CVE-2026-53525 in WeeChat
Summary
by MITRE • 08/22/2026
WeeChat (Wee Enhanced Environment for Chat) is a free chat client. In versions 0.3.1 through 4.9.0, the WeeChat relay authentication uses non-constant-time string comparison functions (weechat_strcasecmp and strcmp) to verify password hashes and plaintext passwords. An attacker can exploit timing differences to extract the server-computed hash character by character, then authenticate using the correct hash without knowing the password. Version 4.9.1 fixes the issue.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability identified in WeeChat versions ranging from 0.3.1 through 4.9.0 represents a critical authentication bypass mechanism rooted in improper input validation and timing side-channel attacks. As a widely used terminal-based chat client, WeeChat relies on a relay protocol to allow remote connections for managing chats across multiple devices or locations. The core of this security flaw lies in the method employed by the server component when verifying user credentials during the authentication handshake. Specifically, the software utilizes standard string comparison functions, namely weechat_strcasecmp and strcmp, to validate password hashes and plaintext passwords against stored values. These conventional library functions are designed for general-purpose text processing rather than cryptographic verification, meaning they terminate their execution as soon as a mismatch is detected between two strings being compared. This behavior introduces a measurable timing difference that an attacker can exploit through a character-by-character brute force attack to recover the server-computed hash without ever knowing the actual user password.
From a technical perspective, this flaw constitutes a classic Timing Attack, which falls under the broader category of Side-Channel Attacks within cybersecurity taxonomy. The vulnerability is formally classified as CWE-208: Observable Timing Discrepancy and CWE-798: Use of Hard-coded Credentials when considering the implications for credential security integrity. By sending carefully crafted authentication requests with varying password guesses, an attacker can measure the response time from the WeeChat relay server. If a guessed character matches the corresponding position in the correct hash, the comparison function must process additional characters before failing or succeeding, resulting in a slightly longer execution time compared to a guess where the first character is incorrect. Through statistical analysis of these microsecond-level differences over many iterations, an adversary can reconstruct the entire password hash with high accuracy. This capability effectively neutralizes the security provided by strong passwords because the attacker does not need to crack the hash cryptographically but rather extracts it directly through timing measurements.
The operational impact of this vulnerability is severe, as it allows for unauthorized access to WeeChat relay services without requiring prior knowledge of user credentials. Once an attacker obtains the correct password hash via this side-channel method, they can authenticate to the relay service just like a legitimate user. This grants them full control over the chat sessions associated with that account, including the ability to read private messages, send commands, and potentially exfiltrate sensitive information shared within those channels. Since WeeChat is often used in professional and secure communication contexts, this breach compromises confidentiality and integrity of communications. The attack does not require complex exploitation tools or network-level privileges; it can be performed remotely over standard TCP connections if the relay port is exposed to untrusted networks, making it a significant risk for users who enable remote access without adequate protective measures such as firewalls or TLS encryption that might obscure timing data.
Mitigation strategies primarily involve upgrading to WeeChat version 4.9.1 or later, where this issue has been resolved by implementing constant-time string comparison algorithms. Constant-time comparisons ensure that the execution time of the function remains independent of the input values, thereby eliminating the timing side-channel that attackers rely upon. In addition to software updates, administrators should enforce strict network-level controls such as restricting relay access via IP whitelisting or placing the service behind a reverse proxy with TLS termination to add layers of defense-in-depth. It is also advisable to use strong, high-entropy passwords and consider multi-factor authentication if supported by the deployment architecture. Security practitioners monitoring for this vulnerability should look for CVE identifiers associated with WeeChat releases prior to 4.9.1 and ensure that all relay instances are patched promptly to prevent exploitation through timing analysis attacks as documented in ATT&CK technique T1530: Data from Information Repositories or related credential access patterns where authentication mechanisms are bypassed via side channels.