CVE-2025-66199

MEDIUM EPSS 32.1%
Published Jan 27, 20265mo ago · Modified Jun 17, 20262w ago
5.9 CVSS 3.1
Medium
Find Similar
Published Jan 27, 2026 5mo ago
Last Modified Jun 17, 2026 2w ago

Description

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.

CVSS Details

Base Score
5.9
Exploitability
2.2
Impact
3.6
Vector string
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack Vector Network
Attack Complexity High
Privileges Required None
User Interaction None
Scope Unchanged
Confidentiality None
Integrity None
Availability High

Threat Intelligence

EPSS Exploit Probability
32.1% percentile
Exploit & Patch Status
No Known Exploit
Patch Available

Weaknesses 1

CWE-789

Affected Products 4

VendorProductVersionRange
opensslopenssl*≥3.3.0  –  <3.3.6
opensslopenssl*≥3.4.0  –  <3.4.4
opensslopenssl*≥3.5.0  –  <3.5.5
opensslopenssl*≥3.6.0  –  <3.6.1

References 5

  • github.com https://github.com/openssl/openssl/commit/3ed1f75249932b155eef993a8e66a99cb98bfef4
    Patch
  • github.com https://github.com/openssl/openssl/commit/6184a4fb08ee6d7bca570d931a4e8bef40b64451
    Patch
  • github.com https://github.com/openssl/openssl/commit/895150b5e021d16b52fb32b97e1dd12f20448be5
    Patch
  • github.com https://github.com/openssl/openssl/commit/966a2478046c311ed7dae50c457d0db4cafbf7e4
    Patch
  • openssl-library.org https://openssl-library.org/news/secadv/20260127.txt
    Vendor Advisory

Remediation

  • github.com https://github.com/openssl/openssl/commit/3ed1f75249932b155eef993a8e66a99cb98bfef4
    Patch
  • github.com https://github.com/openssl/openssl/commit/6184a4fb08ee6d7bca570d931a4e8bef40b64451
    Patch
  • github.com https://github.com/openssl/openssl/commit/895150b5e021d16b52fb32b97e1dd12f20448be5
    Patch
  • github.com https://github.com/openssl/openssl/commit/966a2478046c311ed7dae50c457d0db4cafbf7e4
    Patch