CVE-2026-53524 in WeeChat
Summary
by MITRE • 08/22/2026
WeeChat (Wee Enhanced Environment for Chat) is a free chat client. In versions 4.3.0 through 4.9.0, the WeeChat relay module's WebSocket permessage-deflate decompression function relay_websocket_inflate() has no upper bound on output size. An authenticated relay user can send a small compressed WebSocket frame (~100 bytes) that decompresses to gigabytes, exhausting all server memory and crashing the entire WeeChat process. The api protocol enables permessage-deflate and requires authentication before WebSocket upgrade. Version 4.9.1 patches the issue.
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in WeeChat versions 4.3.0 through 4.9.0 represents a critical resource exhaustion flaw within the relay module's handling of WebSocket connections, specifically targeting the permessage-deflate decompression mechanism. This security defect is rooted in the implementation of the relay_websocket_inflate function, which processes compressed data frames sent by clients over WebSocket protocols. The core technical failure lies in the absence of an upper bound check on the size of the decompressed output buffer. In a standard secure implementation, any compression algorithm must enforce strict limits on the ratio between compressed input and uncompressed output to prevent abuse. However, due to this missing validation logic, the application fails to verify whether the resulting plaintext data exceeds available system resources before allocating memory for it. This architectural oversight allows an attacker to exploit the mathematical properties of certain compression algorithms where a very small amount of compressed data can expand into a massive volume of uncompressed text when crafted specifically for that purpose.
From an operational perspective, this flaw enables a highly efficient Denial of Service attack against WeeChat relay servers. An authenticated user, who is required by the API protocol to authenticate before establishing a WebSocket upgrade connection, can transmit a maliciously constructed compressed frame as small as one hundred bytes. Upon receipt, the server attempts to decompress this payload using the permessage-deflate extension without checking for size constraints. The decompression process rapidly consumes gigabytes of RAM on the host system, leading to immediate memory exhaustion. Since WeeChat is designed as a single-process application in many configurations or relies heavily on shared resources within its relay architecture, this sudden spike in memory usage causes the entire WeeChat process to crash. This results in a complete service outage for all connected users and disrupts chat functionality until the server administrator manually restarts the daemon, causing significant downtime and potential loss of session state depending on configuration settings.
This vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, as well as CWE-770, Allocation of Resources Without Limits or Throttling. The attack vector is classified under MITRE ATT&CK technique T1499, Endpoint Denial of Service, specifically leveraging resource exhaustion via compression bombs. Although the requirement for authentication provides a layer of defense by restricting access to legitimate users, it does not mitigate the risk entirely as insider threats or compromised accounts can still exploit this flaw. The severity is heightened because WebSocket connections are often long-lived and persistent, meaning that even brief exploitation attempts can lead to sustained service disruption if the server fails to recover gracefully from memory allocation failures.
To address this vulnerability, organizations running WeeChat versions 4.3.0 through 4.9.0 must immediately upgrade to version 4.9.1 or later, where the developers have implemented proper bounds checking on decompressed data sizes. In environments where upgrading is not immediately feasible, administrators should consider implementing network-level protections such as rate limiting for WebSocket frames and monitoring memory usage patterns of relay processes. Additionally, configuring Web Application Firewalls to inspect and limit the size of incoming compressed payloads can provide an additional layer of defense against compression bomb attacks. It is also advisable to review authentication policies to ensure that only trusted entities have access to the relay module, thereby reducing the attack surface for potential exploitation by malicious insiders or compromised credentials.