---
id: "CVE-2026-99083"
kind: "vulnerability"
aliases: ["CVE-2026-99083"]
title: "Linux kernel ksmbd: compounded SMB request overflows heap during Kerberos login"
severity: "unknown"
cvss: null
epss: 0.00209
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["kerberos", "ksmbd", "linux-kernel", "runtime", "self-hosted", "smb-server"]
not_affected_if: ["ksmbd-not-enabled", "samba-userspace"]
published_at: "2026-10-23T13:17:32+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99083", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99083", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel ksmbd: compounded SMB request overflows heap during Kerberos login

In the Linux kernel's ksmbd SMB server, a Kerberos session setup sent as part of a compounded SMB2 request miscalculates the free space in the response buffer. That lets memcpy() write the authentication reply past the end of the buffer into adjacent kernel heap memory.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check whether the kernel has ksmbd built in or loaded with `lsmod | grep ksmbd`
- Check whether the in-kernel SMB server is running with `systemctl status ksmbd` or by looking for the ksmbd process
- Compare the running kernel with the fixed release in the kernel git log for the ksmbd fix (unknown from the sources)

## Fix

- Update the Linux kernel to a release that includes the upstream ksmbd fix (the exact version is unknown from the sources)
- If the ksmbd server is not needed, unload it with `modprobe -r ksmbd` or disable the service

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99083
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99083

## From the sources

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

ksmbd: fix heap out-of-bounds write in krb5_authenticate()

In krb5_authenticate(), out_len is calculated to determine available
headroom in response_buf for the incoming Kerberos AP-REP token. However,
when SMB2_SESSION_SETUP is processed as a non-first element of a compounded
SMB2 request, the calculation omits work->next_smb2_rsp_hdr_off.

This causes out_len to overstate remaining buffer headroom by
the cumulative size of prior responses in the compound chain. Consequently,
the length check in ksmbd_krb5_authenticate()
(*out_len <= resp->spnego_blob_len) passes erroneously, allowing memcpy()
to write the AP-REP blob past the end of response_buf into adjacent kernel
heap memory.

Fix this by subtracting work->next_smb2_rsp_hdr_off when computing out_len,
ensuring it accurately reflects physical remaining buffer space.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_