IPv6 stable and temporary interface identifiers derived from a non-cryptographic PRNG secret key

Summary

CVE	CVE-2026-19742
State	PUBLISHED
Assigner	zephyr
Source Priority	CVE Program / NVD first with legacy fallback
Published	2026-10-26 17:17:27 UTC
Updated	2026-11-04 05:18:51 UTC
Description	The IPv6 stack derived the long-lived secret key used to compute RFC 7217 stable interface identifiers (gen_stable_iid() in subsys/net/ip/ipv6.c) and RFC 8981 temporary interface identifiers (gen_temporary_iid() in subsys/net/ip/ipv6_pe.c) with sys_rand_get(). That API is documented as not cryptographically secure, while both RFCs require this key to be an unpredictable random value of at least 128 bits — the unpredictability of every generated IID rests entirely on it. Depending on the platform, sys_rand_get() resolves either to a hardware entropy device or, on boards whose entropy driver sets CONFIG_ENTROPY_NEEDS_PRNG, to the non-cryptographic xoshiro128++ generator in subsys/random/random_xoshiro128.c, which keeps a single global 128-bit state shared with every other sys_rand_get() consumer. Many of those consumers put their output directly on the wire (ephemeral source ports, DNS query IDs, CoAP message IDs and tokens, DHCPv6 transaction IDs, WebSocket masking keys), and the xoshiro state transition is invertible, so an observer of a device's traffic can recover the generator state and reconstruct the IID secret key. Additionally, with the entropy-device backend sys_rand_get() silently falls back to k_cycle_get_32() samples when the entropy driver fails, and returns no error, so the key could be derived from the cycle counter without the caller noticing. With the key known, stable IIDs become directly computable for any prefix (the remaining HMAC inputs — prefix, interface index, interface name, DAD counter — are guessable), and temporary IIDs become linkable after a cheap search over the mixed-in k_uptime_get_32() value. This defeats the properties both features exist to provide: an attacker can track a device across networks and prefixes, and can predict its address inside a /64 without scanning, removing the usual barrier to directly targeting the host. There is no memory-safety, integrity, or availability impact. The fix generates the key with sys_csrand_get() and aborts IID generation with -EIO when no cryptographic randomness is available, instead of silently deriving addresses from a predictable key. Both CONFIG_NET_IPV6_IID_STABLE and CONFIG_NET_IPV6_PE are experimental and disabled by default; builds using CONFIG_TEST_RANDOM_GENERATOR remain predictable by design, as sys_csrand_get() is deliberately routed back to the test generator there.

Risk And Classification

Primary CVSS: v3.1 3.7 LOW from [email protected]

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS: 0.002480000 probability, percentile 0.162140000 (date 2026-11-04)

Problem Types: CWE-338 | CWE-338 crypto

Version	Source	Type	Score	Severity	Vector
3.1	[email protected]	Secondary	3.7	LOW	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N`
3.1	CNA	CVSS	3.7	LOW	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N`

CVSS v3.1 Breakdown

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

None

Availability

None

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Vendor Declared Affected Products

Source	Vendor	Product	Version	Platforms
CNA	Zephyrproject	Zephyr	affected 3.7.0 4.3.2 semver	Not specified
CNA	Zephyrproject	Zephyr	affected 4.4.0 4.5.0 semver	Not specified

References

Reference	Source	Link	Tags
github.com/zephyrproject-rtos/zephyr/commit/253315829d3cb5906c660d8418d8...	[email protected]	github.com
github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-xfm2-gr93-...	[email protected]	github.com
CVE Program record	CVE.ORG	www.cve.org	canonical
NVD vulnerability detail	NVD	nvd.nist.gov	canonical, analysis

No vendor comments have been submitted for this CVE.

There are currently no legacy QID mappings associated with this CVE.