Linux Kernel net/sched act_ct: Use-after-free via shared unconfirmed conntrack entry
Summary
The Linux kernel traffic control conntrack action act_ct can modify an unconfirmed conntrack entry that is shared between two clones of the same packet. This can lead to a use-after-free during destruction, which may cause a crash or memory corruption. Only hosts that attach act_ct to tc rules are affected, and the attack conditions are not fully clear from the sources.
Affected
- Linux kernel from 6.2 onward, with the bug introduced by commit a21b06e7319129994f339ed47f512bbe57b77f5b (OSV ranges: 6.2.0 to before 6.12.112; 6.13.0 to before 6.18.55; 6.19.0 to before 7.2.9)
- Sources disagree on the exact range: the CNA record lists affected versions differently, and VulDB lists up to 6.12.111, 6.18.54 and 7.2.8
- Ubuntu 24.04 (kernel 6.8.0-148.148) and 26.04 (kernel 7.0.0-42.42) are marked vulnerable
- Ubuntu 22.04 and older are marked not affected
- Only systems that use act_ct in tc rules are relevant
Fixed in
- Fixed in stable kernels 6.6.160, 6.12.112, 6.18.55 and 7.2.9 (per OSV)
- Fixing commits: 2b783c562c3238ff50c34fd7a72a735c9cc73d9a, 4a04ed91575819fb1859311113e95700b709bec7, b097d9d7a5b09c8ec5c4c1a08e35ac5a66b06b44, c812bb4aea13d385d048b5b24756aee2edf6f99e, f85009dfcd65e5969526b0db7a49b5413746e630
- Ubuntu 24.04 and 26.04 fixes are still in progress
- Fixed package versions for Debian, Red Hat and SUSE: Unknown.
Workaround
- Unknown. No workaround is documented in the sources.
- Own assessment (not from a source): if hosts do not need act_ct in tc rules, remove those rules until the kernel is patched.
Exploitation
- The description says the trigger is possible but no known practical network pipeline requires it.
- Not listed in CISA KEV (snapshot from Strix; recheck).
- No public proof of concept found. This does not prove that none exists.
- VulDB exploit price estimates are a market figure, not evidence of exploitation.
Our recommendation
- Check whether any host uses act_ct in tc rules (tc filter show, act_ct action).
- If not needed, remove those rules now.
- Apply the distribution kernel update or a stable kernel that includes the fix as soon as it is available.
- Confirm the kernel version after patching and reboot.
- Track the related Open vSwitch CVE-2026-98491 if you run OVS conntrack.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- kernel.org stable commit f85009d git.kernel.org
- kernel.org stable commit 2b783c5 git.kernel.org
- OSV CVE-2026-98441 osv.dev
- Ubuntu Security CVE-2026-98441 ubuntu.com
- Red Hat CVE data via Vulners vulners.com
- VulDB 421775 vuldb.com
- Strix CVE-2026-98441 www.strix.ai
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 23:46.
Original description (NVD)
In the Linux kernel, the following vulnerability has been resolved: net/sched: act_ct: avoid modifying shared unconfirmed ct entry In a case where skb with an unconfirmed ct entry gets cloned, we may end up processing both again but with different sets of extensions. The series of events: 1. The first clone wants to commit and runs the helpers wiring up the extension pointer into the expectation list. 2. Then it looses the confirmation keeping the entry unconfirmed. 3. Second clone now wants to commit labels or run NAT and adds the new extension for that breaking the pointer in the expectation list causing UAF on the destruction path later. While this is possible to trigger, there should be no practical network pipeline where we need to process both clones without modifications in the same zone. So, let's just reset the entry in case for some reason we got an skb with a shared one. This doesn't affect any known use cases, but avoids any potential problems with sharing and modification of the unconfirmed ct entry. Unlike openvswitch module, act_ct allows for NAT without commit. Changing that would be a uAPI break. So, act_ct needs to reset on NAT regardless of the commit flag to avoid reallocation of the extension space. This, however, doesn't really change the picture for sensible networking cases as there should be no need to run the same packet twice (before and after the clone) through conntrack without packet header or zone changes and without commit. The fixes tag points to the introduction of helpers, since that's the main UAF trigger for the sharing.
NVD references (5)
- https://git.kernel.org/stable/c/2b783c562c3238ff50c34fd7a72a735c9cc73d9a
- https://git.kernel.org/stable/c/4a04ed91575819fb1859311113e95700b709bec7
- https://git.kernel.org/stable/c/b097d9d7a5b09c8ec5c4c1a08e35ac5a66b06b44
- https://git.kernel.org/stable/c/c812bb4aea13d385d048b5b24756aee2edf6f99e
- https://git.kernel.org/stable/c/f85009dfcd65e5969526b0db7a49b5413746e630
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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