---
title: "CVE-2026-84783: OpenSSL - Use After Free"
description: "OpenSSL 4.0 contains a use-after-free vulnerability caused by concurrent caching of X.509 certificate extensions in multi-threaded environments, letting remote unauthenticated peers cause denial of service by crashing TLS clients or servers, exploit requires concurrent chain building to the same trusted CA certificate."
url: https://panop.io/threat-heartbeat/CVE-2026-84783/
---

# CVE-2026-84783 OpenSSL - Use After Free

OpenSSL 4.0 contains a use-after-free vulnerability caused by concurrent caching of X.509 certificate extensions in multi-threaded environments, letting remote unauthenticated peers cause denial of service by crashing TLS clients or servers, exploit requires concurrent chain building to the same trusted CA certificate.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.23% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-84783)

## Details

Type

Use After Free

Impact

Denial Of Service

CVSS vector

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

Published

29 September 2026

Updated

15 October 2026

Weakness

[CWE-416](https://cwe.mitre.org/data/definitions/416.html)

## Vendor fix

Upgrade to OpenSSL 4.0.3 or later.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-84783](https://nvd.nist.gov/vuln/detail/CVE-2026-84783)
- [https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081](https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081)
- [https://openssl-library.org/news/secadv/20260929.txt](https://openssl-library.org/news/secadv/20260929.txt)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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