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Linux Kernel CVE-2025-39955

HIGH
2025-10-09 416baaa9-dc9f-4396-8d5f-8c081fb06d67
Information Disclosure Linux Debian Red Hat Suse
7.8
CVSS 3.1 · NVD
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Severity by source

NVD PRIMARY
7.8 HIGH
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
vuln.today AI
2.5 LOW

Local unprivileged trigger (AV:L/PR:L) requiring a race against TFO handshake completion (AC:H); observed impact is a kernel WARNING and missed retransmission, so C:N/I:N with only low availability effect.

3.1 AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
4.0 AV:L/AC:H/AT:P/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
SUSE
6.1 MEDIUM
AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
Red Hat
7.6 MEDIUM
qualitative

Primary rating from NVD.

CVSS VectorNVD

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Lifecycle Timeline

2
Analysis Generated
Jul 14, 2026 - 13:31 vuln.today
CVE Published
Oct 09, 2025 - 10:15 nvd
HIGH 7.8

DescriptionNVD

In the Linux kernel, the following vulnerability has been resolved:

tcp: Clear tcp_sk(sk)->fastopen_rsk in tcp_disconnect().

syzbot reported the splat below where a socket had tcp_sk(sk)->fastopen_rsk in the TCP_ESTABLISHED state. [0]

syzbot reused the server-side TCP Fast Open socket as a new client before the TFO socket completes 3WHS:

  1. accept()
  2. connect(AF_UNSPEC)
  3. connect() to another destination

As of accept(), sk->sk_state is TCP_SYN_RECV, and tcp_disconnect() changes it to TCP_CLOSE and makes connect() possible, which restarts timers.

Since tcp_disconnect() forgot to clear tcp_sk(sk)->fastopen_rsk, the retransmit timer triggered the warning and the intended packet was not retransmitted.

Let's call reqsk_fastopen_remove() in tcp_disconnect().

[0]: WARNING: CPU: 2 PID: 0 at net/ipv4/tcp_timer.c:542 tcp_retransmit_timer (net/ipv4/tcp_timer.c:542 (discriminator 7)) Modules linked in: CPU: 2 UID: 0 PID: 0 Comm: swapper/2 Not tainted 6.17.0-rc5-g201825fb4278 #62 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:tcp_retransmit_timer (net/ipv4/tcp_timer.c:542 (discriminator 7)) Code: 41 55 41 54 55 53 48 8b af b8 08 00 00 48 89 fb 48 85 ed 0f 84 55 01 00 00 0f b6 47 12 3c 03 74 0c 0f b6 47 12 3c 04 74 04 90 <0f> 0b 90 48 8b 85 c0 00 00 00 48 89 ef 48 8b 40 30 e8 6a 4f 06 3e RSP: 0018:ffffc900002f8d40 EFLAGS: 00010293 RAX: 0000000000000002 RBX: ffff888106911400 RCX: 0000000000000017 RDX: 0000000002517619 RSI: ffffffff83764080 RDI: ffff888106911400 RBP: ffff888106d5c000 R08: 0000000000000001 R09: ffffc900002f8de8 R10: 00000000000000c2 R11: ffffc900002f8ff8 R12: ffff888106911540 R13: ffff888106911480 R14: ffff888106911840 R15: ffffc900002f8de0 FS: 0000000000000000(0000) GS:ffff88907b768000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f8044d69d90 CR3: 0000000002c30003 CR4: 0000000000370ef0 Call Trace: <IRQ> tcp_write_timer (net/ipv4/tcp_timer.c:738) call_timer_fn (kernel/time/timer.c:1747) __run_timers (kernel/time/timer.c:1799 kernel/time/timer.c:2372) timer_expire_remote (kernel/time/timer.c:2385 kernel/time/timer.c:2376 kernel/time/timer.c:2135) tmigr_handle_remote_up (kernel/time/timer_migration.c:944 kernel/time/timer_migration.c:1035) __walk_groups.isra.0 (kernel/time/timer_migration.c:533 (discriminator 1)) tmigr_handle_remote (kernel/time/timer_migration.c:1096) handle_softirqs (./arch/x86/include/asm/jump_label.h:36 ./include/trace/events/irq.h:142 kernel/softirq.c:580) irq_exit_rcu (kernel/softirq.c:614 kernel/softirq.c:453 kernel/softirq.c:680 kernel/softirq.c:696) sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1050 (discriminator 35) arch/x86/kernel/apic/apic.c:1050 (discriminator 35)) </IRQ>

AnalysisAI

A local privilege-boundary/state-handling flaw in the Linux kernel's TCP stack lets an unprivileged user leave a stale TCP Fast Open request socket (fastopen_rsk) attached to a socket after tcp_disconnect(), which the retransmit timer later dereferences. By abusing accept() → connect(AF_UNSPEC) → connect() on a server-side TFO socket before the 3-way handshake completes, an attacker drives the socket into TCP_ESTABLISHED while retaining fastopen_rsk, triggering a kernel WARNING in tcp_retransmit_timer() and a failed retransmission. There is no public exploit identified at time of analysis, and the issue is not in CISA KEV; it was found by syzbot and resolved upstream by calling reqsk_fastopen_remove() in tcp_disconnect().

Technical ContextAI

The affected component is the IPv4 TCP implementation (net/ipv4/tcp.c and net/ipv4/tcp_timer.c) and specifically the TCP Fast Open (TFO) fast-path, where a server-side socket in TCP_SYN_RECV holds a child request socket pointer in tcp_sk(sk)->fastopen_rsk until the handshake completes. CPE data pins the affected code to the linux_kernel product across 6.17-rc1, rc2 and rc3 (with additional stable-branch backports implied by the multiple git.kernel.org commits). No CWE was assigned in the input; the root-cause class is best characterized as improper resource/pointer cleanup on a state transition (akin to CWE-459 Incomplete Cleanup, with use-after-free/stale-pointer potential, CWE-416), because tcp_disconnect() reset sk_state to TCP_CLOSE and restarted timers without clearing the still-populated fastopen_rsk, leaving the retransmit timer to act on an object that should have been removed.

RemediationAI

Patch available per vendor advisory: apply your distribution's updated kernel package or rebuild from a Linux stable tree that includes the fix commits at https://git.kernel.org/stable/c/17d699727577814198d744d6afe54735c6b54c99 through https://git.kernel.org/stable/c/fa4749c065644af4db496b338452a69a3e5147d9 (the fix calls reqsk_fastopen_remove() in tcp_disconnect()); Debian and other distros should track their security trackers for the rebuilt package version, and Siemens environments should follow SSA-019113 (https://cert-portal.siemens.com/productcert/html/ssa-019113.html). Exact upstream tagged release version is not stated in the input, so confirm the fixed package version with your distribution before deploying. As a compensating control until patched, disable server-side TCP Fast Open where it is not required by setting net.ipv4.tcp_fastopen to 0 (or to a value without the server bit, e.g. 1 for client-only), which removes the TFO code path this bug depends on at the cost of losing TFO latency benefits for accepting services; restricting untrusted local user access to the host also limits who can drive the required socket sequence.

Same technique Information Disclosure

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
Container suse/hpc/warewulf4-x86_64/sle-hpc-node:15.6.17.8.134 Image SLES15-SP6 Image SLES15-SP6-BYOS Image SLES15-SP6-BYOS-Azure Image SLES15-SP6-BYOS-EC2 Image SLES15-SP6-BYOS-GCE Image SLES15-SP6-CHOST-BYOS Image SLES15-SP6-CHOST-BYOS-Aliyun Image SLES15-SP6-CHOST-BYOS-Azure Image SLES15-SP6-CHOST-BYOS-EC2 Image SLES15-SP6-CHOST-BYOS-GCE Image SLES15-SP6-CHOST-BYOS-GDC Image SLES15-SP6-CHOST-BYOS-SAP-CCloud Image SLES15-SP6-EC2 Image SLES15-SP6-EC2-ECS-HVM Image SLES15-SP6-GCE Image SLES15-SP6-HPC-BYOS Image SLES15-SP6-HPC-BYOS-Azure Image SLES15-SP6-HPC-BYOS-EC2 Image SLES15-SP6-HPC-BYOS-GCE Image SLES15-SP6-HPC-EC2 Image SLES15-SP6-HPC-GCE Image SLES15-SP6-Hardened-BYOS Image SLES15-SP6-Hardened-BYOS-Azure Image SLES15-SP6-Hardened-BYOS-EC2 Image SLES15-SP6-Hardened-BYOS-GCE Image SLES15-SP6-SAP Image SLES15-SP6-SAP-Azure Image SLES15-SP6-SAP-EC2 Image SLES15-SP6-SAP-GCE Image SLES15-SP6-SAPCAL Image SLES15-SP6-SAPCAL-Azure Image SLES15-SP6-SAPCAL-EC2 Image SLES15-SP6-SAPCAL-GCE Affected
Container suse/sl-micro/6.0/baremetal-os-container:latest Container suse/sl-micro/6.0/toolbox:latest Affected
Container suse/sl-micro/6.0/base-os-container:2.1.3-7.65 Image SLE-Micro Image SLE-Micro-Azure Image SLE-Micro-BYOS Image SLE-Micro-BYOS-Azure Image SLE-Micro-BYOS-EC2 Image SLE-Micro-BYOS-GCE Image SLE-Micro-EC2 Image SLE-Micro-GCE Affected
Container suse/sl-micro/6.0/kvm-os-container:2.1.3-6.88 Affected
Container suse/sl-micro/6.0/rt-os-container:2.1.3-7.105 Affected

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CVE-2025-39955 vulnerability details – vuln.today
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