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Fencer Vulns
medium 5.9

CVE-2025-66199 json

TLS 1.3 CompressedCertificate excessive memory allocation

published 2026-01-27 9 mo ago modified 2026-07-25 3 mo ago also EUVD-2025-206398 , GHSA-5888-36j9-c92p , JLSEC-2026-260

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 40%
public exploits
0
none known

Summary

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.

Patch / fix

  • julia OpenSSL_jll 3.5.5+0
  • OpenSSL OpenSSL 3.3.6, 3.4.4, 3.5.5, 3.6.1
  • openssl Debian 13 3.5.4-1~deb13u2
  • openssl Debian 14 3.5.5-1

debian advisories DEBIAN-CVE-2025-66199

  • openssl Ubuntu 25.10 3.5.3-1ubuntu3
  • openssl Ubuntu 26.04 LTS 3.5.5-1ubuntu1

ubuntu advisories UBUNTU-CVE-2025-66199

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • openssl hummingbird 1 3.5.6-0.1.hum1
  • openssl-libs hummingbird 1 3.5.6-0.1.hum1
  • openssl-perl hummingbird 1 3.5.6-0.1.hum1
  • openssl-config-fips hummingbird 1 3.5.6-0.1.hum1
  • openssl-devel-engine hummingbird 1 3.5.6-0.1.hum1
  • openssl RHEL 9 1:3.5.1-7.el9_7
  • openssl-libs RHEL 9 1:3.5.1-7.el9_7
  • openssl-perl RHEL 9 1:3.5.1-7.el9_7

red hat advisories RHSA-2026:7261RHSA-2026:1473RHSA-2026:1472

  • openssl-3 SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 3.5.0-160000.5.1

suse advisories SUSE-SU-2026:20211-1SUSE-SU-2026:20223-1

  • openssl-3 openSUSE Tumbleweed 3.5.3-2.1
  • openssl-3 openSUSE Leap 16.0 3.5.0-160000.5.1

opensuse advisories openSUSE-SU-2026:10237-1openSUSE-SU-2026:20152-1

  • openssl AlmaLinux 10 1:3.5.1-7.el10_1.alma.1
  • openssl-libs AlmaLinux 10 1:3.5.1-7.el10_1.alma.1
  • openssl-perl AlmaLinux 10 1:3.5.1-7.el10_1.alma.1
  • openssl AlmaLinux 9 1:3.5.1-7.el9_7
  • openssl-libs AlmaLinux 9 1:3.5.1-7.el9_7
  • openssl-perl AlmaLinux 9 1:3.5.1-7.el9_7

almalinux advisories ALSA-2026:1472ALSA-2026:1473

  • openssl Rocky Linux 10 1:3.5.1-7.el10_1
  • openssl Rocky Linux 9 1:3.5.1-7.el9_7

rocky linux advisories RLSA-2026:1472RLSA-2026:1473

  • openssl Alpine 3.21, Alpine 3.20 3.3.6-r0
  • openssl Alpine 3.24, Alpine 3.23, Alpine 3.22 3.5.5-r0

alpine advisories ALPINE-CVE-2025-66199

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • openssl Azure Linux 3.0 3.3.5-3

azure linux advisories AZL-75284

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2025-66199 updated cvss 5.9

openssl: OpenSSL: Denial of Service due to excessive memory allocation in TLS 1.3 certificate compression

  • Red Hat JBoss Core Services openssl
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Cert Manager support for Red Hat OpenShift release 1.18 cert-manager/cert-manager-operator-rhel9 1789029076, cert-manager/jetstack-cert-manager-rhel9 1791459711, cert-manager/cert-manager-istio-csr-rhel9 1791459759, cert-manager/cert-manager-operator-bundle 1791966259, cert-manager/jetstack-cert-manager-acmesolver-rhel9 1791459707
    fixed: vendor fix RHSA-2026:82354
  • Red Hat Cost Management 4 costmanagement 1770836349
    fixed: vendor fix RHSA-2026:3228
  • Red Hat Discovery 2 discovery 1769104765, discovery 1769111774
    fixed: vendor fix RHSA-2026:1736
  • Red Hat Enterprise Linux AppStream (v. 9) openssl-perl 3.5.1-7.el9_7
    fixed: vendor fix RHSA-2026:1473
  • Red Hat Enterprise Linux AppStream (v. 10) openssl-perl 3.5.1-7.el10_1
    fixed: vendor fix RHSA-2026:1472
  • Red Hat Enterprise Linux BaseOS (v. 9) openssl 3.5.1-7.el9_7, openssl-libs 3.5.1-7.el9_7
    fixed: vendor fix RHSA-2026:1473
  • Red Hat Enterprise Linux BaseOS (v. 10) openssl 3.5.1-7.el10_1, openssl-libs 3.5.1-7.el10_1
    fixed: vendor fix RHSA-2026:1472
show 4 more
  • Red Hat Hardened Images openssl 3.5.6-0.1.hum1, openssl-libs 3.5.6-0.1.hum1, openssl-perl 3.5.6-0.1.hum1, openssl-config-fips 3.5.6-0.1.hum1, openssl-devel-engine 3.5.6-0.1.hum1
    fixed: vendor fix RHSA-2026:7261
  • Red Hat Insights proxy 1.5 insights-proxy 1770740405
    fixed: vendor fix RHSA-2026:2485
  • Red Hat Update Infrastructure 5 rhui5 1770646925, rhui5 1773670073, rhui5 1773670137, rhui5 1773672059
    fixed: vendor fix RHSA-2026:4943
  • Red Hat Cost Management 4 costmanagement 1770837277 Red Hat Enterprise Linux 6 openssl, openssl-perl Red Hat Enterprise Linux 7 ovmf, OVMF, AAVMF, openssl, openssl-libs, openssl-perl Red Hat Enterprise Linux 8 edk2, shim, openssl, shim-x64, edk2-ovmf, shim-aa64, +9 more Red Hat Enterprise Linux 9 edk2, shim, shim-x64, edk2-ovmf, shim-aa64, edk2-tools, +4 more Red Hat Enterprise Linux 10 edk2, shim, shim-unsigned-x64, shim-unsigned-aarch64
    +2 more products
    Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9 Red Hat Update Infrastructure 5 rhui5 1770807477, rhui5 1770808689, rhui5 1770808765, rhui5 1773668803
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
9 mo ago

Exploit prediction

0.46% chance of exploitation in the next 30 days, higher than 40% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
OSV 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
julia OpenSSL_jll
>=3.5.0+0, <3.5.5+0
3.5.5+0

Affected products

  • OpenSSL OpenSSL

Classification

Weakness
  • CWE-789 Memory Allocation with Excessive Size Value
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.