# CVE-2025-21993: iscsi_ibft: Fix UBSAN shift-out-of-bounds warning in ibft_attr_show_nic()

> A shift-out-of-bounds error in the Linux kernel iSCSI boot process can trigger a UBSAN warning, potentially leading to a denial of service.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Apr 2025
- **Updated:** 11 May 2026
- **Also known as:** AZL-59769, AZL-59892, BELL-CVE-2025-21993, DEBIAN-CVE-2025-21993, GHSA-ghxc-pm62-h5fx, UBUNTU-CVE-2025-21993

## What is CVE-2025-21993?

CVE-2025-21993 is a high-severity vulnerability in Linux kernel, published on 2 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

iscsi_ibft: Fix UBSAN shift-out-of-bounds warning in ibft_attr_show_nic()

When performing an iSCSI boot using IPv6, iscsistart still reads the
/sys/firmware/ibft/ethernetX/subnet-mask entry. Since the IPv6 prefix
length is 64, this causes the shift exponent to become negative,
triggering a UBSAN warning. As the concept of a subnet mask does not
apply to IPv6, the value is set to ~0 to suppress the warning message.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2025-21993?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21993?

Apply the vendor's patch.

## Is CVE-2025-21993 being exploited?

There is no known exploitation of CVE-2025-21993 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21993 and fix it if it is affected.

What it is: CVE-2025-21993 is a high-severity vulnerability in Linux kernel, published on 2 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21993 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-21993/ (Markdown: https://insights.sbomify.com/CVE-2025-21993/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21993
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21993
- OSV: https://osv.dev/vulnerability/CVE-2025-21993

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21993/
Summaries are generated and may contain mistakes; the official description above is the record.