CVE-2025-66199 Details
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.
View at NVD
Exploit prediction scoring system (EPSS) score for CVE-2025-66199
EPSS (Exploit Prediction Scoring System): EPSS predicts the likelihood that a vulnerability will be exploited in the wild. A higher percentage means a greater chance of an exploit occurring. The EPSS model produces a probability score between 0 and 1 (0 and 100%).
0.06 Probability of exploitation activity being observed over the next 30 days (20th percentile)
CVSS score for CVE-2025-66199
CVSS (Common Vulnerability Scoring System): An open framework owned and managed by FIRST.Org, Inc. that assigns a numerical score from 0 to 10 to software vulnerabilities to indicate their severity.
5.9 Medium
Products affected by CVE-2025-66199
OpenSSL 3.4.2
OpenSSL 3.5.2
OpenSSL 3.5.3
OpenSSL 3.5.4
OpenSSL 3.3.5
DVE Score by Bitsight AI
DVE scores complement traditional static scoring systems like CVSS with Bitsight AI powered real-time threat intelligence and underground cyber criminal activity so that security teams can reduce alert fatigue and focus on vulnerabilities most likely to be exploited.
Low
More on DVE scores

CVE-2025-66199 Global Footprint

Top 10 Identified Countries

Country Observations Percentage
US 10,367 29.46%
DE 3,635 10.33%
JP 3,283 9.33%
FR 1,378 3.92%
GB 1,297 3.69%
CA 1,107 3.15%
RU 1,035 2.94%
NL 1,017 2.89%
IT 930 2.64%
IN 718 2.04%

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CVE-2025-66199 Industry Footprint

Top 10 Identified Industries

*Service provider organizations (typically Technology and Telecommunications) are disproportionally represented in the results given their upstream ownership of end-user infrastructure. See our FAQs.

Industry* Observations Percentage
Technology 15,548 63.91%
Telecommunications 4,264 17.53%
Education 2,632 10.82%
Government/Politics 562 2.31%
Business Services 212 0.87%
Manufacturing 168 0.69%
Finance 149 0.61%
Healthcare/Wellness 100 0.41%
Transportation 96 0.39%
Utilities 86 0.35%

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