CVE-2026-90668 in UnrealIRCd
Summary
by MITRE • 09/13/2026
The webserver in UnrealIRCd 6.0.5 through 6.2.6 before 6.2.7 does not limit the number of HTTP request headers, which allows remote attackers to cause a denial of service (memory consumption and unresponsive server) via an HTTP request with an unlimited number of headers, if a websocket or JSON-RPC listener is enabled (disabled by default).
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Analysis
by VulDB Data Team • 09/13/2026
The vulnerability identified in UnrealIRCd versions 6.0.5 through 6.2.6 prior to the release of version 6.2.7 represents a significant resource exhaustion flaw within the integrated web server component. This issue specifically affects configurations where either WebSocket or JSON-RPC listeners are enabled, although it is important to note that these features are disabled by default in standard installations. The core technical deficiency lies in the absence of any upper bound on the number of HTTP request headers that the server will process during a single connection attempt. In modern web architectures, particularly those utilizing WebSocket upgrades or JSON-RPC protocols over HTTP, servers must parse incoming header fields to establish connections and route requests appropriately. However, UnrealIRCd fails to implement a counter or limit mechanism to restrict this parsing operation, allowing an attacker to supply a virtually unlimited number of headers in a single HTTP request.
From a technical perspective, the exploitation of this flaw relies on the server's inability to reject malformed or excessively large header sections during the initial handshake phase. When a remote attacker sends an HTTP request containing thousands or even millions of custom headers, the web server attempts to allocate memory for each individual header field and its associated value. This process triggers a linear increase in memory consumption proportional to the number of headers provided. As the allocation continues without interruption, the server's available memory is rapidly depleted. This resource exhaustion leads directly to system instability, manifesting as unresponsiveness or complete denial of service conditions where legitimate users are unable to connect to the IRC network because the underlying web infrastructure has become saturated and non-functional.
The operational impact of this vulnerability extends beyond simple temporary downtime. Because the attack targets the core listening process for WebSocket and JSON-RPC services, it can effectively neutralize any functionality that relies on these protocols. For networks that utilize UnrealIRCd's built-in features such as web-based client interfaces, real-time status updates via WebSockets, or remote administration tools powered by JSON-RPC, this vulnerability renders those capabilities entirely unusable during an attack. Even if the primary IRC daemon process remains technically alive, the inability to handle new connections through these specific listeners disrupts service availability and can degrade the overall user experience for administrators and clients depending on these features.
This flaw is classified under CWE-787: Out-of-bounds Write or CWE-400: Uncontrolled Resource Consumption, as it involves an attacker controlling a resource (memory) without proper limits imposed by the application logic. In terms of offensive security frameworks, this vulnerability aligns with MITRE ATT&CK technique T1499: Endpoint Denial of Service, specifically under sub-techniques related to resource exhaustion via volumetric attacks or service disruption through configuration abuse. The lack of input validation regarding header count is a common oversight in lightweight web servers that prioritize simplicity over robustness against maliciously crafted requests.
Mitigation for this vulnerability requires immediate action by system administrators and developers. For users currently running affected versions, the most effective temporary mitigation is to disable any enabled WebSocket or JSON-RPC listeners within the UnrealIRCd configuration file until an upgrade can be performed. By turning off these specific services, the attack vector is effectively closed since the vulnerable code path will not be executed during incoming HTTP requests. The permanent solution involves upgrading the server software to version 6.2.7 or later, where this limit has been implemented and enforced by default. Organizations should also consider implementing network-level rate limiting or web application firewall rules that restrict the size of HTTP headers as an additional layer of defense against similar resource exhaustion attempts in other services.