CVE-2018-21035 in Qt
Summary
by MITRE
In Qt through 5.14.1, the WebSocket implementation accepts up to 2GB for frames and 2GB for messages. Smaller limits cannot be configured. This makes it easier for attackers to cause a denial of service (memory consumption).
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Analysis
by VulDB Data Team • 04/07/2024
The vulnerability identified as CVE-2018-21035 resides within the Qt framework's WebSocket implementation and affects versions through 5.14.1. This issue represents a significant security concern that stems from the framework's inability to configure reasonable limits for WebSocket frame and message sizes. The WebSocket protocol within Qt operates with fixed maximum values of 2 gigabytes for individual frames and 2 gigabytes for complete messages, creating a fundamental design flaw that leaves applications vulnerable to memory exhaustion attacks. This particular implementation does not allow developers to adjust these limits through configuration parameters, forcing all applications using Qt's WebSocket functionality to accept potentially maliciously large data transfers that can overwhelm system resources.
The technical nature of this vulnerability aligns with CWE-400, which addresses Uncontrolled Resource Consumption, specifically focusing on the lack of proper bounds checking for resource allocation within network protocols. The flaw operates at the protocol implementation level where the WebSocket parser does not enforce reasonable size constraints on incoming data frames, allowing attackers to craft malicious WebSocket messages that consume excessive memory resources. When a WebSocket client or server receives a message that approaches or exceeds the 2 gigabyte limit, the system attempts to allocate memory for processing, potentially leading to system instability, application crashes, or complete system resource exhaustion. This behavior creates a predictable attack vector where adversaries can systematically consume available memory through carefully constructed WebSocket traffic, making it particularly dangerous in environments where Qt-based applications handle untrusted network input.
From an operational perspective, this vulnerability creates substantial risk for applications that utilize Qt's WebSocket capabilities, particularly those deployed in production environments where memory resources are constrained or where multiple concurrent connections exist. The denial of service impact manifests when attackers send oversized WebSocket frames that cause memory allocation failures, application crashes, or system-wide resource exhaustion that affects other services running on the same system. The vulnerability is especially concerning for web applications, IoT devices, and server applications that rely on Qt's WebSocket implementation for real-time communication protocols. Attackers can exploit this weakness by establishing WebSocket connections and sending progressively larger messages that push the system memory limits, potentially causing cascading failures that affect the entire application stack or system availability.
The mitigation strategy for CVE-2018-21035 requires immediate attention from developers and system administrators who utilize Qt frameworks in their applications. The most effective approach involves upgrading to Qt versions 5.14.2 or later, where the framework implements configurable limits for WebSocket frame and message sizes. Organizations should conduct comprehensive vulnerability assessments to identify all applications using affected Qt versions and prioritize their remediation efforts. Additionally, network-level protections such as rate limiting, connection pooling, and memory monitoring can provide temporary defenses while long-term upgrades are implemented. Security teams should also consider implementing network segmentation and monitoring for unusual WebSocket traffic patterns that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1499.004, which covers Resource Exhaustion, and T1566.001, which addresses spearphishing attachments, as attackers can leverage this weakness to consume system resources and cause service disruption. Organizations implementing security controls should also consider the principle of least privilege and ensure that WebSocket endpoints are properly isolated and monitored to prevent exploitation of this memory exhaustion vulnerability.