{"id":"CVE-2026-19742","kind":"vulnerability","aliases":["CVE-2026-19742"],"related":[],"title":"Zephyr IPv6: predictable key makes stable interface identifiers trackable and guessable","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.\n\nDepending 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.\n\nWith 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.\n\nThe 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.","summary":"Zephyr's IPv6 stack seeded the secret key for RFC 7217 and RFC 8981 interface identifiers with a non-cryptographic random source, which can be recovered from observed traffic. An attacker who learns the key can compute a device's stable addresses for any prefix, track it across networks, and predict its address inside a /64.","severity":"low","cvss":3.7,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["embedded","iot","ipv6","runtime","zephyr"],"not_affected_if":["default-config","ipv6-iid-features-disabled"],"how_to_check":"- Check whether the build enables `CONFIG_NET_IPV6_IID_STABLE` or `CONFIG_NET_IPV6_PE` (both are experimental and disabled by default).\n- Check whether the board's entropy driver sets `CONFIG_ENTROPY_NEEDS_PRNG`, which selects the xoshiro128++ generator.\n- Check whether the build uses `CONFIG_TEST_RANDOM_GENERATOR`.","fix":"- Upgrade Zephyr to a release containing the fix that generates the key with `sys_csrand_get()` (the sources do not name the version: unknown).\n- If the fix is not available, leave `CONFIG_NET_IPV6_IID_STABLE` and `CONFIG_NET_IPV6_PE` disabled.","status":"open","published_at":"2026-10-26T17:17:27+00:00","first_seen_at":"2026-10-26T17:31:22+00:00","triaged_at":"2026-10-26T17:33:33+00:00","updated_at":"2026-10-26T17:33:33+00:00","category":"runtime","confidence":0.7,"triage":{"at":"2026-10-26T17:33:33Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Zephyr to a release containing the fix that generates the key with `sys_csrand_get()` (the sources do not name the version: unknown).\n- If the fix is not available, leave `CONFIG_NET_IPV6_IID_STABLE` and `CONFIG_NET_IPV6_PE` disabled.","reach":"network","title":"Zephyr IPv6: predictable key makes stable interface identifiers trackable and guessable","impact":"low","summary":"Zephyr's IPv6 stack seeded the secret key for RFC 7217 and RFC 8981 interface identifiers with a non-cryptographic random source, which can be recovered from observed traffic. An attacker who learns the key can compute a device's stable addresses for any prefix, track it across networks, and predict its address inside a /64.","category":"runtime","nicknames":[],"who_cares":["zephyr","embedded","iot","ipv6"],"confidence":0.7,"needs_auth":"no","worst_case":"Attacker tracks devices across networks and predicts their IPv6 addresses.","analyst_note":"","how_to_check":"- Check whether the build enables `CONFIG_NET_IPV6_IID_STABLE` or `CONFIG_NET_IPV6_PE` (both are experimental and disabled by default).\n- Check whether the board's entropy driver sets `CONFIG_ENTROPY_NEEDS_PRNG`, which selects the xoshiro128++ generator.\n- Check whether the build uses `CONFIG_TEST_RANDOM_GENERATOR`.","needs_analyst":false,"not_affected_if":["default-config","ipv6-iid-features-disabled"],"who_is_affected":"Only builds that enable stable or temporary IPv6 interface identifiers are exposed, and those features are experimental and off by default. Devices on boards using the xoshiro PRNG backend, or that silently fall back to cycle-counter samples, are the most exposed. Teams not using these options can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"low"},"cost_usd":0.000769,"input_tokens":4083,"output_tokens":722,"severity_before":"low"},"analyst_status":null,"analyst_note":null,"search":"'-19742':14A,17A '-2026':13A,16A '/64':79B,369C '128':141C,192C '32':274C,341C '7217':33B,94C '8981':36B,105C 'abort':401C 'across':71B,358C 'addit':257C 'address':65B,76B,366C,416C 'api':119C 'attack':54B,353C 'avail':389C,411C 'back':269C,455C 'backend':263C 'barrier':375C 'becom':305C,327C 'bit':142C,193C 'board':169C 'build':438C 'caller':297C 'cheap':331C 'coap':220C 'comput':60B,92C,307C 'config':174C,422C,428C,440C 'consum':202C,206C 'could':288C 'counter':294C,321C 'cryptograph':43B,124C,182C,408C 'csrand':398C,450C 'cve':12A,15A 'cycl':272C,293C 'dad':320C 'default':437C 'defeat':344C 'deliber':453C 'depend':153C 'deriv':83C,290C,415C 'design':447C 'devic':62B,166C,243C,262C,357C 'dhcpv6':225C 'direct':210C,306C,377C 'disabl':435C 'dns':217C 'document':121C 'driver':172C,279C 'eio':405C 'either':161C 'embed':18B 'entir':150C 'entropi':165C,171C,175C,261C,278C 'entropy-devic':260C 'ephemer':214C 'error':284C 'everi':146C,197C 'exist':349C 'experiment':433C 'fail':280C 'fall':268C 'featur':348C 'fix':392C 'gen':98C,109C 'generat':147C,184C,249C,393C,403C,443C,459C 'get':117C,159C,201C,266C,273C,340C,399C,451C 'global':191C 'guessabl':11A,323C 'hardwar':164C 'hmac':313C 'host':380C 'id':219C,222C,227C 'identifi':8A,38B,97C,108C 'iid':100C,111C,148C,254C,304C,326C,402C,425C 'impact':390C 'index':317C 'input':314C 'insid':77B,367C 'instead':412C 'integr':387C 'interfac':7A,37B,96C,107C,316C,318C 'invert':237C 'iot':19B 'ipv6':2A,20B,25B,81C,424C,430C 'k':271C,338C 'keep':188C 'key':4A,30B,58B,89C,131C,230C,256C,287C,301C,395C,420C 'known':302C 'learn':56B 'least':140C 'linkabl':328C 'live':87C 'long':86C 'long-liv':85C 'make':5A 'mani':203C 'mask':229C 'memori':385C 'memory-safeti':384C 'messag':221C 'mix':336C 'mixed-in':335C 'name':319C 'need':176C 'net':423C,429C 'network':72B,359C 'non':42B,181C 'non-cryptograph':41B,180C 'notic':298C 'observ':51B,240C 'output':209C 'pe':431C 'platform':156C 'port':216C 'predict':3A,74B,364C,419C,445C 'prefix':68B,310C,315C,361C 'prng':177C 'properti':346C 'provid':351C 'put':207C 'queri':218C 'rand':116C,158C,200C,265C 'random':44B,136C,409C,442C 'reconstruct':252C 'recov':49B,247C 'remain':312C,444C 'remov':372C 'requir':129C 'resolv':160C 'rest':149C 'return':282C 'rfc':32B,35B,93C,104C 'rfcs':128C 'rout':454C 'runtim':21B 'safeti':386C 'sampl':275C 'scan':371C 'search':332C 'secret':29B,88C,255C 'secur':125C 'seed':27B 'set':173C 'share':195C 'silent':267C,414C 'singl':190C 'sourc':45B,215C 'stabl':6A,64B,95C,99C,303C,426C 'stack':26B,82C 'state':194C,234C,250C 'subsys/net/ip/ipv6.c':102C 'subsys/net/ip/ipv6_pe.c':113C 'subsys/random/random_xoshiro128.c':186C 'sys':115C,157C,199C,264C,397C,449C 'target':378C 'temporari':106C,110C,325C 'test':441C,458C 'token':224C 'track':69B,355C 'trackabl':9A 'traffic':52B,245C 'transact':226C 'transit':235C 'unpredict':135C,144C 'uptim':339C 'use':90C,439C 'usual':374C 'valu':137C,342C 'websocket':228C 'whose':170C 'wire':213C 'without':295C,370C 'xoshiro':233C 'xoshiro128':183C 'zephyr':1A,22B,23B","nicknames":[],"mitigations":[],"who_affected":"Only builds that enable stable or temporary IPv6 interface identifiers are exposed, and those features are experimental and off by default. Devices on boards using the xoshiro PRNG backend, or that silently fall back to cycle-counter samples, are the most exposed. Teams not using these options can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-26T17:17:27+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N","cvss_by":"vulnerabilities@zephyrproject.org","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-19742","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-19742","severity":null,"published_at":"2026-10-26T17:17:27+00:00","modified_at":"2026-10-26T17:17:27+00:00","first_seen_at":"2026-10-26T17:31:22+00:00","fetched_at":"2026-10-26T17:31:22+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}