Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-09-29T15:32:24 openssl: openssl: Denial of Service via race condition in certificate extension caching 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H CWE-825

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

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.

This vulnerability is rated as Moderate impact for Red Hat products because exploitation requires precise race timing during initial concurrent TLS handshakes, resulting in a denial of service without compromising system integrity or confidentiality. The condition is limited to multi-threaded environments where several connections concurrently decode and cache extensions of the same shared Certificate Authority certificate on first use. Because the resulting memory read invalidation causes an application abort rather than remote code execution, the blast radius is confined to process crashes during simultaneous startup traffic bursts. Red Hat Hardened Images 2026-08-25T00:00:00Z RHSA-2026:59641 openssl-main-3.5.8-0.1.hum1 Red Hat Hardened Images 2026-09-23T00:00:00Z RHSA-2026:70734 openssl3-main-3.5.8-1.hum1 Red Hat Developer Hub 1 Fix deferred rhdh/rhdh-hub-rhel9 Red Hat Enterprise Linux 10 Fix deferred edk2 Red Hat Enterprise Linux 10 Affected openssl Red Hat Enterprise Linux 10 Fix deferred shim Red Hat Enterprise Linux 10 Fix deferred shim-unsigned-aarch64 Red Hat Enterprise Linux 10 Fix deferred shim-unsigned-x64 Red Hat Enterprise Linux 6 Out of support scope openssl Red Hat Enterprise Linux 7 Fix deferred openssl Red Hat Enterprise Linux 7 Fix deferred ovmf Red Hat Enterprise Linux 7 Fix deferred shim-signed Red Hat Enterprise Linux 8 Fix deferred compat-openssl10 Red Hat Enterprise Linux 8 Fix deferred edk2 Red Hat Enterprise Linux 8 Fix deferred mingw-openssl Red Hat Enterprise Linux 8 Fix deferred openssl Red Hat Enterprise Linux 8 Fix deferred shim Red Hat Enterprise Linux 8 Fix deferred shim-unsigned-aarch64 Red Hat Enterprise Linux 8 Fix deferred shim-unsigned-x64 Red Hat Enterprise Linux 9 Fix deferred compat-openssl11 Red Hat Enterprise Linux 9 Fix deferred edk2 Red Hat Enterprise Linux 9 Not affected openssl Red Hat Enterprise Linux 9 Fix deferred shim Red Hat Enterprise Linux 9 Fix deferred shim-unsigned-aarch64 Red Hat Enterprise Linux 9 Fix deferred shim-unsigned-x64 Red Hat Hardened Images Not affected unbound Red Hat JBoss Core Services Fix deferred jbcs-httpd24-openssl Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-9 Red Hat OpenShift Container Platform 5 Fix deferred openshift/ose-rhel-coreos-9 Red Hat Satellite 6 Fix deferred openvox-agent Red Hat Satellite 6 Fix deferred puppet-agent https://www.cve.org/CVERecord?id=CVE-2026-84783 https://nvd.nist.gov/vuln/detail/CVE-2026-84783 https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081 https://openssl-library.org/news/secadv/20260929.txt