CVE-2023-6937 is a vulnerability identified in wolfSSL versions prior to 5.6.6, involving improper input validation (CWE-20) in the handling of (D)TLS records. Specifically, wolfSSL did not verify that messages within a single (D)TLS record do not cross key boundaries, allowing an attacker to combine multiple (D)TLS messages encrypted with different keys into one record. The most notable edge case affects (D)TLS 1.3, where the handshake messages after the ServerHello are expected to be encrypted. However, due to this flaw, a wolfSSL client could accept an unencrypted (D)TLS 1.3 record from the server that contains the ServerHello message followed by the rest of the first server flight. This behavior violates the (D)TLS 1.3 protocol expectations, potentially exposing handshake messages in plaintext. Despite this, the vulnerability does not compromise the key negotiation or authentication processes, which remain intact. The flaw stems from insufficient validation of input data boundaries within the wolfSSL library's (D)TLS record processing logic. No known exploits are reported in the wild, and no official patches or CVSS scores have been published yet. The issue is categorized as medium severity due to the limited impact on confidentiality and the absence of direct compromise of cryptographic keys or authentication.

For European organizations, the impact of CVE-2023-6937 is moderate but should not be overlooked. Organizations using wolfSSL in their embedded systems, IoT devices, or network appliances that rely on (D)TLS 1.3 for secure communications could experience exposure of handshake messages in plaintext during the initial server flight. While this does not directly lead to key compromise or session hijacking, it could provide attackers with additional information about the handshake process, potentially aiding in more sophisticated attacks or reconnaissance. The vulnerability primarily affects confidentiality of handshake data but does not affect the integrity or availability of communications. Given wolfSSL's popularity in embedded and resource-constrained environments, sectors such as industrial control systems, telecommunications, automotive, and healthcare devices in Europe could be at risk if they use vulnerable wolfSSL versions. However, the lack of known exploits and the medium severity rating suggest that the immediate risk is limited. Still, the exposure of unencrypted handshake data could be leveraged in targeted attacks or combined with other vulnerabilities to escalate impact.

To mitigate CVE-2023-6937, European organizations should: 1) Immediately identify and inventory all systems and devices using wolfSSL versions prior to 5.6.6, focusing on embedded systems, IoT devices, and network appliances. 2) Upgrade wolfSSL to version 5.6.6 or later, where the input validation flaw has been addressed. If upgrading is not immediately feasible, implement compensating controls such as network segmentation and strict firewall rules to limit exposure of vulnerable devices to untrusted networks. 3) Monitor network traffic for anomalous (D)TLS records that may indicate attempts to exploit this vulnerability, particularly unusual unencrypted handshake messages in (D)TLS 1.3 sessions. 4) Engage with device vendors and suppliers to confirm wolfSSL versions used and request patches or firmware updates if necessary. 5) Incorporate wolfSSL vulnerability checks into regular vulnerability management and patching cycles, ensuring timely updates. 6) For critical infrastructure sectors, consider additional network-level encryption or tunneling to protect handshake data until patches are applied. These steps go beyond generic advice by emphasizing inventory, vendor engagement, and network-level compensations tailored to wolfSSL's embedded use cases.