Zephyr Project up to 4.4.2 TCP ISN Implementation subsys/net/ip/tcp.c tcp_init_isn random values

Summaryinfo

A vulnerability, which was classified as problematic, was found in Zephyr Project Zephyr up to 4.4.2. Impacted is the function tcp_init_isn of the file subsys/net/ip/tcp.c of the component TCP ISN Implementation. Executing a manipulation can lead to random values. This vulnerability is registered as CVE-2026-19735. It is possible to launch the attack remotely. No exploit is available. It is advisable to implement a patch to correct this issue.

Detailsinfo

A vulnerability has been found in Zephyr Project Zephyr up to 4.4.2 and classified as problematic. This vulnerability affects the function tcp_init_isn of the file subsys/net/ip/tcp.c of the component TCP ISN Implementation. 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 RFC 6528 initial-sequence-number implementation in subsys/net/ip/tcp.c derived every TCP ISN from SHA-256(unique_key || four-tuple) plus a uptime-derived offset, where unique_key is a 128-bit secret filled once by sys_csrand_get(). The return value of that call was discarded and the once guard was latched to true even when the call failed. Because sys_csrand_get() leaves the destination buffer untouched on failure (it deliberately propagates the entropy-driver error rather than filling the buffer), a single failed call left unique_key as its all-zero BSS content for the remainder of the boot, with no retry, no log message and no fallback. A failure of the cryptographic random source is required to reach the weak state — for example -ENODEV/-EIO from the entropy driver in subsys/random/random_entropy_device.c (the in-tree comment notes that the hardware RNG "might still be gathering entropy during early boot situations"), or psa_generate_random() failing in subsys/random/random_psa.c when PSA crypto is not initialised or is backed by a Bluetooth-HCI entropy device that is not yet up. The first TCP connection is what triggers key generation, so a remote peer that reaches a listening port immediately after boot, or that can provoke a reboot, has indirect influence over whether generation coincides with that window. tcp_init_isn() is called for every passive open in tcp_conn_new() and every active open in net_tcp_connect(), so the poisoned key governs all TCP connections for that boot. With an all-zero key the ISN becomes a public function of the connection four-tuple plus a device-wide time offset. An off-path attacker can compute the hash term offline for any four-tuple and recover the shared time offset from a single observed ISN, after which the ISN the device will pick for other four-tuples is predictable. That defeats exactly the protection RFC 6528 provides: blind TCP connection spoofing against peers that trust the source address, and blind data injection into or reset of connections whose four-tuple can be guessed. Devices whose entropy source never errors were never in the weak state. The fix moves key generation into a single guarded helper that latches only on success, logs the error otherwise, and makes tcp_init_isn() fall back to sys_rand32_get() — the behaviour already used when CONFIG_NET_TCP_ISN_RFC6528 is disabled — instead of hashing with a known-constant key.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-19735 since 08/13/2026. The exploitation appears to be difficult. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1600.001.

Applying the patch 700888e3ecff8474f4fe7dfa8782b8869053222b is able to eliminate this problem.

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Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.4.0
• 4.4.1
• 4.4.2

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CPE 2.2info

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