# CVE-2026-39399: NuGet Gallery: Arbitrary Blob Overwrite via Nuspec Confusion and URI Fragment Truncation

> Attackers can overwrite arbitrary storage blobs via crafted NuGet package metadata, potentially leading to remote code execution or content tampering.

- **Severity:** Critical 9.6 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Apr 2026
- **Updated:** 15 Apr 2026
- **Also known as:** GHSA-9r3h-v4hx-rhfr

## What is CVE-2026-39399?

CVE-2026-39399 is a critical path traversal vulnerability in NuGetGallery, published on 14 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

NuGet Gallery is a package repository that powers nuget.org. A security vulnerability exists in the NuGetGallery backend job’s handling of .nuspec files within NuGet packages. An attacker can supply a crafted nuspec file with malicious metadata, leading to cross package metadata injection that may result in remote code execution (RCE) and/or arbitrary blob writes due to insufficient input validation. The issue is exploitable via URI fragment injection using unsanitized package identifiers, allowing an attacker to control the resolved blob path. This enables writes to arbitrary blobs within the storage container, not limited to .nupkg files, resulting in potential tampering of existing content. This issue has been patched in commit 0e80f87628349207cdcaf55358491f8a6f1ca276.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2026-39399?


## How do I fix CVE-2026-39399?

No fix has been published for CVE-2026-39399 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-39399 being exploited?

There is no known exploitation of CVE-2026-39399 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39399 and fix it if it is affected.

What it is: CVE-2026-39399 is a critical path traversal vulnerability in NuGetGallery, published on 14 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-39399 in the wild and no public exploit code.

Affected product: NuGetGallery. 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-2026-39399/ (Markdown: https://insights.sbomify.com/CVE-2026-39399/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39399
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39399
- OSV: https://osv.dev/vulnerability/CVE-2026-39399

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39399/
Summaries are generated and may contain mistakes; the official description above is the record.