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CVE-2026-89631

Critical Vulnerability: CVE-2026-89631

In the Linux kernel, the following vulnerability has been resolved: smb: client: reject a tree connect response whose byte count is too small CIFSTCon() bounds its strnlen() over the byte area with the server's ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int and converts to a huge size_t. The later subtraction wraps the __u16 bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of

Wednesday, October 21, 2026Vulnios Threat Intelligence
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Vulnerability Snapshot

CVE-2026-89631 is rated CRITICAL — exploitation is trivial or already observed in the wild and impact is severe. Patch immediately, not on the next maintenance window.

Executive Summary

In the Linux kernel, the following vulnerability has been resolved:

smb: client: reject a tree connect response whose byte count is too small

CIFSTCon() bounds its strnlen() over the byte area with the server's

ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int

and converts to a huge size_t. The later subtraction wraps the __u16

bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of

up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the

slab object, and the bytes reach userspace through tcon->nativeFileSystem

in /proc/fs/cifs/DebugData.

Reject a byte area too small for what the parser consumes. Two bytes is

the least it can consume, and no conformant response carries fewer. The

new trace point is the 129th smb_eio_trace entry, which __mode(byte)

cannot represent, so the attribute goes with it.

Why It Matters

CVE-2026-89631 is rated CRITICAL severity, requiring immediate attention from security teams.

CVSS Base Score: 9.1/10

🛡️What Defenders Should Check

  • Check if you are affected — Review your asset inventory for products listed in CVE-2026-89631.
  • Apply available patches — Visit vendor advisories for the latest security updates.
  • Monitor for exploitation — Check your SIEM/IDS logs for related indicators.

    • Use Vulnios to continuously monitor your exposure to CVE-2026-89631 and similar vulnerabilities.

    References & Sources

  • https://git.kernel.org/stable/c/2ae219762ebac929bf6ab3473681f10ed566af08
  • https://git.kernel.org/stable/c/3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d
  • https://git.kernel.org/stable/c/411e484fe71a7f1028de617447edad9cb6d9d68a
  • https://git.kernel.org/stable/c/65deb18359341141d37dc86fc7853511be3c87a7
  • https://git.kernel.org/stable/c/d837645a719151ed392d4d9cab4571c2753dab12

    • How Vulnios Detects This

    Vulnios scans for this vulnerability using Trivy and Grype for SBOM-based CVE matching and Vulnios CVE feed continuous monitoring against your asset inventory. Run a scan against your environment to see whether you are exposed; findings are linked back to the original CVE record so triage starts with the patch path already known.

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    Sources

    Related Threat Alerts

    Frequently Asked Questions

    What is CVE-2026-89631?

    CVE-2026-89631 is a critical-severity vulnerability tracked under the Common Vulnerabilities and Exposures program. In the Linux kernel, the following vulnerability has been resolved:

    smb: client: reject a tree connect response whose byte count is too small

    CIFSTCon() bounds its strnlen() over the byte area with

    Am I affected?

    Affected technology is listed in the Affected Products section above. If your asset inventory contains any of them, assume in-scope until you can prove otherwise.

    How urgent is the response?

    Critical: do not wait for your normal patch cycle. Verify exposure today, apply the vendor patch immediately, and add detection rules for any post-exploit indicators.

    How do I remediate?

    Apply the vendor patch listed in the upstream advisory linked under Sources. If the patch is not yet available, follow the vendor-supplied workaround (often a config flag or feature disable) and add detections for the published exploit pattern in your SIEM. Re-scan after the patch lands to confirm the finding clears.

    Where can I track exploitation activity?

    Watch CISA's Known Exploited Vulnerabilities catalog for CVE-2026-89631. Cross-reference with public exploit databases and your own SIEM/IDS for indicator-of-compromise patterns. Vulnios tracks KEV status automatically and surfaces it on the asset findings view.

    How does Vulnios help with this?

    Vulnios continuously cross-references your asset inventory against the live CVE feed (NVD, vendor advisories, CISA KEV, and curated OSINT). When a new CVE matches your environment, you get a prioritized finding with the severity, KEV status, exploit-prediction (EPSS), and a direct path to the vendor patch. You can start a free scan from the homepage.

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