dbcve.org
OpenSSLFramework / library

CVE-2025-66199

MEDIUM · 5.9 CVSS v3.1 Published 2026-01-27
Fix available
A fix is available. Upgrade to 3.3.6 / 3.4.4 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis

A detailed technical summary for this CVE is being prepared.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
OpenSSLFramework / library
Affected:>= 3.3.0, < 3.3.6>= 3.4.0, < 3.4.4>= 3.5.0, < 3.5.5>= 3.6.0, < 3.6.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.3.6 / 3.4.4 / 3.5.5 or later
Fixed in 3.3.63.4.43.5.5
Vendor patch github.com →
Recommended fix High confidence

OpenSSL 3.3.6, 3.4.4, 3.5.5, or 3.6.1 (depending on your current major.minor version)

  1. 1. Identify the currently installed OpenSSL version using `openssl version` or checking package manager.
  2. 2. For systems running OpenSSL 3.3.x: upgrade to version 3.3.6 or later.
  3. 3. For systems running OpenSSL 3.4.x: upgrade to version 3.4.4 or later.
  4. 4. For systems running OpenSSL 3.5.x: upgrade to version 3.5.5 or later.
  5. 5. For systems running OpenSSL 3.6.x: upgrade to version 3.6.1 or later.
  6. 6. Verify the upgrade was successful by running `openssl version` and confirming the version number matches or exceeds the appropriate fixed version for your branch.
  7. 7. Alternatively, if upgrading is not immediately feasible, apply the mitigation by setting the SSL_OP_NO_RX_CERTIFICATE_COMPRESSION option in your application code or SSL context configuration to disable receiving compressed certificates.

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,600. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-66199 in production — separate from our analysis above.

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