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CVE-2026-74714
bpf: tcp: Fix use-after-free in bpf_iter_tcp_established_batch()
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem net/ipv4. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74714 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
reqsk_queue_hash_req() publishes a TCP_NEW_SYN_RECV request_sock onto the ehash chain, drops the bucket lock, and only afterwards sets rsk_refcnt to 3. Lockless readers such as __inet_lookup_established() handle this with refcount_inc_not_zero(), but bpf_iter_tcp_established_batch() uses plain sock_hold() while holding the bucket lock, on the assumption that the lock guarantees sk_refcnt > 0. That assumption does not hold for request_sock: CPU 0 CPU 1 ----- ----- tcp_conn_request() reqsk_queue_hash_req() inet_ehash_insert(req) spin_lock(bucket) __sk_nulls_add_node_rcu(req) // rsk_refcnt == 0 spin_unlock(bucket) bpf_iter_tcp_established_batch() spin_lock(bucket) sock_hold(req) <-- addition on 0 spin_unlock(bucket) refcount_set(&req->rsk_refcnt, 3) // clobbers saturated value which surfaces as: refcount_t: addition on 0; use-after-free. WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x48/0x90, CPU#1 Call Trace: bpf_iter_tcp_established_batch+0x14e/0x170 bpf_iter_tcp_batch+0x53/0x200 bpf_iter_tcp_seq_next+0x27/0x70 bpf_seq_read+0x107/0x410 vfs_read+0xb9/0x380 The iterator's stolen reference is lost when the publishing CPU's refcount_set() overwrites the count, leaving the socket one reference short. When the last legitimate owner drops its reference the reqsk is freed while still reachable, leading to use-after-free. This reproduces in seconds with tcp_syncookies=0, a handful of threads doing connect()/close() to a local listener while others read an iter/tcp link in a tight loop. Use refcount_inc_not_zero() and skip the socket on failure. A skipped socket is still part of the bucket, so keep counting it in expected. The reallocations are sized from expected, and a request sock whose refcount gets published while the lock is held across the last realloc must already have room. A skipped socket is counted in expected but never batched, so end_sk can be short of expected on a batch that is actually complete. Decide completeness by whether the walk left any socket behind instead. The WARN after the locked realloc checks the same, replacing an end_sk == expected check that could not hold on that path since commit cdec67a489d4 ("bpf: tcp: Make sure iter->batch always contains a full bucket snapshot"). If every matching socket in a bucket is mid-init (refcount 0), end_sk stays 0. Advance to the next bucket rather than returning a batch entry that was never filled this round.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/ipv4/tcp_ipv4.c
- What this is
- NETWORKING [TCP]MAINTAINERS section matched on net/ipv4/tcp*.c · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_INET /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_INET
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-22 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv4
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.15 | 6.1.183 | cc0295f89296 |
| 5.15 | 6.6.152 | ddbe966b5d1f |
| 5.15 | 6.12.104 | 7d2b60a4bc04 |
| 5.15 | 6.18.45 | cefcbbe20846 |
| 5.15 | 7.1.9 | 97e74d3e45d6 |
| 5.15 | 7.2 | e5fd3f514e27 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- bpf_iter_tcp_established_batch() is reached only via local bpf() to load a BPF_TRACE_ITER TCP program, create an iterator link/fd, and read() it (bpf_seq_read->bpf_iter_tcp_batch); per kernel CNA guidance BPF iterator paths are Local even though concurrent TCP_NEW_SYN_RECV creation can be driven by network SYNs.
- AC:LAttack complexity, low
- The fix commit reproduces in seconds with threads doing connect()/close() while others read iter/tcp in a tight loop; the attacker controls both the BPF iterator and TCP connection churn, so the refcount race between sock_hold() and refcount_set() is reliably winnable without uncontrollable victim state.
- PR:LPrivileges required, low
- Loading BPF_PROG_TYPE_TRACING/BPF_TRACE_ITER requires CAP_BPF and CAP_PERFMON at bpf_prog_load(); per kernel CNA guidance these are Low privileges because BPF tokens and user namespaces can delegate them to non-init-namespace users, not only real root.
- UI:NUser interaction, none
- No victim interaction is required; the attacker loads their own iter/tcp BPF program and link, then triggers the race with self-generated connect/close traffic or SYN activity to listeners in the iterated network namespace.
- S:UScope, unchanged
- Impact is a kernel heap use-after-free and memory corruption within the same host kernel security authority; this is standard local kernel compromise/crash, not a VM escape, sandbox boundary cross, or IOMMU bypass.
- C:HConfidentiality impact, high
- The iterator retains dangling request_sock pointers after refcount_set() clobbers a sock_hold() taken on sk_refcnt==0; subsequent bpf_iter_tcp_seq_show/put_batch dereference freed kernel objects, giving UAF read primitives per kernel CNA UAF guidance.
- I:HIntegrity impact, high
- Freed request_sock slabs can be reallocated while the iterator batch still references them; UAF enables corrupted socket metadata, refcount abuse, and control-flow hijacking via heap grooming, meeting High integrity under kernel CNA memory-corruption guidance.
- A:HAvailability impact, high
- The bug surfaces as refcount_warn_saturate and leaves the reqsk one reference short so it is freed while still reachable from the iterator batch, causing kernel oops/panic; the tight-loop repro shows reliable, repeatable host denial of service.