# CVE-2026-55781: NanaZip: Unbounded memory allocation (DoS) in NanaZip UFS parser via unvalidated fs_bsize/fs_fsize superblock fields

> Attackers can cause memory exhaustion and application crash in NanaZip by crafting malicious UFS images with invalid superblock fields.

- **Severity:** Low 2.4 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 10 Jul 2026
- **Updated:** 10 Jul 2026
- **Also known as:** GHSA-m34h-jf84-m74h

## What is CVE-2026-55781?

CVE-2026-55781 is a low-severity denial of service vulnerability in M2Team NanaZip, published on 10 Jul 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

NanaZip is the 7-Zip derivative intended for the modern Windows experience. Prior to 6.5.1749.0, NanaZip's UFS and FFS image handler in NanaZip.Codecs.Archive.Ufs.cpp validates the superblock block size only against the MINBSIZE lower bound and does not validate the fs_fsize fragment size, allowing attacker-controlled 32-bit fields to flow into indirect-block, directory, and extraction buffer allocations. A tiny crafted UFS image can force multi-gigabyte allocations during open or extraction, causing memory exhaustion or process termination. This issue is fixed in version 6.5.1749.0.

Weakness: CWE-789 (Memory Allocation with Excessive Size Value), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-55781?


## How do I fix CVE-2026-55781?

No fix has been published for CVE-2026-55781 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-55781 being exploited?

There is no known exploitation of CVE-2026-55781 in the wild and no public exploit code.

Public exploit code:

- Unbounded memory allocation in NanaZip's UFS handler via an attacker-controlled `fs_bsize` field. (github_poc): https://github.com/NT2DEV/CVE-2026-55781-poc

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55781 and fix it if it is affected.

What it is: CVE-2026-55781 is a low-severity denial of service vulnerability in M2Team NanaZip, published on 10 Jul 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-55781 in the wild and no public exploit code.

Affected product: M2Team NanaZip. 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-2026-55781/ (Markdown: https://insights.sbomify.com/CVE-2026-55781/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55781
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55781
- OSV: https://osv.dev/vulnerability/CVE-2026-55781

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55781/
Summaries are generated and may contain mistakes; the official description above is the record.