CVE-2026-8426Network attack vector

Remote code execution via unchecked upgrade request

Published May 21, 2026 · Updated May 21, 2026

Cross-site request forgery in Concrete CMS 9.5.0 and earlier allows remote attackers to execute code through a crafted browser navigation. The prepare_remote_upgrade route accepts a request without validating a CSRF token, overwrites an installed package's PHP with attacker-controlled marketplace content, and invokes its upgrade method. Exploitation requires an authorized package installer to navigate to the request, a marketplace-connected site, and attacker control of the installed item's returned package.

CVSS severity7.5
High
EPSS probability0.17%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site request forgery in Concrete CMS 9.5.0 and earlier allows remote attackers to execute code through a crafted browser navigation. The prepare_remote_upgrade route accepts a request without validating a CSRF token, overwrites an installed package's PHP with attacker-controlled marketplace content, and invokes its upgrade method. Exploitation requires an authorized package installer to navigate to the request, a marketplace-connected site, and attacker control of the installed item's returned package.

The record

CVE
CVE-2026-8426
Published
May 21, 2026
Updated
May 21, 2026
Vendor
Concrete CMS
Product
Concrete CMS
Classifications
CWE-829, CWE-352, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 21, 2026CVE publishedPublication date reported by the CVE source.
  2. May 21, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=5.0 <=9.5.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Concrete CMS · Concrete CMSversion=5.0 <=9.5.0

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
7.5ConcreteCMSCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-829Inclusion of Functionality from Untrusted Control Sphere
CWE-352Cross-Site Request Forgery (CSRF)
T1204.001User Execution: Malicious Link

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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