CVE-2025-66199Network attack vectorHinoki check available

Service interruption via unchecked certificate length

Published Jan 27, 2026 · Updated Jan 27, 2026

Excessive allocation in OpenSSL certificate decompression allows remote attackers to interrupt services via compressed TLS 1.3 certificates. The TLS 1.3 CompressedCertificate handler grows a heap buffer from the peer-supplied uncompressed length before decompression without enforcing max_cert_list. Reachability requires compiled-in certificate compression, an available compression algorithm, and extension negotiation; clients can accept a malicious server certificate, while servers require mutual TLS, and each failed handshake can consume about 22 MiB plus CPU.

CVSS severity5.9
Medium
EPSS probability0.42%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

Excessive allocation in OpenSSL certificate decompression allows remote attackers to interrupt services via compressed TLS 1.3 certificates. The TLS 1.3 CompressedCertificate handler grows a heap buffer from the peer-supplied uncompressed length before decompression without enforcing max_cert_list. Reachability requires compiled-in certificate compression, an available compression algorithm, and extension negotiation; clients can accept a malicious server certificate, while servers require mutual TLS, and each failed handshake can consume about 22 MiB plus CPU.

The record

CVE
CVE-2025-66199
Published
Jan 27, 2026
Updated
Jan 27, 2026
Vendor
OpenSSL Project
Product
OpenSSL
Classifications
CWE-789, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 27, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 27, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=3.3.0 <3.3.6
  2. Affected versionversion=3.4.0 <3.4.4
  3. Affected versionversion=3.5.0 <3.5.5
  4. Affected versionversion=3.6.0 <3.6.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

OpenSSL Project · OpenSSLversion=3.3.0 <3.3.6; version=3.4.0 <3.4.4; version=3.5.0 <3.5.5; version=3.6.0 <3.6.1

Published CVSS scores

5.9CISA-ADPCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-789Memory Allocation with Excessive Size Value
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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