CVE-2026-39890: PraisonAI Affected by Remote Code Execution via YAML Deserialization in Agent Definition Loading

Summary The AgentService.loadAgentFromFile method uses the js-yaml library to parse YAML files without disabling dangerous tags (such as !!js/function and !!js/undefined). This allows an attacker to craft a malicious YAML file that, when parsed, executes arbitrary JavaScript code. An attacker can exploit this vulnerability by uploading a malicious agent definition file via the API endpoint, leading to remote code execution (RCE) on the server.

Details The vulnerability exists in the YAML deserialization process. The js-yaml library's load function is used without specifying a safe schema (e.g., JSONSCHEMA or DEFAULTSAFESCHEMA). This enables the parsing of JavaScript functions and other dangerous types. When a malicious YAML file containing a !!js/function tag is parsed, the function is evaluated, leading to arbitrary code execution.

The vulnerable code is located in src/agents/agent.service.ts at line 55.

PoC An attacker can create a malicious agent YAML file with the following content: yaml !!js/function > function(){ require('childprocess').execSync('touch /tmp/pwned') } Then, upload this file as an agent definition via the API endpoint that uses AgentService.loadAgentFromFile. When the agent is loaded (either during startup or via an API call that triggers loading), the payload will execute the command touch /tmp/pwned, demonstrating arbitrary code execution.

Impact This vulnerability allows an unauthenticated attacker (if the API endpoint is unprotected) or an authenticated attacker with the ability to upload agent definitions to execute arbitrary code on the server. This can lead to complete compromise of the server, data theft, or further network penetration.

Recommended Fix Replace the unsafe load method with a safe alternative. Specifically, use the load method with a safe schema, such as JSONSCHEMA or DEFAULTSAFESCHEMA. For example:

typescript import yaml from 'js-yaml'; import { JSONSCHEMA } from 'js-yaml';

// In the loadAgentFromFile method const agent = yaml.load(fileContent, { schema: JSONSCHEMA });

Alternatively, if the application requires only a subset of YAML features, consider using the safeLoad method from an older version (though note it was deprecated). The key is to avoid loading tags that can execute code.

Additionally, validate and sanitize all user input, especially file uploads. Ensure that agent definition files are only uploaded by trusted users and consider storing them in a secure location with proper access controls.