Bleichenbacher Padding Oracle Vulnerability in wolfSSL PKCS#7 Decryption

CVE-2026-6291

What is CVE-2026-6291?

A vulnerability exists in the wolfSSL library that allows an attacker to exploit distinguishable error codes during the PKCS#7 decryption process. Specifically, when the library handles RSA PKCS#1 v1.5 key transport, the error codes issued differ based on the success of RSA padding validation versus other decryption errors. An attacker capable of sending modified EnvelopedData messages and analyzing the responses can utilize this discrepancy as a padding oracle, facilitating the stepwise recovery of the encrypted Content Encryption Key (CEK). To mitigate this risk, a recent patch introduces a mechanism that generates a deterministic pseudo-random fake CEK in the event of padding failures while ensuring that all operations are executed in constant time, thus normalizing error reporting across all failure paths.

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