Bug 2543243 (CVE-2026-84783)

Summary:	CVE-2026-84783 openssl: openssl: Denial of Service via race condition in certificate extension caching
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	abarbaro, akhatavk, alizardo, anthomas, aos-team-art-private, asdas, csutherl, dpaolell, ehelms, ggainey, jchui, jclere, jdelft, jhe, jpasqual, jupierce, juwatts, ktsao, lgarciaa, mbiarnes, mdellweg, mhulan, nboldt, nmoumoul, oaljalju, osousa, pcreech, pjindal, plodge, ppalepu, ppostler, prdhamdh, psrna, rchan, rhel-process-autobot, sghai, sidsharm, smallamp, suppawar, szappis, tmalecek, vchlup, vlaad, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	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.	Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-29 15:52:14 UTC

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