CVE-2026-19735: crypto in zephyrproject zephyr

The vulnerability in Zephyr's TCP ISN generation (RFC 6528 implementation) occurs because the unique 128-bit secret key is generated once at boot using sys_csrand_get(). If this call fails, the key remains zeroed with no retry or error logged, causing all TCP ISNs to be derived from a predictable function of the connection four-tuple and a device-wide time offset. This predictable ISN allows off-path attackers to compute ISNs for arbitrary connections after observing one, defeating RFC 6528's protection against blind TCP spoofing and injection. The failure can be triggered by entropy driver errors during early boot or uninitialized PSA crypto. The fix moves key generation into a guarded helper that only latches on success, logs errors, and falls back to sys_rand32_get() when necessary.

An attacker can predict TCP initial sequence numbers if the cryptographic random source fails during boot, enabling blind TCP connection spoofing and data injection or reset attacks against trusted peers. This compromises the integrity and trust of TCP connections relying on ISN randomness. Devices with no entropy source failures are not affected.

A fix is available that ensures the secret key for TCP ISN generation is only latched on successful cryptographic random generation, logs errors on failure, and falls back to a safer random method. Users should upgrade to a version including this fix. Until patched, monitoring for entropy source failures during boot and avoiding exposure of listening TCP ports immediately after reboot may reduce risk.