OpenSSL's 29 September Advisory: 14 CVEs, and Most Need DTLS or QUIC

On 29 September 2026 the OpenSSL project published a security advisory covering 14 CVEs and released OpenSSL 4.0.3, 3.6.5, 3.5.9 and 3.4.8. One issue is rated High, one Moderate and twelve Low. For the 3.0, 1.1.1 and 1.0.2 lines, the advisory lists fixed releases (3.0.23, 1.1.1zj and 1.0.2zs) for premium support customers only.

OpenSSL is linked into web servers, language runtimes, databases and most container base images, so a 14 CVE advisory produces a lot of scanner findings. But the High is in DTLS, five of the Lows are in the QUIC stack, and several more need a specific API call or algorithm. Most TLS-over-TCP services reach only a few of them.

What's affected

The High, CVE-2026-84782. DTLS can write a handshake message in fragments and suspend part-way with WANT_WRITE. If the retransmission timer fired during that pause, OpenSSL resent an earlier message from the wrong buffer offset. The advisory says the retransmitted message "can disclose a heap memory to the peer as plaintext handshake data" or crash the process. Every line from 1.0.2 to 4.0 is affected.

The Moderate, CVE-2026-84783. On OpenSSL 4.0 only, the first concurrent use of one X.509 certificate by several threads could free cached extension data that another thread was still reading. A remote peer could crash a multi-threaded TLS client, or a multi-threaded server that requests client certificates, when several connections first build chains to the same CA at once. 3.6 and earlier are not affected.

Lows worth a second look. CVE-2026-35189 needs no special feature: a certificate under the roughly 100 KiB peer limit could cause several hundred MiB of memory allocation during a normal handshake. It affects clients, and servers that ask for client certificates. CVE-2026-72897 matters for servers that switch contexts with SSL_set_SSL_CTX(), a call commonly made in SNI callbacks. The advisory says apps that never call it are not affected.

Four of the five QUIC issues are memory and CPU exhaustion bugs, with peaks the advisory puts at around 100 MB and 400 MB for two of them. The fifth lets a QUIC server that skips address validation be used to amplify a DDoS attack. The advisory notes the QUIC stack is outside the FIPS module boundary.

How to check if you're exposed

Find which OpenSSL each component actually loads:

openssl version -a
python3 -c "import ssl; print(ssl.OPENSSL_VERSION)"
node -p "process.versions.openssl"
ldd "$(command -v nginx)" 2>/dev/null | grep -E "libssl|libcrypto"

Inside container images, look for every copy, including ones vendored by language packages:

find / -name "libssl.so*" -o -name "libcrypto.so*" 2>/dev/null

Then ask the feature questions:

• Do you speak DTLS? WebRTC media stacks, some VPNs and constrained-device protocols use it. Plain HTTPS servers do not.
• Do you use OpenSSL's QUIC implementation? Many HTTP/3 deployments use a different QUIC library. Check how your server or client was built before assuming either way.
• Is anything multi-threaded on OpenSSL 4.0? Most servers and many clients are, so 4.0 users should treat CVE-2026-84783 as reachable.
• Do you request client certificates? That makes CVE-2026-35189 and CVE-2026-84783 reachable on the server side.

What to do now

1. Move to 4.0.3, 3.6.5, 3.5.9 or 3.4.8, or to your distribution's patched package. Most Linux vendors backport fixes into their own version strings, so check their advisory rather than comparing upstream numbers.
2. If you rely on 3.0 or 1.1.1 from upstream, note that fixed releases for those lines are listed for premium support customers only. Use a vendor that maintains your line, or plan the move to a supported release.
3. Prioritize DTLS endpoints for the High. The heap disclosure goes to the peer, so any DTLS service facing untrusted clients comes first.
4. Upgrade OpenSSL 4.0 users quickly because of CVE-2026-84783, which needs no unusual feature.
5. Rebuild images and restart processes. A running process keeps the old library in memory until it restarts.

Why it matters

A long advisory is not the same as a big one. Fourteen CVEs read as a crisis on a dashboard. Sorted by feature, most of them fall away for a typical HTTPS service: no DTLS, no OpenSSL QUIC, no SM2, no CMP client. What remains is a short list that needs attention, and the right engineers can focus on it.

The flip side is that the DTLS and QUIC users get a dense set of issues at once. If your product uses those protocols, this advisory is mostly about you.

Try it yourself

For one production host, run the version commands above and fill in a two-column list: each CVE from the table, and "reachable" or "not reachable" based on the features you use. Then compare it with what your scanner reported for the same host.

Click through it at your own pace, or open it full screen.

Quick check

Sources

• OpenSSL Security Advisory, 29 September 2026
• OpenSSL vulnerabilities list
• OpenSSL security policy