High Insecure deserialization

CVE-2026-8135 — Concrete CMS 9.5.0 and below is vulnerable to RCE due to insecure deserialization occurring in the ExpressEntryList block controller.

Concrete CMS 9.5.0 and below is vulnerable to Remote Code Execution due to insecure deserialization occurring in the ExpressEntryList block controller. An rogue administrator with privileges to add blocks to an area can bypass the intended protection mechanism (_fromCIF === true), which normally restricts malicious inputs over form POST requests, by leveraging the REST API functionality. Because the REST API parses requests using json_decode(), the string "true" is evaluated as a strict PHP Boolean(true). This bypass allows the attacker to inject a malicious serialized payload into the block's filterFields database column. The payload will subsequently be executed when the block's data is viewed or edited by an administrator leading to complete server takeover (RCE).The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 8.9 with a vector of CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H. Thanks Nguyễn Văn Thiện https://github.com/Thien225409 for reporting

Published May 21, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: ConcreteCMS

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.7% (30-day exploitation probability)+8
Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

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

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.74%

ahead of 54% of scored CVEs · Oct 14, 2026

Affects

Concretecms

Concrete Cms

CVSS base

7.2

High · 3 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

None

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Concretecms Concrete Cms).
No fix documented in the sources: reduce exposure and monitor the vendor advisory.
Search available logs for exploit probes matching the vulnerability class (insecure deserialization), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Concretecms	Concrete Cms	≤ 9.5.0	Vulnerable	NIST National Vulnerability Database
Concretecms	Concrete Cms	≥ 5.0 and ≤ 9.5.0	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

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

Attack vector

Network

Attack complexity

Low

Privileges required

High

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Every base score collected

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

Score	Version	Severity	Expl.	Impact	Scored by
8.9	CVSS 4.0	High	—	—	ff5b8ace-8b95-4078-9743-eac1ca5451de via NIST National Vulnerability Database
8.9	CVSS 4.0	High	—	—	ConcreteCMS via CVE Program (cve.org) e CISA ADP
7.2	CVSS 3.1	High	1.2	5.9	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H (score shown at the top: nvd@nist.gov)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-502 · Deserialization of Untrusted Data

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.74%

Detection

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

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Monitor serialized objects in requests and unexpected processes spawned by the runtime.

web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)application logs

Timeline

What happened to this CVE, newest first.

2026
Affected products added · concrete cms/concrete cms
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
CVSS changed · 7.2
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
May 21, 2026 · ConcreteCMS

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

1 on the record

documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes
Release Notes, release-notes, NIST National Vulnerability Database

Elsewhere on this site

Concretecms
every CVE for this vendor

Insecure deserialization
same class

CWE-502
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-85387 Concrete CMS before 9.5.4 re-authorized OAuth REST API requests from the bearer token… 29d
Medium CVE-2026-18120 Missing Authorization in Concretecms Concrete Cms 29d
Low CVE-2026-87031 Missing Authorization in Concretecms Concrete Cms 29d
Medium CVE-2026-87028 Missing Authorization in Concretecms Concrete Cms 29d
Critical CVE-2026-85385 Cross-site Scripting (XSS) in Concretecms Concrete Cms 29d
Medium CVE-2026-85386 Cross-site Scripting (XSS) in Concretecms Concrete Cms 29d

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 fixed version, patch or mitigation documented.
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 record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-8135 is "Concrete CMS 9.5.0 and below is vulnerable to RCE due to insecure deserialization occurring in the ExpressEntryList block controller.", rated High. It affects Concrete Cms (Concretecms); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.2 describes technical severity. EPSS 0.74% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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