CVE-2025-11677

Executive Summary

CVE-2025-11677 is a Use After Free (UAF) vulnerability in the WebSocket server implementation of the warmcat libwebsockets library. The flaw resides in the lws_handshake_server function and can be triggered when a user provides a callback function that handles the LWS_CALLBACK_HTTP_CONFIRM_UPGRADE event. Under specific configurations, an attacker can cause a denial of service (DoS) by exploiting the use of freed memory. No CVSS score, EPSS score, or public exploit code has been released at this time. The vendor has not yet provided a patch.

Supply Chain & Package Security

This vulnerability affects the libwebsockets library, which is a C library for WebSocket clients and servers. It is commonly used in embedded systems, IoT devices, and applications requiring real-time communication. Organizations using libwebsockets in their software supply chain should monitor for updates and assess their exposure.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Not yet assigned)
• Attack Vector: Network (likely)
• Attack Complexity: High (requires specific callback configuration)
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High (Denial of Service)
• Threat Level: Medium (due to limited exploitability and no public PoC)

Deep Dive & Mechanics

The vulnerability is a classic Use After Free (CWE-416) in the WebSocket handshake process. When a server application registers a callback for LWS_CALLBACK_HTTP_CONFIRM_UPGRADE, the library may free a memory buffer during the handshake but later attempt to access it. This can occur if the callback performs an action that deallocates memory (e.g., closing the connection or modifying internal state) while the handshake is still in progress.

The specific code path is in lws_handshake_server, which processes incoming HTTP upgrade requests. If the user-provided callback triggers a reentrant or asynchronous operation that frees the connection context, subsequent access to that context leads to a use-after-free condition. This can crash the server or cause unpredictable behavior.

Vulnerable vs. Patched Code

Since no patch is available, a direct comparison cannot be made. However, the vulnerable pattern typically involves:

// Vulnerable pattern (pseudo-code)
int lws_handshake_server(struct lws *wsi) {
    // ... allocate memory for wsi->user_space
    if (user_callback(wsi, LWS_CALLBACK_HTTP_CONFIRM_UPGRADE, ...) < 0) {
        // callback may free wsi->user_space
        // but code continues to use it
    }
    // Use after free: accessing wsi->user_space after callback
    memcpy(wsi->user_space, ...);
}

A patched version would either:

• Check if the memory is still valid after the callback.
• Prevent the callback from freeing memory during the handshake.
• Use reference counting to ensure memory is not freed prematurely.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A server using libwebsockets with a callback for LWS_CALLBACK_HTTP_CONFIRM_UPGRADE.
• Ability to send a crafted HTTP upgrade request.

Exploitation Steps:

1. Identify a target server that uses libwebsockets and has a custom callback for LWS_CALLBACK_HTTP_CONFIRM_UPGRADE.
2. Send a WebSocket upgrade request (HTTP GET with Upgrade: websocket and Connection: Upgrade headers).
3. The server's callback will be invoked. If the callback performs an action that frees memory (e.g., closing the connection), the subsequent use of that memory will cause a crash.

Example Request:

GET /chat HTTP/1.1
Host: target.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

Note: No public exploit code is available. This is a theoretical exploitation path.

Blue Team

Detection:

• Monitor for crashes or segmentation faults in processes using libwebsockets.
• Look for repeated HTTP upgrade requests followed by connection resets.
• Use memory debugging tools (e.g., AddressSanitizer) to detect use-after-free in development.

Mitigation:

• If using libwebsockets, avoid freeing memory in the LWS_CALLBACK_HTTP_CONFIRM_UPGRADE callback.
• Implement a workaround by deferring memory deallocation until after the handshake completes.
• Apply any vendor patch as soon as it becomes available.

Indicators of Compromise (IOCs)

• Network: HTTP requests with Upgrade: websocket header targeting servers with custom callbacks.
• System: Application crashes with segmentation faults in lws_handshake_server.
• Logs: Error messages indicating use-after-free or invalid memory access in libwebsockets.

Detection Engineering

YARA Rule

rule CVE_2025_11677_Libwebsockets_UAF {
    meta:
        description = "Detects use-after-free pattern in libwebsockets handshake"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-11677"
    strings:
        $s1 = "lws_handshake_server" ascii wide
        $s2 = "LWS_CALLBACK_HTTP_CONFIRM_UPGRADE" ascii wide
        $s3 = "user_callback" ascii wide
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-11677 - Use After Free in libwebsockets
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential exploitation of CVE-2025-11677 via crash logs
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: libwebsockets
detection:
    selection:
        EventID: 1000  # Application Error (Windows)
        Message|contains: "lws_handshake_server"
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-11677 - Potential libwebsockets UAF Exploit"; flow:to_server; content:"Upgrade"; http_header; content:"websocket"; http_header; content:"Sec-WebSocket-Key"; http_header; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: If you use libwebsockets with a custom LWS_CALLBACK_HTTP_CONFIRM_UPGRADE callback, review the callback code to ensure it does not free memory that is still needed by the handshake process.
2. Short-term: Monitor the vendor's repository for patches. Consider disabling the WebSocket upgrade feature if not required.
3. Long-term: Apply the official patch from warmcat/libwebsockets once released. Update to the latest version of the library.
4. Compensating Controls: Use a Web Application Firewall (WAF) to filter malicious upgrade requests. Implement memory safety checks (e.g., AddressSanitizer) in development environments.

Valters IT
DevSecOps & Cyber Threat Intelligence