CVE-2026-18426VulnerabilityPublished 15 Sep 2026Updated 21 Sep 2026

Concrete CMS 9.0.0 through 9.5.2 did not enforce a block-level edit-permission check on the Express Form block's

Source GCVE Database

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Not known to be exploited · CVSS 6.5 Medium.

Data as of 31 Oct 2026, 20:20 UTC

CVSS 3.1 base score

6.5Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 21% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 15 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 31 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-g5mv-jcqh-ch66

What to do

AI-suggested

Until a fix is available, restrict edit permissions to trusted users only and monitor for unusual form control changes. Avoid reusing CSRF tokens across different blocks or forms if possible. Follow official Concrete CMS security updates for any released patches or workarounds.

Description

Concrete CMS versions 9.0.0 through 9.5.2 have a vulnerability where the Express Form block's control-management actions do not enforce block-level edit-permission checks. Instead, they rely solely on CSRF token validation, which is bound to the user and action but not to specific blocks, pages, or forms. This allows an authenticated user with edit access to one Express Form to reuse a valid token to modify controls on other Express Forms they should not have permission to edit, potentially injecting controls that lead to stored cross-site scripting (XSS). The vulnerability has a low CVSS v4.0 score of 2.0. No known exploits are reported in the wild, and no patch information is provided.

Impact

An authenticated user with edit access to one Express Form block can manipulate controls on other Express Forms they are not authorized to edit by reusing CSRF tokens. This can lead to unauthorized addition, modification, or deletion of form controls, including injection of malicious HTML content resulting in stored cross-site scripting (XSS). The impact is limited by the requirement for authenticated edit access and the low CVSS score reflects the limited scope and complexity of exploitation.

Summary

Concrete CMS versions 9.0.0 through 9.5.2 lack proper enforcement of block-level edit-permission checks on the Express Form block's control-management actions. The system relies only on CSRF token validation, which is user- and action-bound but not tied to specific blocks or forms. This flaw enables an authenticated user with edit rights on one Express Form to reuse a CSRF token to add, modify, or delete controls on other Express Forms without authorization. Such unauthorized modifications can include injecting controls whose values are rendered as HTML, leading to stored XSS vulnerabilities. The issue is tracked as CVE-2026-18426 and has a CVSS v4.0 score of 2.0, indicating low severity.

CWE-862Missing Authorization (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #4OWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The product does not perform an authorization check when an actor attempts to access a resource or perform an action.
Architecture and Design
Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) to enforce the roles at the appropriate boundaries.
Architecture and Design
Ensure that access control checks are performed related to the business logic. These checks may be different than the access control checks that are applied to more generic resources such as files, connections, processes, memory, and database records.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
OWASP Cheat Sheets
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V8.3.8 (opens in a new tab)
V13.4.2 (opens in a new tab)
V7.2.2 (opens in a new tab)
Attack patterns
Exploitation of Thunderbolt Protection Flaws (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Description

AI analysisMachine-generated, updated 16 Sep 2026, 03:17 UTC

WeaknessesCWE-862

Technical details

References

Community reviews

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