How to Fix CVE-2026-31478: ksmbd: replace hardcoded hdr2_len with offsetof() in Linux

By Sai Kiran Pandrala

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Last verified: 2026-05-25

CVE-2026-31478 is a ksmbd: replace hardcoded hdr2_len with offsetof() in Linux Linux. Fix it by upgrading to 5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11.

What is CVE-2026-31478?

CVE-2026-31478 is a ksmbd: replace hardcoded hdr2_len with offsetof() flaw in Linux Linux. It carries a CVSS base score of 9.8 (critical). 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: replace hardcoded hdr2_len with offsetof() in smb2_calc_max_out_buf_len()

After this commit (e2b76ab8b5c9 "ksmbd: add support for read compound"),

response buffer management was changed to use dynamic iov array.

In the new design, smb2_calc_max_out_buf_len() expects the second

argument (hdr2_len) to be the offset of ->Buffer field in the

response structure, not a hardcoded magic number.

Fix the remaining call sites to use the correct offsetof() value.

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-2026-31478

Apply the vendor patch. Target the build named in the Fixed in row above (5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11). 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 5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11.

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-2026-31478 being exploited in the wild?

As of 2026-05-25, CVE-2026-31478 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-2026-31478?

The CVSS base score is 9.8 (Critical).

What version fixes this?

Upgrade to 5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11.

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/70b4c414889492c522b6e4331562360f49be2361
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-31478
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: https://git.kernel.org/stable/c/9a7166f0ef8cbb7bb48dd05e2471d995566003f5
• Additional reference: https://git.kernel.org/stable/c/c3a89e3ec1ccf64fa6a34e391e1581ebbcba8683
• Additional reference: https://git.kernel.org/stable/c/6aef1765d6807e0f027cd87f6ac973eb0879a46d
• Additional reference: https://git.kernel.org/stable/c/80824c7e527b70cf9039534e60aff592e8f209d1

*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.*