OpenSSL 4.0.2 Patches 11 Vulnerabilities, Keeps the QUIC Stack on the Hot Seat
The August update lands alongside parallel fixes for OpenSSL 3.6, 3.5, 3.4, 3.0, and extended support branches.
OpenSSL has dropped 4.0.2, and if you run anything built on the cryptographic library, you should patch it. The release addresses eleven vulnerabilities, with the most severe rating a solid Moderate. No critical paths. No remote code execution. Just a long list of memory mismanagement, format string slips, and QUIC stack growing pains.
The release came out August 25, 2026, marking the second coordinated patch of the month for the 4.0 and 3.6 lines. OpenSSL also pushed 3.6.4, 3.5.8, 3.4.7, and 3.0.22 on the same day. Even the legacy 1.1.1 and 1.0.2 branches got their extended-support updates. It's a reminder that cryptography libraries do not stop working just because the headlines moved on.
In the months since 4.0.1 shipped in June, the project has settled into a two-per-month cadence that feels almost aggressive. You get used to it. The 4.0 branch has been quietly carrying more protocol weight than most people realize, and this update proves it. The advisory confirms this is the first major coordinated disclosure for the 4.0 branch alongside parallel fixes across the supported lines. Keep in mind that the underlying architecture is taking on more load than it did two releases ago.
The QUIC Stack Takes the Heat
Five of the eleven CVEs touch QUIC. CVE-2026-18798 is the one that actually gets a Moderate rating, fixing a double-free in the server's default packet handler. A malformed INITIAL packet with a short destination connection ID trips the error path, freeing a QRX object twice. Heap corruption follows. Process dies. Remote code execution remains highly improbable, but denial of service is the headline.
There is also CVE-2026-63075, which keeps metadata for every ACK-only packet the stack sends. A peer can force your connection into a memory leak loop. Combine that with the double-free, and you have a protocol that is still maturing under a microscope. Not that the other fixes are small.
The DTLS record buffering issue (CVE-2026-54874) is a low-severity memory amplifier that retains roughly 16 KB per buffered record instead of just the actual payload. Up to 100 records per connection means about 1.7 MB of retained memory from relatively small network traffic. It is a 1,200× amplification factor, which sounds harmless until your server starts trading RAM for CPU cycles.
CVE-2026-63072 covers a heap buffer overflow in CMS key unwrapping. Changing an OID from AES-wrap to AES-wrap-pad makes the library write eight bytes past the allocated buffer. Deterministic. Denial of service. No RCE.
Three CVEs target CMP, which handles automated certificate enrollment. It is a niche protocol, but the bugs are real. One leaks memory when extra certificates are not cleared. Another crashes the server because a password protection algorithm pointer never gets type-checked. The third feeds an untrusted sender DN straight into a format string. Percent characters survive the conversion. You will not get code execution, but you will get a crash.
The remaining low-severity items round out the advisory. AEAD ciphers skip tag verification when handed an empty ciphertext (CVE-2026-75803). OCSP response checking leaks memory on malformed stapled responses (CVE-2026-54876). Raw public key servers crash when a peer requests the signature_algorithms_cert extension (CVE-2026-14457). All of it lands in the Low bucket.
Trail of Bits found four of the eleven issues. AWS, OpenAI, the ZKSC Institute, Red Hat, and Rochester Institute of Technology each contributed a fix or discovery. The OpenSSL project's advisory makes it clear that fuzzing is doing the heavy lifting here. These are resource management and boundary-check bugs caught before they could snowball into something worse.
It is a rather thorough update for what it looks like on the surface. Eleven patches, all non-critical, but the QUIC and CMP surfaces are still wide open. The library is growing faster than the attack surface is being hardened. That is not unique to OpenSSL, but it is worth tracking. Every month feels like a new protocol layer gets pulled in, and every new layer brings its own memory management headaches. Fuzzers are catching the resource leaks early, but the underlying architecture is taking on more load than it did two releases ago.
If you are running OpenSSL 4.0.0 through 4.0.1, upgrade to 4.0.2. If you are on 3.6 through 3.0, grab the matching point release for your branch. Legacy 1.1.1 and 1.0.2 users should pull 1.1.1zi and 1.0.2zr.
Head here to review the full advisory and CVE list and here to the GitHub release page.
