CVE-2026-8135

CVE-2026-8135 is a high-severity vulnerability in Concretecms Concrete Cms with a CVSS 3.x base score of 7.2. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-502.

Key facts

Description

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

Frequently asked questions

What is CVE-2026-8135?
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
How severe is CVE-2026-8135?
CVE-2026-8135 has a CVSS 3.x base score of 7.2, rated high severity. It is exploitable over network with low attack complexity, requires high privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-8135 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (37th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-8135?
CVE-2026-8135 affects Concretecms Concrete Cms. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-8135?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-8135 have an EU (EUVD) identifier?
Yes. CVE-2026-8135 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-31336.
When was CVE-2026-8135 published?
CVE-2026-8135 was published on 2026-05-21 and last updated on 2026-06-17.

References

Affected products (1)

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