CVE-2026-87043 in Zephyr
Summary
by MITRE • 11/04/2026
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.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability resides in the entropy-device random number generator implementation within the Zephyr RTOS subsystem, specifically affecting the rand_get function logic found in subsys/random/random_entropy_device.c. The core technical flaw is a premature return condition where the system returns -ENODEV immediately upon detecting that the entropy device is not ready via device_is_ready(). This early exit prevents execution from reaching the critical non-cryptographic system-timer fallback mechanism located further down in the same function. Because the internal implementation z_impl_sys_rand_get() has a void return type, it discards this error code entirely. Furthermore, the public API sys_rand_get is also declared as void, meaning there is no standard mechanism to propagate failure states back to the caller. Consequently, when the entropy source is unavailable, the output buffer provided by the caller remains untouched and uninitialized. The thin wrapper functions such as sys_rand32_get simply declare a local uint32_t variable without initialization, invoke sys_rand_get with that variable's address, and then return it directly. This results in the function returning whatever stale data previously existed at that memory location on the stack or heap, effectively creating an uninitialized read vulnerability where random number generation silently degrades into returning garbage values rather than failing securely.
This issue is particularly dangerous because the unready state of the entropy device is a normal runtime condition during system initialization and can also occur permanently if specific hardware drivers fail to initialize correctly. Entropy drivers typically register at PRE_KERNEL_1 priority, which defaults to 50 in Zephyr configurations. However, many kernel objects and other subsystems run at lower priorities such as 30, meaning they may attempt to request random numbers before the entropy driver has completed its initialization sequence. While sibling implementations like subsys/random/random_xoshiro128.c and z_early_rand_get already handle this race condition by falling back to a system timer source, the primary entropy device path lacked this safeguard. Additionally, on certain hardware platforms such as STM32 with RNG clock configuration errors, PSA Crypto initialization failures, or specific configurations for nRF CRACEN, ESP32, and Virtio devices, the entropy driver may fail permanently during boot. In these scenarios, every subsequent call to a non-cryptographic random number generator will consistently return uninitialized memory contents rather than generating new randomness.
The operational impact of this vulnerability is severe across multiple networking and security protocols that rely on unpredictable values for session management and authentication. For instance, the coap_next_token function in subsys/net/lib/coap/coap.c fills a static buffer with random data; due to this flaw, every CoAP token generated becomes identical because it reads from an uninitialized .bss constant or stale stack memory. Similarly, tcp_init_isn which generates TCP initial sequence numbers returns uninitialized stack words, compromising the security of TCP connections against prediction attacks. Other critical components such as DHCPv4 and DHCPv6 transaction IDs, WebSocket masking keys, RTP SSRCs, and Pointer Authentication Keys for ARM64 architectures are all derived from this compromised source. When these values are transmitted over the network in protocols like SSH packet padding, netmidi2 session nonces, or DTLS connection identifiers on builds without CONFIG_CSPRNG_ENABLED, an attacker can observe these predictable or stale values to infer internal state information or hijack sessions. This represents a significant degradation of security posture where cryptographic strength is silently lost without any error indication to the application layer.
From a classification perspective, this vulnerability aligns with CWE-457 which describes the use of an uninitialized variable that can lead to unpredictable behavior and potential information disclosure. It also relates to CWE-330 regarding insufficient randomness when generating values for security purposes. In terms of MITRE ATT&CK mapping, this flaw facilitates techniques associated with T1608 Weaponized Files or general reconnaissance where predictable network identifiers allow an attacker to map the environment more effectively than intended by the protocol design. The vulnerability is not exploitable through active manipulation of the device state since an attacker cannot force the entropy driver into a not-ready state; however, it can be exploited remotely once triggered during boot or initialization phases.
The remediation strategy involves modifying the rand_get function to ensure that if the entropy device is not ready, execution falls through to the system-timer fallback rather than returning early with an error code that gets discarded by void functions. The fix initializes the return variable explicitly and ensures that a valid random value is always written to the caller's buffer regardless of whether the primary entropy source is available. This maintains backward compatibility while ensuring consistent behavior across all initialization phases. It is important to note that this vulnerability affects only non-cryptographic random number generation; the cryptographically secure path sys_csrand_get remains unaffected as it properly propagates errors and does not silently degrade, thereby preserving security for applications explicitly requesting high-entropy data.