CVE-2025-39955: Linux kernel · tcp: Clear tcp_sk(sk)->fastopen_rsk in tcp_disconnect().
Affects Linux kernelSource CVE Database V5
Not known to be exploited · fixed in fa4749c065644af4db496b338452a69a3e5147d9 : update Linux kernel when convenient.
Data as of 10 Nov 2026, 12:50 UTC
Rated Medium by Threat Radar's AI analysis · No CVSS score from NVD or the CNA
Thin evidence
One scorer or none- CISA SSVC
- Exploitation
- No evidence of active exploitation and no public proof of concept
- Automatable
- Attackers cannot reliably automate steps 1–4 of the kill chain
- Technical impact
- Total control over the software or total information disclosure
CISA SSVC assessment, 2026-06-10.
- Patch
- Available – fixed in fa4749c065644af4db496b338452a69a3e5147d9, dfd06131107e7b699ef1e2a24ed2f7d17c917753, ae313d14b45eca7a6bb29cb9bf396d977e7d28fb +5
Exploitation evidence
Disclosed: evidence found.
CVE published 9 Oct 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 10 Nov 2026.
What to do
Upgrade to a fixed release listed under Affected software.
Affected software
| Product | Affected | Fixed in |
|---|---|---|
| LinuxLinux | Affected
| Fixed in
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| LinuxLinux | Affected
| Fixed in
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| LinuxLinux | Affected
| Fixed in
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| LinuxLinux | Affected
| Fixed in
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| LinuxLinux | Affected
| Fixed in
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| LinuxLinux | Affected
| Fixed in
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AI-suggested
This vulnerability has been fixed in the Linux kernel. Users should update to a kernel version that includes the fix identified by the listed commit hashes. No additional mitigation actions are required once the kernel is updated.
Description
A vulnerability in the Linux kernel TCP implementation was fixed by clearing the tcp_sk(sk)->fastopen_rsk pointer in tcp_disconnect(). The flaw caused a warning and prevented retransmission of intended packets when a TCP Fast Open socket was reused improperly before completing the three-way handshake. This issue was identified by syzbot and fixed in subsequent Linux kernel commits.
References
- git.kernel.org/stable/c/7ec092a91ff351dcde89c23e795b73a328274db6 (opens in a new tab)
- git.kernel.org/stable/c/a4378dedd6e07e62f2fccb17d78c9665718763d0 (opens in a new tab)
- git.kernel.org/stable/c/33a4fdf0b4a25f8ce65380c3b0136b407ca57609 (opens in a new tab)
- git.kernel.org/stable/c/17d699727577814198d744d6afe54735c6b54c99 (opens in a new tab)
- git.kernel.org/stable/c/dfd06131107e7b699ef1e2a24ed2f7d17c917753 (opens in a new tab)
- git.kernel.org/stable/c/fa4749c065644af4db496b338452a69a3e5147d9 (opens in a new tab)
- git.kernel.org/stable/c/ae313d14b45eca7a6bb29cb9bf396d977e7d28fb (opens in a new tab)
- git.kernel.org/stable/c/45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01 (opens in a new tab)
- cert-portal.siemens.com/productcert/html/ssa-019113.html (opens in a new tab)
- cert-portal.siemens.com/productcert/html/ssa-457936.html (opens in a new tab)