CVE-2026-42768: Issue summary: The CMS_decrypt and PKCS7_decrypt functions are vulnerable to Bleichenbacher-style attack when an attacker is able to provide the CMS or S/MIME m

Issue summary: The CMS_decrypt and PKCS7_decrypt functions are vulnerable to Bleichenbacher-style attack when an attacker is able to provide the CMS or S/MIME m

Issue summary: The CMS_decrypt and PKCS7_decrypt functions are vulnerable to Bleichenbacher-style attack when an attacker is able to provide the CMS or S/MIME messages and observe the error code and/or decryption output. Impact summary: The Bleichenbacher-style attack allows an attacker to use the victim's vulnerable application as a way to decrypt or sign messages with the victim's private RSA key. The attack is possible in 2 variants. 1. The decryption API (CMS_decrypt(), PKCS7_decrypt()) is used without providing the recipient certificate. In this case OpenSSL iterates over every KeyTransRecipientInfo (KTRI) without stopping at the first success. An attacker who authors a message with two KTRI entries — the first one wrapping a real CEK under the victim's public key, the second with an arbitrary probe ciphertext — obtains opportunity to iterate the 2nd KTRI to get a valid PKCS#1 v1.5 padding if the error code of the application is available. That is a Bleichenbacher oracle (Bleichenbacher, CRYPTO '98): an adaptive-chosen-ciphertext side channel from which the attacker decrypts any RSA ciphertext to the victim's key or forges any PKCS#1 v1.5 signature under it. 2. When the

Description from the National Vulnerability Database, published by openssl-security@openssl.org

What it means for a small business

OpenSSL runs on web servers, so this is mostly your hosting company's to fix. On shared hosting you can't patch it yourself, but you can ask.

Ask your host whether your server runs it, whether it's updated, and on what date. A good host answers that in a day.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 0.80%, higher than 56% of CVEs. That’s low, but low isn’t zero, and the odds jump the day someone publishes an exploit. Scored October 28, 2026, updated daily.

Check your own setup

Website check: reads what your site runs from the outside (WordPress version, plugins it can see, old server software) and matches it against this database.
Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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