Use-After-Free in X.509 Extension Cache Under Concurrent Use (CVE-2026-84783)

What is affected, what to do

A flaw was found in OpenSSL. A remote, unauthenticated attacker can cause a Denial of Service (DoS) by initiating concurrent connections to a multi-threaded Transport Layer Security (TLS) client or server verifying certificates.

Update OpenSSL to version 4.0.3.

Affected

• OpenSSL lt 4.0.0..4.0.3

Fixed in

• OpenSSL 4.0.3

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Metrics

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

A flaw was found in OpenSSL. A remote, unauthenticated attacker can cause a Denial of Service (DoS) by initiating concurrent connections to a multi-threaded Transport Layer Security (TLS) client or server verifying certificates. Due to improper synchronization when multiple threads simultaneously decode and cache certificate extensions for a shared Certificate Authority (CA) certificate, cached memory can be freed while still in use by another thread. This use-after-free condition results in an invalid memory read, causing the application to crash.

Source: redhat_csaf, cvelistv5, NVD (NIST), BSI WID Portal, BSI CSAF

Vendors and products

• Freebsd Project Freebsd OS
• Open Source Openssl
• OpenSSL OpenSSL

Weakness classes (CWE)

• CWE-416Use After Free

  Variant

  The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

  Entry at MITRE CWE-416

References & sources

• openssl-library.orgOpenSSL AdvisoryVendor advisory
• github.com4.0.3 git commitPatch

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

1. 08.10.2026Initial analysis by NVDNVD

   Newly listed as affected:

   • OpenSSL (4.0.0 – before 4.0.3)

   New references:

   • https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081
   • https://openssl-library.org/news/secadv/20260929.txt
   Show 3 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Product configuration	added	—	OR *cpe:2.3:a:openssl:openssl:*:*:*:*:*:*:*:* versions from (including) 4.0.0 up to (excluding) 4.0.3
   Reference type	added	—	OpenSSL Software Foundation: https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081 Types: Patch
   Reference type	added	—	OpenSSL Software Foundation: https://openssl-library.org/news/secadv/20260929.txt Types: Vendor Advisory

   Source identifier: nvd@nist.gov

2. 29.09.2026Record modifiedCISA ADP

   CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

   CISA assessment (SSVC): exploitation: none known, automatable: yes

   Show 2 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
   SSVC assessment	added	—	{"id":"CVE-2026-84783","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-29T16:42:36.722506Z"}

   Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

3. 29.09.2026Receivedopenssl.org

   Weakness type: CWE-416

   Description added or changed

   New references:

   • https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081
   • https://openssl-library.org/news/secadv/20260929.txt
   Show 5 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Description	added	—	Issue summary: The first concurrent use of the same X.509 certificate by several threads may cause its cached extension data to be freed while another thread is still using it. Impact summary: A remote, unauthenticated peer could crash a multi-threaded TLS client, or a multi-threaded TLS server that requests client certificates, if the first certificate chains built to the same trusted CA certificate are built by several connections at the same time. This is a use-after-free read, which is likely to crash the process, resulting in a Denial of Service. CWE: CWE-416: Use After Free Description: OpenSSL caches the decoded values of a certificate's X.509v3 extensions inside the X509 object the first time they are needed. In OpenSSL 4.0 this cache is built in two phases: the extension values are computed while holding a read lock on the certificate, and the results are then installed into the certificate under a write lock. Because a read lock does not exclude other readers, several threads can compute the cache for the same certificate at the same time. Each thread that subsequently acquires the write lock installs its own results and frees the values installed by the thread before it, even though that earlier thread has already marked the cache as complete and may have returned pointers into it to its caller. A caller still using those pointers then reads freed memory. Any certificate shared between threads is exposed the first time its extensions are decoded. In TLS the certificates at risk are the trusted CA certificates supplied for chain verification, by whatever means, since these are shared by every connection and their extensions are decoded and cached the first time a chain is built to them. Certificates sent by the peer are decoded separately for each connection and are not shared, so they are not affected. In a TLS client verifying server certificates, or a TLS server that requests and verifies client certificates, the use-after-free could only occur if the first chains built to the same trusted CA are built by several connections at the same time. FIPS impact: no The FIPS module is not affected as X.509 certificate handling is outside of the OpenSSL FIPS module boundary. OpenSSL 4.0 is vulnerable to this issue. OpenSSL 3.6, 3.5, 3.4, 3.0, 1.1.1 and 1.0.2 are not affected by this issue. OpenSSL 4.0 users should upgrade to OpenSSL 4.0.3. This issue was reported on 27 August 2026 by Tim Becker (Xint.io) and independently in a public report on 31 August 2026 by aydinmercan. The fix has been developed by Bob Beck. -- cut (non-publishing metadata for internal use) -- Reported by: Tim Becker (Xint.io), aydinmercan Fixed by: Bob Beck
   Weakness type (CWE)	added	—	CWE-416
   Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/84xxx/CVE-2026-84783.json">CVE-2026-84783</a>
   Reference	added	—	https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081
   Reference	added	—	https://openssl-library.org/news/secadv/20260929.txt

   Source identifier: openssl-security@openssl.org