SSRF in The Nu HTML Checker
CVE-2025-15104
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).
Vulnerability class: SSRF (Server-Side Request Forgery)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N.
EPSS exploit prediction
EPSS: 0.005 (38.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 39th percentile — 38.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.000
Affected products
- The Nu HTML Checker — versions latest - commit:23f090a11bab8d0d4e698f1ffc197a4fe226a9cd
- Validator
Weakness classification (CWE)
References
- fluidattacks.com/advisories/europe (Exploit, Third Party Advisory, third-party-advisory)
- github.com/validator/validator (Product, product)
Frequently asked questions
- What is CVE-2025-15104?
- CVE-2025-15104 is a medium-severity vulnerability in The Nu HTML Checker, classified under Server-Side Request Forgery (SSRF). CVSS score: 5.3/10. Published 2026-01-16.
- How severe is CVE-2025-15104?
- Medium severity. CVSS v3 base score is 5.3 out of 10.