Highly exploitable, CVE-2026-84783 poses a critical security risk that could lead to severe breaches.
Status: Received on 29 Sep 2026, 16:17 UTC
Published on: 29 Sep 2026, 16:17 UTC
CVSS Release: version 3
134c704f-9b21-4f2e-91b3-4a467353bcc0
Secondary
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVE-2026-84783: 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
The exploitability of CVE-2026-84783 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).
With low attack complexity and no required privileges, CVE-2026-84783 is an easy target for cybercriminals. Organizations should prioritize immediate mitigation measures to prevent unauthorized access and data breaches.
A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.
Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.
Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.
Above is the CVSS Sub-score Breakdown for CVE-2026-84783, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.
Below is the Impact Analysis for CVE-2026-84783, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.
Stay updated with real-time CVE vulnerabilities and take action to secure your systems. Enhance your cybersecurity posture with the latest threat intelligence and mitigation techniques. Develop the skills necessary to defend against CVEs and secure critical infrastructures. Join the top cybersecurity professionals safeguarding today's infrastructures.