How severe is CVE-2021-42656?

CVE-2021-42656 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-42656?

CVE-2021-42656 affects the following packages: SSCMS (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2021-42656?

The records we use list no patched version of SSCMS for CVE-2021-42656. Check the maintainer's advisory, and where your build allows, override or pin the dependency away from the vulnerable range.

How do I detect CVE-2021-42656 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for SSCMS, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-42656 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.

Is CVE-2021-42656 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-42656, including 1 in-the-wild exploitation 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-2021-42656?

CVE-2021-42656 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 54th 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. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-42656?

CVE-2021-42656 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.

When was CVE-2021-42656 published, and has it been updated?

CVE-2021-42656 was published on May 24, 2022 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET

.NET NuGet

Not in CISA KEV

MEDIUM severity

CVE-2021-42656 — SSCMS

MEDIUM

CVE-2021-42656 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in SSCMS. 1 public exploit or proof-of-concept reference is 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.

Cross site scripting in SiteServer CMS

Also known asGHSA-2xwp-7j3p-c78x

Published

May 24, 2022

Updated

Oct 20, 2026

Affected

1 pkg

Patched

See advisory

Exploits

1 known

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs54th percentile — riskier than 54% 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

Real-World Exposure

1 pkg affected

.NETSSCMS

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

SiteServer CMS V6.15.51 is affected by a Cross Site Scripting (XSS) vulnerability.

Affected Packages

1 total

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`SSCMS`	all versions	No fix

Affected Products

1 product · 1 configurations

Application

siteserver cmssscms

1 version

6.15.51

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.

SiteServer CMS V6.15.51 is affected by a Cross Site Scripting (XSS) vuln…

github.comNVDJun 2022

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for SSCMS, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Remediation status
The records we use list no patched version of SSCMS for CVE-2021-42656. Check the maintainer's advisory, and where your build allows, override or pin the dependency away from the vulnerable range.
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.

Frequently Asked Questions

SiteServer CMS V6.15.51 is affected by a Cross Site Scripting (XSS) vulnerability.

O3 Security · Impact-Aware SCA

Is CVE-2021-42656 in your dependencies?

Find it across NuGet, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

5.4/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

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

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No Nuclei template or Metasploit module yet. 1 public proof-of-concept reference is listed on this page.

OpenSSF Scorecard

Package health score

cms

2.3/10

Code-Review0/10

Maintained6/10

Branch-Protection0/10

Assessed Nov 2, 2026

Timeline

Disclosed

May 24, 2022

First exploit

Jun 1, 2022

7 days after disclosure — rapid exploitation

Last updated

Oct 20, 2026

Identifiers

CVE-2021-42656

GHSA-2xwp-7j3p-c78x

References

Advisorygithub.com · cms Advisorynvd.nist.gov · CVE 2021 42656 Fixgithub.com · siteserver install.zip Reportgithub.com

Related vulnerabilities

Also affects SSCMS

This CVE is medium severity; 3 of 3 other CVEs affecting SSCMS are rated as high or higher (top: high).

CVE-2023-2862CVE-2022-30349CVE-2021-42655

Same weakness: CWE-79

This CVE is medium severity; 5 of 5 other CVEs with CWE-79 are rated as high or higher (top: critical).

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

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 20, 2026