CVE-2026-117470 in Psono
Summary
by MITRE • 10/27/2026
Psono Server CE & EE versions 3.1.0 through 7.2.3 did not consume a stored WebAuthn challenge after assertion verification. An unauthenticated remote attacker who possessed the user's password and one captured valid WebAuthn assertion could replay that assertion with separate login tokens while the stored challenge remained unchanged, bypassing the WebAuthn second factor. The attack required capture of a valid assertion and did not itself disclose plaintext secrets.
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Analysis
by VulDB Data Team • 10/27/2026
The vulnerability identified in Psono Server Community Edition and Enterprise Edition versions 3.1.0 through 7.2.3 represents a critical failure in the implementation of WebAuthn authentication protocols, specifically concerning the handling of cryptographic challenges during the assertion verification process. In standard WebAuthn operations, the server generates a unique, random challenge for each login attempt to ensure freshness and prevent replay attacks. The client signs this challenge with their private key to produce an assertion, which is then sent back to the server for validation. However, in the affected versions of Psono Server, the system failed to properly consume or invalidate the stored WebAuthn challenge after a successful assertion verification. This architectural flaw means that the cryptographic nonce associated with a specific authentication session remained valid and unchanged even after it had been used once, fundamentally breaking the statelessness and one-time-use requirement inherent in secure multi-factor authentication designs.
This technical deficiency allows an unauthenticated remote attacker to perform a replay attack against WebAuthn protected accounts under specific conditions. If an attacker possesses the user's static password credential and is able to capture a single valid WebAuthn assertion, they can reuse that same signed assertion for subsequent login attempts using different session tokens or authentication contexts. Because the server does not track whether a challenge has already been consumed, it accepts the replayed assertion as legitimate, effectively bypassing the second factor of authentication entirely. The attacker essentially substitutes the dynamic cryptographic proof with a static captured value, exploiting the lack of state management in the verification logic to gain unauthorized access without needing to compromise additional secrets or perform complex cryptanalytic attacks against the WebAuthn protocol itself.
The operational impact of this vulnerability is severe as it undermines the primary security guarantee provided by multi-factor authentication systems. By allowing assertion replay, the attacker can authenticate as any user whose password has been compromised and for whom a valid WebAuthn assertion was previously captured on the network or through other means such as man-in-the-middle attacks during legitimate login attempts. This bypasses the intended protection against credential stuffing and phishing attacks that rely solely on password theft. Although the vulnerability does not directly disclose plaintext secrets like private keys or passwords, it renders the second factor ineffective for any user who has successfully authenticated at least once while using a vulnerable version of the software. The risk is particularly acute in environments where network traffic may be intercepted or where users frequently authenticate from untrusted networks.
From an industry standard perspective, this vulnerability aligns with CWE-294, which describes authentication bypass by capture-replay attack, and specifically relates to CWE-310 regarding cryptographic failure involving the lack of proper state management for nonces. In terms of the MITRE ATT&CK framework, this flaw facilitates the Tactic Initial Access through techniques such as Credential Stuffing or Phishing, where stolen credentials are leveraged due to a weakened second factor mechanism. The specific behavior also reflects CWE-345, which pertains to Insufficient Verification of Data Authenticity, as the server fails to verify that the assertion corresponds to a fresh challenge issued by the server for that specific session.
To mitigate this vulnerability, organizations running Psono Server versions 3.1.0 through 7.2.3 must upgrade immediately to a patched version where the WebAuthn challenge consumption logic has been corrected. Until an upgrade is possible, administrators should consider disabling WebAuthn if feasible and relying on other strong authentication methods that do not suffer from this specific state management flaw. Additionally, implementing network-level protections such as TLS inspection or strict session token rotation policies may help reduce the window of opportunity for assertion capture, although these are compensatory controls rather than definitive fixes. The root cause lies in the server-side logic failing to update its internal state after verification, so ensuring that each challenge is marked as consumed immediately upon successful authentication is essential to restoring the integrity of the multi-factor authentication process.