# CVE-2025-66199: TLS 1.3 CompressedCertificate excessive memory allocation


- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 27 Jan 2026
- **Updated:** 29 Jan 2026
- **Also known as:** ALPINE-CVE-2025-66199, AZL-75284, BELL-CVE-2025-66199, CLSA-2026-1772112014, DEBIAN-CVE-2025-66199, GHSA-5888-36j9-c92p, JLSEC-2026-260, MINI-2vpp-59xh-28xc, MINI-mgc9-4rpq-57xp, UBUNTU-CVE-2025-66199

## What is CVE-2025-66199?

CVE-2025-66199 is a medium-severity vulnerability in Openssl Openssl, published on 27 Jan 2026. 

### Official 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.

Weakness: CWE-789 (Memory Allocation with Excessive Size Value)

## Which versions are affected by CVE-2025-66199?

It affects 1 product.


Products: Openssl Openssl.

## How do I fix CVE-2025-66199?

Upgrade to a fixed version.

## Is CVE-2025-66199 being exploited?

There is no known exploitation of CVE-2025-66199 in the wild and no public exploit code.

## Other vulnerabilities in Openssl Openssl

- [CVE-2009-3245](https://insights.sbomify.com/CVE-2009-3245/): Null dereference in n/a (critical 10.0)
- [CVE-2006-3738](https://insights.sbomify.com/CVE-2006-3738/): Memory corruption in n/a (critical 10.0)
- [CVE-2026-63073](https://insights.sbomify.com/CVE-2026-63073/): Untrusted Sender DN Used as Format String in CMP Response Validation (critical 9.8)
- [CVE-2016-6309](https://insights.sbomify.com/CVE-2016-6309/): Use after free in n/a (critical 9.8)
- [CVE-2016-0799](https://insights.sbomify.com/CVE-2016-0799/): Integer overflow in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-66199 and fix it if it is affected.

What it is: CVE-2025-66199 is a medium-severity vulnerability in Openssl Openssl, published on 27 Jan 2026. 
Exploitation: There is no known exploitation of CVE-2025-66199 in the wild and no public exploit code.

Affected product: Openssl Openssl. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-66199/ (Markdown: https://insights.sbomify.com/CVE-2025-66199/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66199
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66199
- OSV: https://osv.dev/vulnerability/CVE-2025-66199

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-66199/
Summaries are generated and may contain mistakes; the official description above is the record.