CVE-2026-0773: Upsonic Cloudpickle RCE Vulnerability

CVE-2026-0773 Overview

CVE-2026-0773 is a critical insecure deserialization vulnerability affecting Upsonic that allows remote attackers to execute arbitrary code on vulnerable installations without authentication. The vulnerability exists within the add_tool endpoint, which listens on TCP port 7541 by default. Due to improper validation of user-supplied data, attackers can exploit the cloudpickle deserialization mechanism to achieve remote code execution in the context of the service account.

Critical Impact

Unauthenticated remote code execution via insecure deserialization allows complete system compromise without any user interaction.

Affected Products

• Upsonic (versions with vulnerable add_tool endpoint)
• Systems exposing TCP port 7541 with the Upsonic service

Discovery Timeline

• 2026-01-23 - CVE CVE-2026-0773 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2026-0773

Vulnerability Analysis

This vulnerability is classified as CWE-502 (Deserialization of Untrusted Data). The flaw resides in the add_tool endpoint of the Upsonic service, which accepts serialized Python objects via the cloudpickle library without proper validation. Cloudpickle is a powerful serialization library that extends Python's native pickle module to support more complex objects, but this flexibility also introduces significant security risks when processing untrusted input.

The add_tool endpoint, accessible over TCP port 7541, directly deserializes incoming data without verifying its source or contents. When the cloudpickle loads() function processes a maliciously crafted payload, it can instantiate arbitrary Python objects and execute code during the deserialization process. This is particularly dangerous because pickle deserialization in Python can trigger the __reduce__ method on objects, which allows attackers to specify arbitrary functions to execute with controlled arguments.

The vulnerability does not require any authentication to exploit, significantly lowering the barrier for attackers. Successful exploitation grants code execution with the privileges of the Upsonic service account, potentially leading to complete system compromise.

Root Cause

The root cause of this vulnerability is the direct use of cloudpickle deserialization on untrusted network input without implementing any validation, authentication, or integrity checks. The add_tool endpoint blindly trusts incoming serialized data and passes it to the cloudpickle deserializer, which then reconstructs Python objects that can execute arbitrary code during the deserialization process.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can send a specially crafted cloudpickle serialized payload to TCP port 7541 targeting the add_tool endpoint. The malicious payload contains a Python object that, when deserialized, executes attacker-controlled code.

A typical attack flow involves the attacker creating a malicious Python object with a custom __reduce__ method that returns a tuple specifying a function to call (such as os.system) and its arguments (such as a reverse shell command). When the Upsonic service deserializes this payload via cloudpickle, the __reduce__ method is invoked, executing the attacker's payload in the context of the service account.

For detailed technical information about this vulnerability, refer to the Zero Day Initiative Advisory ZDI-26-042.

Detection Methods for CVE-2026-0773

Indicators of Compromise

• Unexpected network connections to TCP port 7541 from external or untrusted sources
• Anomalous process spawning from the Upsonic service process
• Unusual outbound connections or reverse shell activity originating from the service account
• Suspicious file system modifications or new files created by the Upsonic process

Detection Strategies

• Monitor network traffic to TCP port 7541 for unusual connection patterns or large payloads
• Implement application-level logging to capture requests to the add_tool endpoint
• Deploy endpoint detection and response (EDR) solutions to detect anomalous behavior from the Upsonic service process
• Use network intrusion detection systems (NIDS) to identify known pickle/cloudpickle exploit payload patterns

Monitoring Recommendations

• Enable verbose logging on the Upsonic service to capture all incoming requests
• Set up alerts for any child process creation from the Upsonic service
• Monitor for unexpected network connections from the service account
• Implement file integrity monitoring on directories accessible to the Upsonic service

How to Mitigate CVE-2026-0773

Immediate Actions Required

• Restrict network access to TCP port 7541 using firewall rules to allow only trusted sources
• If possible, disable the add_tool endpoint until a patch is available
• Implement network segmentation to isolate systems running Upsonic
• Run the Upsonic service with minimal privileges using a dedicated service account

Patch Information

Consult the Zero Day Initiative Advisory ZDI-26-042 for the latest patch information and vendor updates. Monitor the Upsonic project for security releases addressing this vulnerability.

Workarounds

• Deploy firewall rules to block external access to TCP port 7541
• Implement a reverse proxy with authentication in front of the Upsonic service
• Consider disabling the affected endpoint if not required for operations
• Use network-level access controls to restrict which hosts can communicate with the service

# Example firewall rule to restrict access to TCP port 7541
# Allow only trusted internal IP range
iptables -A INPUT -p tcp --dport 7541 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 7541 -j DROP