CVE-2019-17023 in Firefox
Summary
by MITRE
After a HelloRetryRequest has been sent, the client may negotiate a lower protocol that TLS 1.3, resulting in an invalid state transition in the TLS State Machine. If the client gets into this state, incoming Application Data records will be ignored. This vulnerability affects Firefox < 72.
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Analysis
by VulDB Data Team • 01/09/2020
The vulnerability identified as CVE-2019-17023 represents a critical state machine flaw within the Transport Layer Security implementation of Mozilla Firefox browsers. This issue specifically manifests during the TLS 1.3 handshake process when a HelloRetryRequest message is transmitted by the server to the client. The flaw occurs in the protocol negotiation phase where the client may inadvertently transition to a lower protocol version despite having initiated a TLS 1.3 handshake, creating an inconsistent and invalid state within the TLS state machine architecture. The vulnerability stems from inadequate state validation mechanisms that fail to properly enforce protocol version consistency throughout the handshake lifecycle. This particular weakness falls under the CWE-362 category of Concurrent Execution using Shared Resource Vulnerabilities, as it involves improper handling of shared protocol state information during concurrent handshake operations. The issue is particularly concerning from an adversarial perspective as it aligns with ATT&CK technique T1071.004 for Application Layer Protocol: DNS, since it could potentially be exploited to disrupt legitimate TLS connections through protocol downgrade attacks.
The technical implementation of this vulnerability allows for a state transition anomaly where the client's TLS state machine becomes inconsistent after processing a HelloRetryRequest message. When a client receives a HelloRetryRequest during TLS 1.3 negotiation, it should maintain the TLS 1.3 protocol context for subsequent handshake messages. However, the flaw permits the client to negotiate a lower protocol version such as TLS 1.2 or earlier, which creates a fundamental inconsistency in the state machine. This invalid state transition directly impacts the client's ability to properly process subsequent TLS records, specifically causing the system to ignore incoming Application Data records that would normally be decrypted and delivered to the application layer. The state machine's inability to properly handle this transition creates a denial-of-service condition where legitimate encrypted traffic is silently dropped rather than processed. The vulnerability exists because the TLS implementation does not adequately validate that protocol version consistency is maintained throughout the handshake process, particularly after state transitions initiated by HelloRetryRequest messages.
From an operational impact perspective, this vulnerability creates a significant risk for Firefox users who may experience intermittent connection failures or complete loss of encrypted communication when encountering servers that utilize HelloRetryRequest during TLS 1.3 negotiation. The vulnerability affects all Firefox versions prior to 72.0, making it a substantial concern for organizations that have not yet updated their browser deployments. The impact extends beyond simple connection failures as the silent dropping of Application Data records means that network administrators may not immediately detect the compromise since the connection appears to be established but is actually non-functional. This behavior can mask more serious underlying security issues or make it difficult to diagnose network connectivity problems, particularly in environments where TLS 1.3 is actively used. The vulnerability's exploitation potential increases in scenarios where attackers can force servers to send HelloRetryRequest messages, potentially creating a systematic disruption of TLS communications for affected clients.
Organizations and users should immediately update to Firefox version 72.0 or later to remediate this vulnerability, as Mozilla has addressed the issue through proper state machine validation and protocol version consistency enforcement. The fix implemented by Mozilla involves strengthening the validation logic within the TLS state machine to prevent invalid protocol version transitions after HelloRetryRequest processing, ensuring that once a client has initiated TLS 1.3 negotiation, it maintains that protocol context throughout the handshake process. Security teams should also consider implementing network monitoring to detect potential exploitation attempts that might involve forced protocol downgrades or unusual HelloRetryRequest patterns. Additional mitigations include ensuring that all systems are running patched browser versions and that network security controls are configured to detect and alert on anomalous TLS handshake behaviors. The vulnerability serves as a reminder of the importance of proper state machine implementation in cryptographic protocols and the necessity of thorough testing of edge cases in protocol negotiation sequences. This issue demonstrates how seemingly minor protocol handling inconsistencies can create significant security and operational impacts, particularly in environments where TLS 1.3 adoption is increasing.