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Fencer Vulns
high 7.5

CVE-2026-84783 json

Use-After-Free in X.509 Extension Cache Under Concurrent Use

published 2026-09-29 40 d ago modified 2026-10-08 32 d ago also EUVD-2026-89014

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 21%
public exploits
0
none known

Summary

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

Patch / fix

  • OpenSSL OpenSSL 4.0.3
  • openssl3 hummingbird 1 3.5.8-1.hum1
  • openssl3-libs hummingbird 1 3.5.8-1.hum1
  • openssl3-devel-engine hummingbird 1 3.5.8-1.hum1
  • openssl hummingbird 1 3.5.8-0.1.hum1
  • openssl-libs hummingbird 1 3.5.8-0.1.hum1
  • openssl-perl hummingbird 1 3.5.8-0.1.hum1
  • openssl-config-fips hummingbird 1 3.5.8-0.1.hum1
  • openssl-devel-engine hummingbird 1 3.5.8-0.1.hum1

red hat advisories RHSA-2026:70734RHSA-2026:59641

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-84783 updated cvss 5.9

openssl: openssl: Denial of Service via race condition in certificate extension caching

  • Red Hat Developer Hub 1 rhdh/rhdh-hub-rhel9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Enterprise Linux 6 openssl, openssl-perl, openssl 3.5.8-1.el10_2
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 ovmf, OVMF, shim, AAVMF, mokutil, openssl, +6 more Red Hat Enterprise Linux 8 openssl, shim-x64, edk2-ovmf, shim-aa64, shim-ia32, edk2-aarch64, +13 more Red Hat Enterprise Linux 9 shim-x64, edk2-ovmf, shim-aa64, edk2-tools, edk2-aarch64, compat-openssl11, +5 more Red Hat Enterprise Linux 10 shim 16.1-4.el10, edk2 20251114-5.el10_2.2, shim-unsigned-x64 16.1-1.el10, shim-unsigned-aarch64 16.1-2.el10 Red Hat JBoss Core Services jbcs-httpd24-openssl, jbcs-httpd24-openssl-libs, jbcs-httpd24-openssl-perl Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9
    +2 more products
    Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9 Red Hat Satellite 6 puppet-agent, openvox-agent 8.28.1-1.el9sat
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images openssl3 3.5.8-1.hum1, openssl 3.5.8-0.1.hum1, openssl3-libs 3.5.8-1.hum1, openssl-libs 3.5.8-0.1.hum1, openssl-perl 3.5.8-0.1.hum1, openssl-config-fips 3.5.8-0.1.hum1, +3 more
    fixed: vendor fix RHSA-2026:59641
  • Red Hat Enterprise Linux 9 openssl, openssl-libs, openssl-perl, openssl 3.5.8-1.el10_2 Red Hat Enterprise Linux 10 openssl 3.5.8-1.el10_2 Red Hat Hardened Images unbound 1.26.1-1.hum1
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
40 d ago

Exploit prediction

0.29% chance of exploitation in the next 30 days, higher than 21% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

  • OpenSSL OpenSSL

Classification

Weakness
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.