{"id":"CVE-2026-87043","aliases":["EUVD-2026-116746","GHSA-j6p3-2pm6-rrj9"],"status":"published","title":"sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready","description":"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 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(\u0026ret, 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.","severity":{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.00183,"percentile":0.07972,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"cwes":["CWE-330"],"published":"2026-11-04T06:09:08Z","modified":"2026-11-05T03:31:29Z","products":[{"vendor":"zephyrproject","product":"zephyr"},{"vendor":"zephyrproject-rtos","product":"zephyr"}],"references":[{"url":"https://github.com/zephyrproject-rtos/zephyr/commit/76eb10b5c3e93de5218cc9f1aa313ece3eb5a6e6","tags":["patch","fix"]},{"url":"https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-j6p3-2pm6-rrj9","tags":["advisory"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/87xxx/CVE-2026-87043.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-87043","tags":["advisory"]},{"url":"https://github.com/zephyrproject-rtos/zephyr","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-87043","title":"sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready","description":"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 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(\u0026ret, 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.","status":"published","published":"2026-11-04T06:09:08.6Z","modified":"2026-11-09T21:23:14.313Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-330"],"updated_at":"2026-11-09T22:35:09.300039Z"}],"euvd":[{"id":"EUVD-2026-116746","description":"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 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(\u0026ret, 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.","status":"published","published":"2026-11-04T06:09:08Z","modified":"2026-11-09T21:23:14Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"updated_at":"2026-11-09T22:18:21.405141Z"}],"nvd":[{"id":"CVE-2026-87043","description":"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 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(\u0026ret, 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.","status":"published","published":"2026-11-04T06:17:07.103Z","modified":"2026-11-09T22:18:56.387Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"secondary"}],"cwes":["CWE-330"],"updated_at":"2026-11-09T23:16:04.854042Z"}],"osv":[{"id":"CVE-2026-87043","title":"sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready","description":"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 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(\u0026ret, 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.","status":"published","published":"2026-11-04T06:09:08.6Z","modified":"2026-11-11T07:23:42.170658712Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-330"],"updated_at":"2026-11-10T10:57:25.458985Z"}]}}