What is CVE-2026-81918?

CVE-2026-81918 is Concrete CMS below 9.5.3 is vulnerable to Stored XSS via the Date Format field in the Page Attribute Display block. The supplied enrichment data classifies it as Cross-site Scripting mapped to CWE-79 Cross-site Scripting.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-81918. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-81918 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-81918. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-81918 using CVSS 4.8, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

mediumCross-site Scripting

CVE-2026-81918

CVE-2026-81918 — Concrete CMS below 9.5.3 is vulnerable to Stored XSS via the Date Format field in the Page Attribute Display block

Concrete CMS below 9.5.3 is vulnerable to Stored XSS via the Date Format field in the Page Attribute Display block. A user with edit_page_contents permissions could store a payload which executes in the browser of any visitor who viewed a page where the block was configured to display a date-type attribute. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 4.8 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N. Thanks Oriol Ortiz for reporting.

Published Sep 11, 2026Updated Sep 11, 2026Sources: NVD, Concrete CMS (CNA), GitHub, GitHub Security Advisory, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

ahead of 0% of scored CVEs

Affects

Concrete CMS

CVSS base

4.8

medium

Remediation

The vendor's own words where we have them.

Upgrade Concrete CMS to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Concrete CMS	Concrete CMS	5.0.0 to <= 9.5.2	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack Complexity

Low

Attack Vector

Network

Privileges Req

High

User Interaction

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
4.8	CVSS 4.0	medium	—	—	Concrete CMS (CNA)

CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-79 · Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Bypass Protection Mechanism
Read Application Data
Execute Unauthorized Code or Commands

Mitigation: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples of libraries and frameworks that make it easier to generate properly encoded output include Microsoft's Anti-XSS library, the OWASP ESAPI Encoding module, and Apache Wicket.

Weakness & attack patterns

CWE-79Cross-site Scripting

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-85 · AJAX FingerprintingCAPEC-591 · Reflected XSSCAPEC-63 · Cross-Site Scripting (XSS)CAPEC-209 · XSS Using MIME Type MismatchCAPEC-592 · Stored XSSCAPEC-588 · DOM-Based XSS

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Search application, proxy, and WAF logs for requests touching /AV:N/AC:L/AT:N/PR:H/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N.
Prioritize edge telemetry for network-reachable Concrete CMS.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 4.8 (AV:N)
Sep 11, 2026 · NVD
Initial · CVE published
Sep 11, 2026 · NVD
Added · GHSA-j987-77r9-4hgr published (medium)
Sep 11, 2026 · GitHub Security Advisory
CVE published by MITRE.
Sep 11, 2026 · SOCRadar CTI

References

3 on the record

documentation.concretecms.org/developers/introduction/version-history/953-release-notes
Vendor Advisory
nvd.nist.gov/vuln/detail/CVE-2026-81918
NVD
github.com/advisories/GHSA-j987-77r9-4hgr
Exploit, Third Party Advisory

Elsewhere on this site

Concrete CMSevery CVE for this vendor
Cross-site Scriptingsame class
CWE-79other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-81918 is Concrete CMS below 9.5.3 is vulnerable to Stored XSS via the Date Format field in the Page Attribute Display block, a medium vulnerability affecting Concrete CMS from Concrete CMS. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-81918. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-81918. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 4.8 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.