medium CVSS 6.1 EPSS 0.29%

CVE-2026-26987·PHP vulnerability

LibreNMS is an auto-discovering PHP/MySQL/SNMP based network monitoring tool. Versions 25.12.0 and below are vulnerable to Reflected XSS attacks via email field. This issue has been fixed in version 26.2.0.

Severity
medium
Software
PHP
Fixed in
26.2.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.29% — higher than 21% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

Until: 26.2.0

Fixed in: 26.2.0

How to fix this CVE

LibreNMS instances must be upgraded to version 26.2.0 or later to eliminate the reflected XSS vulnerability in the email field. Organizations running affected versions should prioritize this update to prevent attackers from injecting malicious scripts through unsanitized email parameters. After updating, verify that input validation is properly enforced across all user-facing forms.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

(?:email|mail)=(?:[^&]*?(?:%3c|<)(?:script|img|svg|iframe)[^&]*?(?:%3e|>))

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement strict input validation rules blocking email parameter values containing HTML/JavaScript tags (<, >, script, onerror, onload, etc.). Apply output encoding to all email field displays using HTML entity encoding (e.g., < becomes &lt;). Set Content-Security-Policy headers to restrict inline script execution.

How to check if you are affected

  1. Run `php -v` to verify the installed PHP version and cross-reference against LibreNMS release notes for version 26.2.0 or later
  2. Check the LibreNMS application directory for the version tag in `VERSION` file or via the web interface: `cat /opt/librenms/VERSION | head -1`
  3. Search application access logs for GET/POST requests containing URL-encoded or HTML-encoded characters in the email parameter: `grep -i 'email.*%3c\|email.*&lt;' /var/log/apache2/access.log`
  4. Verify the patch by inspecting the email input handling code in `/opt/librenms/app/Models/User.php` to confirm output escaping is applied to email fields

FAQ

What is CVE-2026-26987?

This is a reflected cross-site scripting (XSS) vulnerability in LibreNMS where the email field fails to properly sanitize user input, allowing attackers to inject arbitrary JavaScript code that executes in the browsers of other users who click a crafted link.

Is CVE-2026-26987 being actively exploited?

No, this vulnerability is not currently listed on the CISA KEV catalog and no public exploits are available, though the attack surface remains if systems remain unpatched.

What versions of PHP are affected by CVE-2026-26987?

LibreNMS versions 25.12.0 and earlier are vulnerable; the fix is available in version 26.2.0 and all subsequent releases.

How do I check if my server is vulnerable to CVE-2026-26987?

Run `cat /opt/librenms/VERSION` and verify the version number is 26.2.0 or higher. If lower, the instance is vulnerable and requires immediate patching.

Does Defensia detect CVE-2026-26987?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP with LibreNMS is installed on a monitored server, CVE-2026-26987 will appear in your dashboard with remediation steps.

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References

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