# CVE-2025-15104: Nu Html Checker (validator.nu) - Restriction bypass vulnerability allowing local SSRF

> Attackers can bypass hostname restrictions to force the validator to make requests to internal resources, potentially exposing sensitive data.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Jan 2026
- **Updated:** 16 Jan 2026
- **Also known as:** DEBIAN-CVE-2025-15104, GHSA-fccg-7w3p-w66f, UBUNTU-CVE-2025-15104

## What is CVE-2025-15104?

CVE-2025-15104 is a medium-severity server-side request forgery vulnerability in The Nu Html Checker, published on 16 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Nu Html Checker (validator.nu) contains a restriction bypass that allows remote attackers to make the server perform arbitrary HTTP/HTTPS requests to internal resources, including localhost services. While the validator implements hostname-based protections to block direct access to localhost and 127.0.0.1, these controls can be bypassed using DNS rebinding techniques or domains that resolve to loopback addresses.This issue affects The Nu Html Checker (vnu): latest (commit 23f090a11bab8d0d4e698f1ffc197a4fe226a9cd).

Weakness: CWE-918 (Server-Side Request Forgery)

## Which versions are affected by CVE-2025-15104?

4 open source packages ship an affected version, including nu.validator:validator and vnu-jar. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nu.validator:validator` | Maven | All versions to 26.1.11 | None |
| `nu.validator:validator` | Maven | All versions to 26.1.11 | None |
| `vnu-jar` | npm | All versions to 26.1.11 | None |
| `vnu-jar` | npm | All versions to 26.1.11 | None |

Products: Validator Validator.

## How do I fix CVE-2025-15104?

No fix has been published for CVE-2025-15104 yet. Limit exposure of the affected software until one is available.

## Is CVE-2025-15104 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-15104 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-15104 and fix it if it is affected.

What it is: CVE-2025-15104 is a medium-severity server-side request forgery vulnerability in The Nu Html Checker, published on 16 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-15104 exists.

Affected packages and fixed versions:
- nu.validator:validator (Maven): affected all versions up to 26.1.11; no fix listed
- nu.validator:validator (Maven): affected all versions up to 26.1.11; no fix listed
- vnu-jar (npm): affected all versions up to 26.1.11; no fix listed
- vnu-jar (npm): affected all versions up to 26.1.11; no fix listed

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-2025-15104/ (Markdown: https://insights.sbomify.com/CVE-2025-15104/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-15104
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-15104
- OSV: https://osv.dev/vulnerability/CVE-2025-15104

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-15104/
Summaries are generated and may contain mistakes; the official description above is the record.