Affected: OpenSSL

Issue summary: A TLS 1.3 connection using certificate compression can be forced to allocate a large buffer before decompression without checking against the configured certificate size limit.

Impact summary: An attacker can cause per-connection memory allocations of up to approximately 22 MiB and extra CPU work, potentially leading to service degradation or resource exhaustion (Denial of Service).

In affected configurations, the peer-supplied uncompressed certificate length from a CompressedCertificate message is used to grow a heap buffer prior to decompression. This length is not bounded by the max_cert_list setting, which otherwise constrains certificate message sizes. An attacker can exploit this to cause large per-connection allocations followed by handshake failure. No memory corruption or information disclosure occurs.

This issue only affects builds where TLS 1.3 certificate compression is compiled in (i.e., not OPENSSL_NO_COMP_ALG) and at least one compression algorithm (brotli, zlib, or zstd) is available, and where the compression extension is negotiated. Both clients receiving a server CompressedCertificate and servers in mutual TLS scenarios receiving a client CompressedCertificate are affected. Servers that do not request client certificates are not vulnerable to client-initiated attacks.

Users can mitigate this issue by setting SSL_OP_NO_RX_CERTIFICATE_COMPRESSION to disable receiving compressed certificates.

The FIPS modules in 3.6, 3.5, 3.4 and 3.3 are n…