# CVE-2026-84783: Use-After-Free in X.509 Extension Cache Under Concurrent Use > A race condition in OpenSSL's X.509 extension cache allows remote attackers to crash multi-threaded TLS clients or servers via use-after-free. - **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 29 Sep 2026 - **Updated:** 29 Sep 2026 - **Also known as:** GHSA-2pj2-4hfp-vxp8, RHSA-2026:70734, UBUNTU-CVE-2026-84783 ## What is CVE-2026-84783? CVE-2026-84783 is a high-severity use after free vulnerability in Openssl Openssl, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description 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: CWE-416 (Use After Free) ## Which versions are affected by CVE-2026-84783? It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `openssl3` | Red Hat:hummingbird:1 | All versions | 0:3.5.8-1.hum1 | | `openssl3-devel` | Red Hat:hummingbird:1 | All versions | 0:3.5.8-1.hum1 | | `openssl3-devel-engine` | Red Hat:hummingbird:1 | All versions | 0:3.5.8-1.hum1 | | `openssl3-libs` | Red Hat:hummingbird:1 | All versions | 0:3.5.8-1.hum1 | Products: Openssl Openssl. ## How do I fix CVE-2026-84783? Apply the vendor's patch. ## Is CVE-2026-84783 being exploited? There is no known exploitation of CVE-2026-84783 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-2026-84783 and fix it if it is affected. What it is: CVE-2026-84783 is a high-severity use after free vulnerability in Openssl Openssl, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-84783 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-2026-84783/ (Markdown: https://insights.sbomify.com/CVE-2026-84783/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84783 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84783 - OSV: https://osv.dev/vulnerability/CVE-2026-84783 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-84783/ Summaries are generated and may contain mistakes; the official description above is the record.