CVE-2025-66199

A flaw was found in OpenSSL. A remote attacker can exploit this vulnerability by sending a specially crafted CompressedCertificate message during the TLS 1.3 handshake. This can cause excessive per-connection memory allocations, leading to resource exhaustion and a Denial of Service (DoS) for affected clients and servers. This issue occurs when TLS 1.3 certificate compression is enabled and negotiated.

This vulnerability is rated Low for Red Hat products. The flaw in OpenSSL 3.3, 3.4, 3.5, and 3.6 allows an attacker to cause excessive memory allocation during TLS 1.3 handshake with certificate compression, potentially leading to a Denial of Service. This affects both clients and servers in mutual TLS scenarios where certificate compression is negotiated. Servers not requesting client certificates are not vulnerable to client-initiated attacks.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, disable the reception of compressed certificates by setting the SSL_OP_NO_RX_CERTIFICATE_COMPRESSION option in OpenSSL configurations. This will prevent the vulnerable code path from being exercised.