# CVE-2026-8135: Concrete CMS Vulnerable to Deserialization of Untrusted Data

> A rogue administrator can achieve remote code execution in Concrete CMS by injecting a malicious serialized payload via the REST API, bypassing input validation.

- **Severity:** High 8.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 May 2026
- **Updated:** 22 May 2026
- **Also known as:** GHSA-pv2v-6w2v-97x6

## What is CVE-2026-8135?

CVE-2026-8135 is a high-severity unsafe deserialization vulnerability in Concretecms Concrete Cms, published on 21 May 2026. An attacker needs administrator access to exploit it, with no action from the victim.

### Official 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

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2026-8135?

2 open source packages ship an affected version, including concrete5/concrete5. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `concrete5/concrete5` | Packagist | All versions | 9.5.1 |
| `concrete5/concrete5` | Packagist | All versions | 9.5.1 |

Products: Concretecms Concrete Cms.

## How do I fix CVE-2026-8135?

Upgrade concrete5/concrete5 to 9.5.1 or later.

## Is CVE-2026-8135 being exploited?

There is no known exploitation of CVE-2026-8135 in the wild and no public exploit code.

## Other vulnerabilities in Concretecms Concrete Cms

- [CVE-2023-48648](https://insights.sbomify.com/CVE-2023-48648/): Concrete CMS allows unauthorized access because directories can be created with insecure permissions (critical 9.8)
- [CVE-2022-21829](https://insights.sbomify.com/CVE-2022-21829/): Code injection in concrete CMS (critical 9.8)
- [CVE-2021-22958](https://insights.sbomify.com/CVE-2021-22958/): Server-Side Request Forgery vulnerability in concrete5 (critical 9.8)
- [CVE-2021-40098](https://insights.sbomify.com/CVE-2021-40098/): Path traversal in n/a (critical 9.8)
- [CVE-2026-8134](https://insights.sbomify.com/CVE-2026-8134/): Concrete CMS Vulnerable to Relative Path Traversal (critical 9.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-8135 and fix it if it is affected.

What it is: CVE-2026-8135 is a high-severity unsafe deserialization vulnerability in Concretecms Concrete Cms, published on 21 May 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-8135 in the wild and no public exploit code.

Affected packages and fixed versions:
- concrete5/concrete5 (Packagist): affected all versions; fixed in 9.5.1
- concrete5/concrete5 (Packagist): affected all versions; fixed in 9.5.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-8135/ (Markdown: https://insights.sbomify.com/CVE-2026-8135/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-8135
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-8135
- OSV: https://osv.dev/vulnerability/CVE-2026-8135

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-8135/
Summaries are generated and may contain mistakes; the official description above is the record.