How severe is CVE-2021-25299?

CVE-2021-25299 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which products are affected by CVE-2021-25299?

CVE-2021-25299 affects the following products: nagios nagios xi.

How do I patch CVE-2021-25299?

The records we use list no fixed version for CVE-2021-25299. Check the nagios nagios xi advisory for a patched release, and apply the workarounds below until you can upgrade.

How do I detect CVE-2021-25299 across my infrastructure?

Inventory every nagios nagios xi deployment and check each version against the affected-products list above.

How do I mitigate CVE-2021-25299 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.

Is CVE-2021-25299 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2021-25299, including 2 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-25299?

CVE-2021-25299 has an EPSS (Exploit Prediction Scoring System) score of 97.8%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

What type of vulnerability is CVE-2021-25299?

CVE-2021-25299 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

Which products and versions are affected by CVE-2021-25299?

CVE-2021-25299 affects nagios nagios xi (1 version, e.g. 5.7.5). Check the affected-products list on this page against your deployed version to confirm exposure.

Is there exploit tooling available for CVE-2021-25299?

A Nuclei detection template exists for CVE-2021-25299, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-25299 published, and has it been updated?

CVE-2021-25299 was published on February 15, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🛡️

Not in CISA KEV

MEDIUM severity

CVE-2021-25299 — Nagios Xi

MEDIUM

CVE-2021-25299 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in nagios nagios xi. 2 public exploit or proof-of-concept references are indexed, so weaponization risk is real. The records list no fixed version yet; check the vendor advisory below and apply the mitigations in the meantime.

Nagios XI version xi-5.7.5 is affected by cross-site scripting (XSS). The vulnerability exists in the file /usr/local/nagiosxi/html/admin/sshterm.php due to improper sanitization of…

Published

Feb 15, 2021

Updated

Jul 9, 2026

Affected

1 product

Patched

See advisory

Exploits

2 known

Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

97.8%probability of exploitation in next 30 days

Very High Risk+12.60%

Lower risk than most CVEs100th percentile — riskier than 100% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Description

Nagios XI version xi-5.7.5 is affected by cross-site scripting (XSS). The vulnerability exists in the file /usr/local/nagiosxi/html/admin/sshterm.php due to improper sanitization of user-controlled input. A maliciously crafted URL, when clicked by an admin user, can be used to steal his/her session cookies or it can be chained with the previous bugs to get one-click remote command execution (RCE) on the Nagios XI server.

Affected Products

1 product · 1 configurations

Application

nagios xinagios

1 version

5.7.5

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Nagios XI version xi-5.7.5 is affected by cross-site scripting (XSS). Th…

github.comNVDMar 2021

Report on packetstormsecurity.com

packetstormsecurity.comNVDMar 2021

Detection & mitigation playbook

Vendor / appliance

Detect
Inventory every nagios nagios xi deployment and check each version against the affected-products list above.
Remediation status
The records we use list no fixed version for CVE-2021-25299. Check the nagios nagios xi advisory for a patched release, and apply the workarounds below until you can upgrade.
Mitigate without a patch
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.

How to detect CVE-2021-25299

A community-maintained Nuclei template exists for this CVE. You can scan for it directly:

nuclei -id cve-2021-25299 -u https://target

Template: Nagios XI 5.7.5 - Cross-Site Scripting
Severity: medium
Impact: Attackers can craft malicious URLs containing JavaScript that executes when clicked by admin users, potentially stealing session cookies that can be chained with other vulnerabilities for one-click remote command execution and complete Nagios XI server compromise.
Remediation: Upgrade Nagios XI to the latest version or apply the provided patch to fix the XSS vulnerability.

Template by ProjectDiscovery nuclei-templates (ritikchaddha), MIT licensed. View the full template. Scan only systems you are authorised to test.

Frequently Asked Questions

O3 Security · Runtime Protection

Is CVE-2021-25299 being exploited in your environment?

Detect and block the exploit chain at execution, on systems you cannot patch yet.

Protect my workloads How runtime protection works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

Timeline

Disclosed

Feb 15, 2021

First exploit

Mar 4, 2021

Last updated

Jul 9, 2026

Identifiers

CVE-2021-25299

References

Exploit / PoCpacketstormsecurity.com · Nagios XI 5.7.5 Remote Code Execution Exploit / PoCgithub.com · README.md Exploit / PoCpacketstormsecurity.com · Nagios XI 5.7.5 Remote Code Execution Exploit / PoCgithub.com · README.md Webassets.nagios.com · versions Webassets.nagios.com · versions

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs with CWE-79 are rated as high or higher (top: critical).

CVE-2026-55649CVE-2026-113787CVE-2026-44267CVE-2026-96632CVE-2026-9086CVE-2026-118246CVE-2026-118251CVE-2026-118253Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026