How to Fix CVE-2026-31611: ksmbd: require 3 sub-authorities before reading sub_auth[2] in Linux

By Sai Kiran Pandrala

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Last verified: 2026-05-25

CVE-2026-31611 is a ksmbd: require 3 sub-authorities before reading sub_auth[2] in Linux Linux. Fix it by upgrading to 6.6.136, 6.12.83, 6.18.24, 6.19.14, 7.0.1, 7.1-rc1.

What is CVE-2026-31611?

CVE-2026-31611 is a ksmbd: require 3 sub-authorities before reading sub_auth[2] flaw in Linux Linux. It carries a CVSS base score of 8.6 (high). It is not currently listed in the CISA Known Exploited Vulnerabilities catalog.

From the source record: In the Linux kernel, the following vulnerability has been resolved:

ksmbd: require 3 sub-authorities before reading sub_auth[2]

parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on

match reads sid.sub_auth[2] as the file mode. If sid_unix_NFS_mode is

the prefix S-1-5-88-3 with num_subauth = 2 then compare_sids() compares

only min(num_subauth, 2) sub-authorities so a client SID with

num_subauth = 2 and sub_auth = {88, 3} will match.

If num_subauth = 2 and the ACE is placed at the very end of the security

descriptor, sub_auth[2] will be 4 bytes past end_of_acl. The

out-of-band bytes will then be masked to the low 9 bits and applied as

the file's POSIX mode, probably not something that is good to have

happen.

Fix this up by forcing the SID to actually carry a third sub-authority

before reading it at all.

Why it matters in practice: The blast radius depends on how the affected service is exposed. An internet-facing instance with no compensating controls is the highest-risk configuration.

Am I affected?

You are affected if your installation of Linux matches a version listed in the Affected row above.

# Debian/Ubuntu
dpkg -s linux | grep Version
# RHEL/Rocky
rpm -q linux

How to fix CVE-2026-31611

Apply the vendor patch. Target the build named in the Fixed in row above (6.6.136, 6.12.83, 6.18.24, 6.19.14, 7.0.1, 7.1-rc1). The runnable command set below covers the most common deployment patterns for Linux.

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade linux
dpkg -s linux | grep Version

RHEL / CentOS / Rocky

sudo dnf upgrade linux -y
rpm -q linux

After applying the patch

1. Restart the service or device so the patched binary loads.
2. Confirm the running version matches the Fixed in row using the verification command below.
3. Rotate credentials and API keys that the affected service could access if the asset was exposed during the disclosure window.

If you can't patch immediately

Until the patch lands, narrow the attack surface with these runnable controls.

Restrict network exposure

Block public access to the affected service at the perimeter. Allow only trusted source IPs.

# Linux iptables: only allow trusted admin subnet
sudo iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -j DROP
sudo iptables-save | sudo tee /etc/iptables/rules.v4

# Windows firewall: only allow trusted admin subnet on management port
New-NetFirewallRule -DisplayName "Restrict-Mgmt-Allow" -Direction Inbound -Action Allow `
  -RemoteAddress 10.10.10.0/24 -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Restrict-Mgmt-Deny"  -Direction Inbound -Action Block `
  -Protocol TCP -LocalPort 443

Mitigations are temporary. Apply the vendor patch as soon as a maintenance window opens.

How to verify the fix worked

Confirm the patched build is the one actually running.

# Debian/Ubuntu
dpkg -s linux | grep Version
# RHEL/Rocky
rpm -q linux

Expected: a version at or above 6.6.136, 6.12.83, 6.18.24, 6.19.14, 7.0.1, 7.1-rc1.

Also worth doing: pull recent log windows for indicators of compromise listed in the vendor advisory, and re-run an authenticated vulnerability scan with up-to-date signatures.

Frequently asked questions

Is CVE-2026-31611 being exploited in the wild?

As of 2026-05-25, CVE-2026-31611 is not listed in the CISA Known Exploited Vulnerabilities catalog. Watch the catalog and patch on a normal cadence; KEV status can change as exploitation evidence emerges.

What is the CVSS score for CVE-2026-31611?

The CVSS base score is 8.6 (High).

What version fixes this?

Upgrade to 6.6.136, 6.12.83, 6.18.24, 6.19.14, 7.0.1, 7.1-rc1.

Will a WAF or IDS rule alone close this?

No. Network filters cut down opportunistic scans but they do not remove the flaw. The vendor patch is the only durable fix.

References

• Official vendor advisory: https://git.kernel.org/stable/c/b5b5d5936a50497fb151c0b122899a6894721c2b
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-31611
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: https://git.kernel.org/stable/c/08f9e6d899b5c834bbcc239eae1bed58d9b15d2c
• Additional reference: https://git.kernel.org/stable/c/d2454f4a002d08560a60f214f392e6491cf11560
• Additional reference: https://git.kernel.org/stable/c/46bbcd3ebfb3549c8da1838fc4493e79bd3241e7
• Additional reference: https://git.kernel.org/stable/c/9401f86a224f37b50e6a3ccf1d46a70d5ef8af0a

*Assembled from the official vendor advisory, the NVD record, and the CISA KEV listing on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*