CVE-2025-15104: Nu Html Checker (validator.nu) contains a restriction bypass that allows remote attackers to make the server p

Q: How do I fix CVE-2025-15104?

validator: upgrade to 2026-01-11. Exact commands for each OS are on this page.

Q: Is CVE-2025-15104 being exploited?

CVE-2025-15104 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2025-15104 affect?

CVE-2025-15104 affects validator. Severity is medium (CVSS 5.3). 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-

Q: How urgent is CVE-2025-15104?

Priority 39/100. Public exploit reference; Remotely exploitable, no login, no user action

This week

MEDIUM 5.3REMOTEUPGRADE

CVE-2025-15104 is a medium-severity flaw (CVSS 5.3) affecting validator. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 Ch

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-918
Published	2026-01-16
Last modified	2026-06-17

Publisher (CNA): Fluid Attacks · Vendor: validator

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-01-16 14:15 UTCPublished CVE record published
2026-06-17 08:37 UTCUpdated Record last modified

validator validator UPGRADE

Affected: < 2026-01-11

Fixed in: 2026-01-11

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy validator || rpm -q validator || validator --version

2. UPGRADE to 2026-01-11 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade validator

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade validator

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-15104?

validator: upgrade to 2026-01-11. Exact commands for each OS are on this page.

Is CVE-2025-15104 being exploited?

CVE-2025-15104 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-15104 affect?

CVE-2025-15104 affects validator. Severity is medium (CVSS 5.3). 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-

How urgent is CVE-2025-15104?

Priority 39/100. Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2025-12758 HIGH 7.5 UPGRADE Versions of the package validator before 13.15.22 are vulnerable to Incomplete Filtering of One or More Instan
CVE-2025-56200 MEDIUM 6.1 UPGRADE A URL validation bypass vulnerability exists in validator.js through version 13.15.15

References

← Back to latest CVEs