CVE-2025-66199

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 not affected by this issue,
as the TLS implementation is outside the OpenSSL FIPS module boundary.

OpenSSL 3.6, 3.5, 3.4 and 3.3 are vulnerable to this issue.

OpenSSL 3.0, 1.1.1 and 1.0.2 are not affected by this issue.

• https://github.com/openssl/openssl/commit/3ed1f75249932b155eef993a8e66a99cb98bfef4

  openssl-security@openssl.org Patch
• https://github.com/openssl/openssl/commit/6184a4fb08ee6d7bca570d931a4e8bef40b64451

  openssl-security@openssl.org Patch
• https://github.com/openssl/openssl/commit/895150b5e021d16b52fb32b97e1dd12f20448be5

  openssl-security@openssl.org Patch
• https://github.com/openssl/openssl/commit/966a2478046c311ed7dae50c457d0db4cafbf7e4

  openssl-security@openssl.org Patch
• https://openssl-library.org/news/secadv/20260127.txt

  openssl-security@openssl.org Vendor Advisory