Zephyr RTOS CVE-2026-19735

Requires CONFIG_NET_TCP_ISN_RFC6528 to be enabled AND the cryptographic random source to FAIL at boot: sys_csrand_get() returning an error (e.g. -ENODEV/-EIO from the entropy driver in random_entropy_device.c while the hardware RNG is still gathering entropy, or psa_generate_random() failing in random_psa.c when PSA crypto is uninitialised or backed by an HCI entropy device that is not yet up). The poisoned all-zero key is only latched if that specific failure occurs during the first TCP connection's key generation; devices whose entropy source never errors are not vulnerable. Limiting factors: the attacker must observe at least one ISN off-path, the victim peer must trust the source address for spoofing to matter, and exploitation is confined to boots where the RNG failure coincides with the key-generation window (an attacker can only indirectly influence this by reaching a listening port right after boot or provoking a reboot). No authentication is needed (PR:N).

The signals converge on a conditional, moderate-severity issue rather than a top priority. The CVSS 3.1 vector (AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N, base 4.8) correctly flags AC:H because exploitation is gated on a precondition outside the attacker's direct control: sys_csrand_get() must fail at boot (e.g. -ENODEV/-EIO from the entropy driver, or psa_generate_random() failing before PSA crypto / an HCI entropy source is up), leaving unique_key as all-zero BSS. Only then does the ISN become a public function of the four-tuple plus a recoverable device time offset, enabling off-path blind spoofing, data injection, and reset - consistent with the C:L/I:L rating. Exploitation status is 'no public exploit identified at time of analysis': the input shows no CISA KEV listing and no POC flag, and no EPSS score was provided, so there is no evidence of active or at-scale exploitation. The vendor patch is confirmed. Net: a real but narrow weakness - devices on robust hardware whose entropy source never errors were never in the weak state, so this is a defense-in-depth fix for a cryptographic-failure edge case rather than a broadly exploitable network flaw. No CVSS-vs-description conflict was found; the AC:H rating is well justified by the entropy-failure dependency.

A Zephyr device using CONFIG_NET_TCP_ISN_RFC6528 boots while its hardware RNG is still gathering entropy, sys_csrand_get() returns an error, and the all-zero key silently governs every TCP connection for that boot. An off-path attacker observes a single ISN from one connection, solves offline for the device-wide time offset, and can then predict the ISN for any other four-tuple to blindly spoof, inject into, or reset TCP connections with address-trusting peers. No public exploit code is identified, and the AC:H rating reflects the required entropy-source failure.