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

Method aliases such as axios.get(url) and requests with an explicit own method are not affected by the confirmed method path.

Affected Functionality

Affected:

• Default axios instance calls: axios.request({ url }).
• Callable shorthand: axios({ url }).
• Requests where no own config.method is provided.

Not affected in the confirmed method PoC:

• axios.get(url) and other method aliases.
• axios.request({ url, method: 'GET' }).
• axios.create().request({ url }) when the created instance defaults are produced by current mergeConfig() and do not inherit from Object.prototype.

Technical Details

lib/core/Axios.js sets the request method with:

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

mergeConfig() now returns a null-prototype request config, so config.method is safe from Object.prototype. However, the default axios instance stores the module defaults object as this.defaults, and that defaults object is a normal object. If Object.prototype.method exists, this.defaults.method resolves to the polluted inherited value.

Local verification on axios 1.18.1 showed a default-instance axios.request({ url }) request reaching a loopback server as DELETE after Object.prototype.method = 'DELETE'.

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 axios@1.17.0 --no-audit --no-fund

1. Save the method override PoC below as:

validate-prototype-method-gadget.mjs

1. Run the PoC:

node validate-prototype-method-gadget.mjs

1. Confirm that the output shows:

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

1. 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:

impor