# CVE-2026-87043: sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready

- Exploited (CISA KEV): no
- Published: 2026-11-04
- CVSS: 4.8 MEDIUM (v3.1)
- EPSS: no score
- CWE: CWE-330

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(&ret, sizeof(ret)) and returns ret.

The "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).

When 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.

The 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.

## Edge WAF coverage (claimed by vendors, not measured)

| Vendor | Status | First rule | Days after publication |
|---|---|---|---|
| Cloudflare | No public claim | — | — |
| AWS | No public claim | — | — |
| Microsoft Azure | No public claim | — | — |
| Google Cloud | No public claim | — | — |
| Fastly | No public claim | — | — |
| Vercel | No public claim | — | — |
| F5 | No public claim | — | — |
| Akamai | No public source | — | — |
| OWASP CRS | No public claim | — | — |

## Social

1 posts seen since 2026-11-04.

- bluesky 2026-11-04: CVE-2026-87043 - sys_rand_get() silently returns an unwritten buffer when the entropy device is not ready CVE ID : CVE-2026-87043    Published : Nov. 4, 2026, 6:17 a.m. \| 47 minutes ago    Description : The entropy-device random number generator in subsys/random/random_entropy... (https://bsky.app/profile/cve.skyfleet.blue/post/3zn27pm3tly6u)

"No public claim" means no vendor statement names this CVE; "Before feed history" means the CVE is older than the vendor feed. In both cases generic rules may still block it.

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