CVE-2026-39890

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., JSON_SCHEMA or DEFAULT_SAFE_SCHEMA). 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:

!!js/function >
  function(){ require('child_process').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.

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.

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

CVE-2026-39890 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.5.115); upgrading removes the vulnerable code path.

Kodem intelligence

Replace the unsafe load method with a safe alternative. Specifically, use the load method with a safe schema, such as JSON_SCHEMA or DEFAULT_SAFE_SCHEMA. For example:

import yaml from 'js-yaml';
import { JSON_SCHEMA } from 'js-yaml';

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

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.

1. What is CVE-2026-39890? CVE-2026-39890 is a critical-severity insecure deserialization vulnerability in praisonai (pip), affecting versions <= 4.5.114. It is fixed in 4.5.115. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
2. How severe is CVE-2026-39890? CVE-2026-39890 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of praisonai are affected by CVE-2026-39890? praisonai (pip) versions <= 4.5.114 is affected.
4. Is there a fix for CVE-2026-39890? Yes. CVE-2026-39890 is fixed in 4.5.115. Upgrade to this version or later.
5. Is CVE-2026-39890 exploitable, and should I be worried? Whether CVE-2026-39890 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39890 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39890? Upgrade praisonai to 4.5.115 or later.