{"id":"CVE-2026-87043","title":"sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready","summary":"The entropy-device random number generator in subsys/random/random_entropy_device.c returned early with -ENODEV from rand_get() as soon as device_is_ready() failed, before reaching the non-cryptographic system-timer fallback further down…","severity":"medium","cvss":4.8,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","cvssSource":"cna","cwe":["CWE-330"],"vendor":"zephyrproject","product":"zephyr","affected":["zephyr >= 4.4.0 < 4.5.0"],"published":"2026-11-04","updated":"2026-11-04","sourceUpdated":"2026-11-04T06:09:08.600Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-87043","references":[{"url":"https://github.com/zephyrproject-rtos/zephyr/commit/76eb10b5c3e93de5218cc9f1aa313ece3eb5a6e6","label":"Fix commit"},{"url":"https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j6p3-2pm6-rrj9","label":"GHSA-j6p3-2pm6-rrj9"}],"tags":["cve.org"],"epss":0.00183,"epssPercentile":0.07973,"ingestedAt":"2026-11-05T20:33:06.721Z","slug":"CVE-2026-87043","body":"## Overview\n\nThe entropy-device random number generator in subsys/random/random_entropy_device.c returned early with -ENODEV from rand_get() as soon as device_is_ready() failed, before reaching the non-cryptographic system-timer fallback further down the same function. Because z_impl_sys_rand_get() returns void and discards that error — and the public sys_rand_get() syscall is itself declared void, so it cannot report failure — the caller's output buffer was left untouched and its previous contents were handed back as random data. The thin wrappers make this a plain uninitialized read: sys_rand32_get() declares uint32_t ret;, calls sys_rand_get(&ret, sizeof(ret)) and returns ret.\n\nThe \"not ready\" state is a normal runtime condition rather than a fault. Entropy drivers initialise at PRE_KERNEL_1 with CONFIG_ENTROPY_INIT_PRIORITY (default 50), so any earlier initialiser — kernel objects run at priority 30 — observes an unready device; the sibling generator subsys/random/random_xoshiro128.c and z_early_rand_get() in kernel/init.c both already carried an explicit timer fallback for exactly this case. The state is also permanent for the boot when an entropy driver's init hook fails, which several in-tree drivers can do on real hardware (entropy_stm32 on an RNG clock configuration error, entropy_psa_crypto when psa_crypto_init() fails, entropy_nrf_cracen, entropy_esp32, entropy_virtio).\n\nWhen this occurs, every non-cryptographic random value in the system is either a constant or stale memory. coap_next_token() in subsys/net/lib/coap/coap.c fills a static buffer, so every CoAP token becomes the same .bss constant; tcp_init_isn() in subsys/net/ip/tcp.c returns an uninitialized stack word as the TCP initial sequence number; DHCPv4/DHCPv6 transaction IDs, WebSocket masking keys and RTP SSRCs are similarly degraded, and arch/arm64/core/pac.c derives per-thread Pointer Authentication keys from the same source. Where the bytes are transmitted directly — SSH packet padding, the netmidi2 session nonce, a DTLS connection ID on builds without CONFIG_CSPRNG_ENABLED — stale caller stack or heap contents are disclosed on the wire.\n\nThe fix initialises ret to -ENODEV and only calls entropy_get_entropy() when the device is ready, so the not-ready case now falls through to the timer fallback and the buffer is always written. The cryptographically secure path is unchanged: sys_csrand_get() still propagates the error to the caller and never silently degrades. An attacker cannot induce the unready state, but can observe and exploit the resulting non-random values remotely on an affected device.\n\n## Affected\n\n- `zephyr >= 4.4.0 < 4.5.0`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":26,"depthScoreParts":{"impact":26.4,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}