Linux KernelOperating system · Linux
CVE-2026-31478
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIn 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceA logic error in the Linux kernel's ksmbd (kernel SMB daemon) module causes incorrect buffer length calculations. The function smb2_calc_max_out_buf_len() expects the offset of the ->Buffer field in the response structure, but some call sites pass hardcoded magic numbers instead of proper offsetof() values. This mismatch in response buffer management (introduced after commit e2b76ab8b5c9 changed to dynamic iov arrays) can lead to improper memory handling during SMB operations.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Linux KernelOperating system
>= 5.15.145, < 5.15.203>= 6.1.71, < 6.1.168>= 6.6.1, < 6.6.131>= 6.7, < 6.12.80>= 6.13, < 6.18.21>= 6.19, < 6.19.11= 6.6= 7.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check if ksmbd SMB server is activeRun 'lsmod | grep ksmbd' to check if the ksmbd module is loaded, or check for ksmbd-related processes. Also check /proc/fs/ksmbd/ for active shares.Affected if ksmbd module is loaded or ksmbd is serving SMB shares - the vulnerability only applies when ksmbd is running as the SMB server
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Verify kernel version against affected rangesRun 'uname -r' or 'cat /proc/version' to get the kernel version, then compare it to the affected ranges: 5.15.145-5.15.202, 6.1.71-6.1.167, 6.6.1-6.6.130, 6.7-6.12.79, 6.13-6.18.20, 6.19-6.19.10, or versions 6.6 and 7.0Affected if kernel version falls within any of the listed affected version ranges
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Check if SMB server functionality is enabledExamine kernel config or running services: check /boot/config-$(uname -r) for CONFIG_KSMBD=y or CONFIG_SMB_SERVER_KBFS, or check if smbd service is running via 'systemctl status smbd' or equivalentAffected if SMB server (ksmbd) is compiled into the kernel or loaded as a module and actively serving files
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Inspect source code for hardcoded hdr2_len valuesIf kernel source is available, search for calls to smb2_calc_max_out_buf_len and check if arguments are hardcoded magic numbers rather than offsetof() macro calls - look for patterns like 'smb2_calc_max_out_buf_len(..., some_number)' near ksmbd source filesAffected if source code contains hardcoded numeric values instead of offsetof() in calls to smb2_calc_max_out_buf_len (this requires source inspection)
Environment is affected if ksmbd SMB server is active AND the kernel version falls within the affected ranges AND the vulnerable call sites with hardcoded values are present in the running kernel binary
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped5.15.2036.1.1686.6.131
Apply the kernel patch to correct the call sites to use offsetof() instead of hardcoded values, then rebuild and deploy the updated kernel. Given CVSS 9.8, prioritize immediate patching on systems running ksmbd as an SMB server.
Upgrade to Linux kernel 5.15.203+, 6.1.168+, 6.6.131+, or 6.12.80+ (depending on which branch is in use)
- Identify the currently running Linux kernel version using 'uname -r'
- Determine which kernel version branch is in use (e.g., 5.15.x, 6.1.x, 6.6.x, or 6.7-6.12.x)
- Upgrade the kernel to a version greater than the upper bound of the affected range: for 5.15.x series upgrade to >= 5.15.203, for 6.1.x upgrade to >= 6.1.168, for 6.6.x upgrade to >= 6.6.131, or for 6.7-6.12.x upgrade to >= 6.12.80
- Reboot the system to load the patched kernel
- Verify the fix is applied by checking the smb2_calc_max_out_buf_len() call sites in the kernel source or by confirming the kernel version
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Linux Kernel
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 h
- Testing6.0 h
- Review / QA3.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,096.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-31478 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-31478 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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