Without the NO_SESSION_CACHE_REF option defined, wolfSSL_get_session() returns a ClientSession reference validated only by a hash of the session ID. Since TLS 1.2 session IDs are server-chosen and sent in cleartext, an attacker can overwrite the client-side cache entry with their own master secret and parameters if they reuse the same session ID. This poisoned cache entry persists globally across WOLFSSL_CTX boundaries until eviction or timeout. Resuming a session via this handle bypasses certificate verification and domain name checks, allowing the attacker to be accepted as the original server. The vulnerability affects wolfSSL builds from v5.3.0 through v5.9.2 with default or certain integration options that leave NO_SESSION_CACHE_REF undefined. The fix involves adding a generation counter to the cache and raising WOLFSSL_CACHE_VERSION from 2 to 3, preventing acceptance of poisoned cache entries from older versions.

An attacker can impersonate the original server during TLS 1.2 or DTLS 1.2 session resumption by poisoning the session cache. This leads to bypass of certificate chain verification and domain name checks, potentially allowing man-in-the-middle attacks or unauthorized access. The vulnerability does not affect TLS 1.3 or ticket-based resumption, which use client-chosen cache keys. The poisoned cache entry persists in the process-global cache and crosses context boundaries until evicted or timed out (default 500 seconds).

A fix is available that adds a per-write generation counter to the session cache and increases the cache version to reject poisoned entries. Users should upgrade to a fixed version beyond v5.9.2. Applications using wolfSSL should avoid configurations that leave NO_SESSION_CACHE_REF undefined or should define NO_SESSION_CACHE_REF or use integration options that do so. wolfSSL_get1_session() is not affected and can be used as a safer alternative. Patch status is confirmed by the vendor's fix described in the advisory.