can lead to memory leaks or faults on protected platforms.

The vulnerability can be triggered remotely by sending a specially crafted TCP SYN packet with a malformed TCP option having zero length. Recovery from this denial-of-service condition requires a device reset or watchdog intervention.

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Impact Analysis

This vulnerability can cause a permanent denial-of-service (DoS) condition on devices running the affected Mongoose Embedded Web Server versions. An attacker can remotely send a single unauthenticated TCP SYN packet with a malformed TCP option that triggers an infinite loop in the TCP option parser.

As a result, the entire single-threaded event loop freezes, blocking all network frame processing, protocol state machines (such as HTTP, MQTT, WebSocket, TLS, DNS), timers, callbacks, and connection lifecycle management.

This means the device or application becomes unresponsive to network requests and requires a power cycle or watchdog reset to recover, potentially causing service outages and operational disruption.

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Detection Guidance

This vulnerability can be detected by monitoring for TCP SYN packets containing malformed TCP options where the TCP option length field is zero. Such packets cause the Mongoose event loop to freeze due to an infinite loop in the TCP option parser.

Detection involves capturing and analyzing network traffic to identify TCP SYN packets with TCP options that have an option length of zero.

Suggested commands include using packet capture tools like tcpdump or Wireshark to filter and inspect TCP SYN packets with suspicious TCP options.

• tcpdump -i <interface> 'tcp[tcpflags] & tcp-syn != 0 and tcp[<offset>] = 0'

Note: The exact tcpdump filter to detect zero-length TCP options requires custom parsing of TCP options, which may not be straightforward with standard tcpdump filters. Instead, capturing SYN packets and analyzing them with Wireshark or a custom script to check TCP option lengths is recommended.

Additionally, monitoring the Mongoose server for symptoms such as the event loop freezing or the mg_mgr_poll() function never returning can indicate exploitation attempts.

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Mitigation Strategies

The immediate and recommended mitigation step is to upgrade the Cesanta Mongoose component to version 7.21 or later, where this vulnerability has been fixed.

Until the upgrade can be applied, consider implementing network-level protections such as filtering or blocking TCP SYN packets with suspicious or malformed TCP options to prevent exploitation.

Monitoring the system for signs of the event loop freezing and having a watchdog or automated recovery mechanism to reset the device can help reduce downtime caused by exploitation.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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