ZephyrProject Zephyr up to 4.4.x DNS Stub Resolver resolve.c dns_resolve_init_locked random values

Summaryinfo

A vulnerability, which was classified as problematic, was found in ZephyrProject Zephyr up to 4.4.x. This vulnerability affects the function dns_resolve_init_locked of the file subsys/net/lib/dns/resolve.c of the component DNS Stub Resolver. Executing a manipulation can lead to random values. This vulnerability is tracked as CVE-2026-79981. The attack can be launched remotely. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in ZephyrProject Zephyr up to 4.4.x. It has been declared as problematic. This vulnerability affects the function dns_resolve_init_locked of the file subsys/net/lib/dns/resolve.c of the component DNS Stub Resolver. The manipulation with an unknown input leads to a random values vulnerability. The CWE definition for the vulnerability is CWE-330. The product uses insufficiently random numbers or values in a security context that depends on unpredictable numbers. As an impact it is known to affect confidentiality. CVE summarizes:

The DNS stub resolver in subsys/net/lib/dns/resolve.c binds one UDP socket per configured server when the resolver context is initialised (dns_resolve_init_locked() -> register_dispatcher() -> dns_dispatcher_register()), with the local port left at 0 so the stack picks an ephemeral port exactly once. dns_write() then sends every subsequent query from that same socket without re-binding, so the source port is constant for the lifetime of the resolver context while only the 16-bit transaction identifier (ctx->queries[i].id, drawn with sys_rand16_get()) varies per query. RFC 5452 section 9.2 requires both to vary; with a fixed port, an off-path attacker has to guess only the identifier. The stable port is cheap to learn. Any single observed query reveals it permanently, and it can also be found by probing the ephemeral UDP range: subsys/net/ip/connection.c answers unmatched unicast UDP with an ICMP port-unreachable, so the resolver's socket is the one port in that range that stays silent. Acceptance of a reply is decided solely by get_slot_by_id() matching the transaction identifier and a CRC-16 of the queried name — the reply's source address is never checked (dispatcher_cb() does ARG_UNUSED(addr)), so forged answers need not even be spoofed to appear to come from the configured server; they only need to reach the resolver's port. An attacker with UDP reachability to the device (same LAN, Wi-Fi or mesh segment, or a globally routable device address) can therefore flood answers for a predictable query name across the 16-bit identifier space during a query window and race the legitimate server. A win makes the application resolve an attacker-chosen address, and with CONFIG_DNS_RESOLVER_CACHE enabled the forged record is retained for its attacker-chosen TTL. The attack still requires winning that race, and applications that validate TLS certificates for the resolved endpoint are largely protected. The fix draws a fresh local port before each query to a server that has nothing outstanding: dns_randomize_source_port() unregisters the dispatcher, closes and reopens the socket, and re-registers it bound to port 0 so the system selects a new port, raising the per-query guessing space from roughly 16 to roughly 31 bits. Multicast DNS and LLMNR are excluded because their ports are protocol-fixed. The behaviour is controlled by CONFIG_DNS_RESOLVER_RANDOMIZE_SOURCE_PORT (default y).

The advisory is available at github.com. This vulnerability was named CVE-2026-79981 since 08/25/2026. The exploitation appears to be difficult. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1600.001 by the MITRE ATT&CK project.

Upgrading to version 4.5.0 eliminates this vulnerability.

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Productinfo

Type

• Project Management Software

Vendor

• ZephyrProject

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4

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CPE 2.2info

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