Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.4",
  "pubDate": "2026-08-21T23:16:26.363Z",
  "pubdate": "2026-08-21T23:16:26.363Z",
  "executiveSummary": "WeeChat (Wee Enhanced Environment for Chat) versions 0.3.1 through 4.9.0 contain a critical timing vulnerability within its relay authentication mechanism. The flaw stems from the utilization of non-constant-time string comparison functions—specifically weechat_strcasecmp and strcmp—when validating plaintext passwords and password hashes. This cryptographic implementation flaw introduces measurable execution time variations dependent upon the number of matching character prefixes presented during authentication attempts. An unauthenticated remote attacker can systematically exploit these timing differentials via side-channel analysis to accurately measure response latencies, thereby incrementally brute-forcing and extracting the server-computed password hash character by character. Upon successful hash extraction, the adversary can bypass standard authentication controls by authenticating directly using the acquired hash without possessing the actual plaintext password. The impact includes full unauthorized access to the WeeChat relay interface, potentially exposing sensitive chat communications, server metadata, and control channels. This vulnerability presents a significant risk to confidentiality and integrity across exposed chat relay deployments, requiring immediate remediation to secure administrative and client communication pathways.",
  "technicalDetails": "The root cause of this vulnerability lies in the implementation of the authentication verification routines within the WeeChat relay component. Specifically, the codebase relies on standard non-constant-time string comparison functions, namely weechat_strcasecmp and strcmp, to evaluate user-supplied credentials against expected password hashes or plaintext passwords. Standard string comparison routines typically terminate execution and return control immediately upon encountering the first non-matching byte. Consequently, comparisons that match a larger prefix of the target string take demonstrably longer to execute than those that fail early.\nAn attacker leverages this timing side-channel by sending a series of crafted authentication requests containing systematically varied candidate characters for each sequential position of the target hash. By transmitting a high volume of requests and conducting statistical analysis on the round-trip network latency or server processing time for each iteration, the adversary can identify which candidate character yields a statistically significant increase in execution time. This indicates a successful prefix match, allowing the attacker to deduce the correct hash value character by character.\nOnce the complete server-computed hash is extracted through this iterative side-channel measurement process, the attacker bypasses the underlying cryptographic password validation by supplying the harvested hash directly as the authentication credential. Because the vulnerable comparison function processes the attacker-supplied hash against the stored hash using non-constant-time logic, or accepts the hash value due to validation logic constraints, the authentication check succeeds.\nThe vulnerable components include the relay authentication functions utilizing weechat_strcasecmp and strcmp. Affected software versions span from 0.3.1 up to and including 4.9.0. The attack vector is remotely exploitable over the network where the WeeChat relay is exposed, typically requiring no prior authentication privileges. Payload behavior consists of high-frequency probing followed by credential substitution using the harvested hash, leading to unauthorized remote access to the relay system and complete compromise of session confidentiality."
}