Description

Handshake messages, such as KeyUpdate, are always considered as state-advancing, regardless of whether a handshake has been completed or not. As a result, a malicious client can keep sending KeyUpdate messages to force the server to keep performing key derivation operations indefinitely.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-08-13T22:17:22.550Z",
  "pubdate": "2026-08-13T22:17:22.550Z",
  "executiveSummary": "This vulnerability involves a denial of service condition stemming from improper state management of handshake messages in secure communication protocols. Specifically, handshake messages such as KeyUpdate are universally classified as state-advancing regardless of whether the cryptographic handshake has achieved completion. Consequently, an unauthenticated or authenticated malicious client can repeatedly transmit KeyUpdate messages to a server.\nThis behavior forces the targeted server to continuously perform resource-intensive cryptographic key derivation operations indefinitely. The primary impact is resource exhaustion, leading to central processing unit saturation, degradation of service availability, and potential denial of service for legitimate clients attempting to establish or maintain secure sessions.\nThe risk implications are severe for exposed endpoints, as the attack requires minimal attacker capabilities—merely the ability to establish a network connection and transmit specially timed handshake frames. Exploitation requirements are low, assuming network reachability to the vulnerable service, and no special privileges or prior authentication are strictly necessary to initiate the flooding of these state-advancing messages.",
  "technicalDetails": "The root cause of the vulnerability lies in the state machine logic governing the processing of handshake messages. Within the affected protocol implementation, incoming messages such as KeyUpdate are evaluated without adequately checking whether the cryptographic handshake phase has successfully completed. The state machine unconditionally treats these frames as state-advancing inputs.\nThe vulnerable component is the handshake state machine and the associated cryptographic subsystem responsible for processing KeyUpdate messages and executing subsequent key derivation functions. Because the implementation lacks rate-limiting or state validation checks for these specific frames during or outside active handshakes, it enters a repetitive processing loop dictated entirely by the attacker's transmission rate.\nThe attack flow proceeds as follows: First, a malicious client initiates a connection to the target server. Instead of proceeding through a standard handshake lifecycle or completing it normally, the client continuously dispatches KeyUpdate handshake messages. Upon receipt, the server's protocol parser identifies the frame as a valid handshake message and forwards it to the state machine. Because KeyUpdate is marked as state-advancing, the server triggers internal cryptographic routines to derive new keys and update its internal cryptographic context. The attacker persists in sending these messages indefinitely, causing the server to expend high volumes of computational cycles on key derivation operations. This relentless consumption of CPU resources starves other concurrent processes and legitimate connection attempts, culminating in a complete denial of service.\nNetwork exposure is inherent to any service accepting incoming secure transport connections. The payload behavior consists of validly formatted KeyUpdate protocol messages sent out of sequence or repeated continuously to manipulate the protocol state machine. Authentication and privilege requirements are negligible, as the flaw can be triggered during the initial phases of connection establishment before session authentication is finalized."
}