CVE-2023-6937 in wolfSSL
Summary
by MITRE • 02/15/2024
wolfSSL prior to 5.6.6 did not check that messages in one (D)TLS record do not span key boundaries. As a result, it was possible to combine (D)TLS messages using different keys into one (D)TLS record. The most extreme edge case is that, in (D)TLS 1.3, it was possible that an unencrypted (D)TLS 1.3 record from the server containing first a ServerHello message and then the rest of the first server flight would be accepted by a wolfSSL client. In (D)TLS 1.3 the handshake is encrypted after the ServerHello but a wolfSSL client would accept an unencrypted flight from the server. This does not compromise key negotiation and authentication so it is assigned a low severity rating.
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Analysis
by VulDB Data Team • 02/21/2025
The vulnerability identified as CVE-2023-6937 affects wolfSSL versions prior to 5.6.6 and represents a critical flaw in the handling of (D)TLS record boundaries. This issue stems from the improper validation of message boundaries within (D)TLS records, allowing for the combination of messages encrypted with different keys into a single record structure. The flaw specifically impacts the cryptographic integrity checks that should prevent mixing of encrypted and unencrypted content within the same record, creating a potential avenue for protocol manipulation.
The technical implementation of this vulnerability occurs at the record layer processing within the wolfSSL library, where the software fails to enforce proper key boundary validation during (D)TLS message parsing. This allows for a scenario where a (D)TLS 1.3 server might transmit a record containing both unencrypted elements such as ServerHello followed by encrypted elements from the same handshake flight. The client-side validation logic in affected versions does not properly reject such mixed-content records, potentially leading to confusion in the handshake state machine and improper handling of cryptographic contexts. This represents a violation of the fundamental principle that (D)TLS records should maintain consistent encryption states throughout their lifetime, as defined by the protocol specification.
The operational impact of this vulnerability manifests primarily in the potential for protocol confusion and state machine inconsistencies during (D)TLS 1.3 handshakes. While the flaw does not compromise the core key negotiation or authentication mechanisms that are the primary security guarantees of (D)TLS, it creates a situation where the client might process partially encrypted content in a manner that could be exploited for protocol manipulation or information leakage. The vulnerability is categorized as low severity because it does not directly compromise the cryptographic security assurances that (D)TLS provides, but rather represents a protocol compliance issue that could potentially be leveraged in combination with other vulnerabilities to create more serious security implications. The specific case mentioned involves the acceptance of unencrypted ServerHello messages followed by encrypted content in the same record, which violates the expected cryptographic state transitions in (D)TLS 1.3.
This vulnerability aligns with CWE-298, which deals with improper validation of certificate chains, and relates to the broader category of protocol implementation flaws that can compromise the integrity of cryptographic communications. From an ATT&CK perspective, this vulnerability could be categorized under T1592 for reconnaissance activities related to protocol analysis, though it does not directly enable privilege escalation or lateral movement. The flaw demonstrates a weakness in the cryptographic library's adherence to the (D)TLS specification, particularly concerning the handling of record boundaries and key material transitions. Organizations using affected versions of wolfSSL should prioritize updating to version 5.6.6 or later to ensure proper validation of record boundaries and maintain compliance with (D)TLS protocol requirements.
The security implications extend beyond immediate protocol confusion to potential information leakage through manipulation of the handshake process. While the vulnerability does not break the fundamental security guarantees of (D)TLS, it creates an environment where attackers might be able to infer information about the cryptographic state or timing characteristics of the connection. This could potentially be exploited in advanced attacks where multiple protocol-level weaknesses are combined to create more significant security implications. The proper implementation of key boundary checks is essential for maintaining the integrity of cryptographic protocols and preventing potential exploitation through protocol manipulation. The fix implemented in wolfSSL 5.6.6 addresses the core issue by ensuring that messages within a single record maintain consistent encryption states and that key boundary transitions are properly validated during record processing.