CVE-2026-10653: The Zephyr net_buf library (lib/net_buf/buf.c) manipulated both of its reference counts -- the per-header buf->ref and the per-data-block ref_count at...

Description

The Zephyr net_buf library (lib/net_buf/buf.c) manipulated both of its reference counts -- the per-header buf->ref and the per-data-block ref_count at the start of each variable/heap data allocation -- with plain non-atomic C operators (buf->ref++, if (--buf->ref > 0), if (--(*ref_count))). The API is documented as self-synchronizing: callers may share one buffer across threads (e.g. via k_fifo) and each holder independently calls net_buf_unref() with no surrounding lock. Under true concurrency (SMP, or single-core preemption between the non-atomic load and store while another context unrefs the same buffer), two holders can both observe the same prior reference value and both conclude they are the last reference. For heap/variable-data pools (mem_pool_data_unref/heap_data_unref, used by zbus message subscribers, the IP stack RX/TX buffers when CONFIG_NET_BUF_FIXED_DATA_SIZE=n, capture, wireguard, ISO-TP and usbip) this produces a double k_heap_free()/k_free() of the same block -- heap-metadata corruption and a use-after-free on the heap-hardening poison pattern. For the per-header refcount the buffer is returned to the pool free LIFO twice for any pool type (including fixed-data pools used by Bluetooth and networking), corrupting the free list so a later allocation hands the same buffer to two owners. The fix converts both refcounts to atomic_inc/atomic_dec (overlaying buf->ref in an atomic_t-sized union and changing the data-block refcount from uint8_t to atomic_t). Impact is gated on genuine concurrency and on an application architecture that shares one buffer among multiple independent unref'ers; the trigger is a refcount/timing race rather than packet content, so an external attacker has at most weak indirect influence over the race window. Affects all Zephyr releases through v4.4.0. This fix is not being backported to v3.7-branch (LTS). The backport was attempted and closed unmerged (#111181): the v3.7 networking tree has diverged from main, and the new atomic word-packing -- together with the assertions it adds -- turns pre-existing v3.7-only reference-counting defects elsewhere in the stack into hard faults, so landing the change faithfully would mean pulling an open-ended set of additional v3.7-only fixes into an LTS branch. v3.7 remains affected. Applications on v3.7 that share one net_buf across threads should serialize their own net_buf_unref() calls rather than rely on the documented self-synchronizing behaviour. The fix is on main and has been backported to v4.3-branch (#110852) and v4.4-branch (#110853).

Metrics

CVSS 3.1

8.1/10

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

EPSS Probability

0.28%

20.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-415

Affected Software

Vendor	Product	Versions
Zephyrproject	Zephyr	>= 2.7.0, <= 4.4.1

References

https://github.com/zephyrproject-rtos/zephyr/commit/9bb2878319d5f46c29ab5fe855a378d87cd75fc3Patch
https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-284j-5jm9-55hhExploit, Patch, Vendor Advisory

Timeline

Published: Jun 30, 2026
Last Modified: Aug 6, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2026-10653?

The Zephyr net_buf library (lib/net_buf/buf.c) manipulated both of its reference counts -- the per-header buf->ref and the per-data-block ref_count at the start of each variable/heap data allocation -- with plain non-atomic C operators (buf->ref++, if (--buf->ref > 0), if (--(*ref_count))). The API is documented as self-synchronizing: callers may share one buffer across threads (e.g. via k_fifo) and each holder independently calls net_buf_unref() with no surrounding lock. Under true concurrency (SMP, or single-core preemption between the non-atomic load and store while another context unrefs the same buffer), two holders can both observe the same prior reference value and both conclude they are the last reference. For heap/variable-data pools (mem_pool_data_unref/heap_data_unref, used by zbus message subscribers, the IP stack RX/TX buffers when CONFIG_NET_BUF_FIXED_DATA_SIZE=n, capture, wireguard, ISO-TP and usbip) this produces a double k_heap_free()/k_free() of the same block -- heap-metadata corruption and a use-after-free on the heap-hardening poison pattern. For the per-header refcount the buffer is returned to the pool free LIFO twice for any pool type (including fixed-data pools used by Bluetooth and networking), corrupting the free list so a later allocation hands the same buffer to two owners. The fix converts both refcounts to atomic_inc/atomic_dec (overlaying buf->ref in an atomic_t-sized union and changing the data-block refcount from uint8_t to atomic_t). Impact is gated on genuine concurrency and on an application architecture that shares one buffer among multiple independent unref'ers; the trigger is a refcount/timing race rather than packet content, so an external attacker has at most weak indirect influence over the race window. Affects all Zephyr releases through v4.4.0. This fix is not being backported to v3.7-branch (LTS). The backport was attempted and closed unmerged (#111181): the v3.7 networking tree has diverged from main, and the new atomic word-packing -- together with the assertions it adds -- turns pre-existing v3.7-only reference-counting defects elsewhere in the stack into hard faults, so landing the change faithfully would mean pulling an open-ended set of additional v3.7-only fixes into an LTS branch. v3.7 remains affected. Applications on v3.7 that share one net_buf across threads should serialize their own net_buf_unref() calls rather than rely on the documented self-synchronizing behaviour. The fix is on main and has been backported to v4.3-branch (#110852) and v4.4-branch (#110853).

How severe is CVE-2026-10653?

CVE-2026-10653 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 0.28% probability of exploitation in the next 30 days.

How do I fix CVE-2026-10653?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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