Axios: Header Injection via Prototype Pollution

Summary

A prototype pollution gadget exists in the Axios HTTP adapter (lib/adapters/http.js) that allows an attacker to inject arbitrary HTTP headers into outgoing requests. The vulnerability exploits duck-type checking of the data payload, where if Object.prototype is polluted with getHeaders, append, pipe, on, once, and Symbol.toStringTag, Axios misidentifies any plain object payload as a FormData instance and calls the attacker-controlled getHeaders() function, merging the returned headers into the outgoing request.

The vulnerable code resides exclusively in lib/adapters/http.js. The prototype pollution source does not need to originate from Axios itself — any prototype pollution primitive in any dependency in the application's dependency tree is sufficient to trigger this gadget.

Prerequisites:

A prototype pollution primitive must exist somewhere in the application's dependency chain (e.g., via lodash.merge, qs, JSON5, or any deep-merge utility processing attacker-controlled input). The pollution source is not required to be in Axios. The application must use Axios to make HTTP requests with a data payload (POST, PUT, PATCH).

Details

The vulnerability is in lib/adapters/http.js, in the data serialization pipeline:

// lib/adapters/http.js 
} else if (utils.isFormData(data) && utils.isFunction(data.getHeaders)) {
    headers.set(data.getHeaders());
    // ...
}

Axios uses two sequential duck-type checks, both of which can be satisfied via prototype pollution:

1. utils.isFormData(data) — lib/utils.js

const isFormData = (thing) => {
  let kind;
  return thing && (
    (typeof FormData === 'function' && thing instanceof FormData) || (
      isFunction(thing.append) && ( 
        (kind = kindOf(thing)) === 'formdata' ||  
        (kind === 'object' && isFunction(thing.toString) && thing.toString() === '[object FormData]')
      )
    )
  )
}

2. utils.isFunction(data.getHeaders) — Duck-type for form-data npm package

// Returns true if Object.prototype.getHeaders is a function
utils.isFunction(data.getHeaders)

PoC

// Simulate Prototype Pollution
Object.prototype[Symbol.toStringTag] = 'FormData';
Object.prototype.append = () => {};
Object.prototype.getHeaders = () => {
    const headers = Object.create(null);
    (.... Introduce here all the headers you want ....)
    return headers;
};
Object.prototype.pipe = function(d) { if(d&&d.end)d.end(); return d; };
Object.prototype.on = function() { return this; };
Object.prototype.once = function() { return this; };

// Legitimate application code
const response = await axios.post('https://internal-api.company.com/admin/delete', 
    { userId: 42 },
    { headers: { 'Authorization': 'Bearer VALID_USER_TOKEN' } }
);

Impact

• Authentication Bypass (CVSS: C:H)
• Session Fixation (CVSS: I:H)
• Privilege Escalation (CVSS: C:H, I:H)
• IP Spoofing / WAF Bypass (CVSS: I:H)

Note on Scope: There is an argument to promote this from S:U to S:C (Scope: Changed), which would raise the score to 10.0. In some architectures, Axios is commonly used for service to service communication where downstream services trust identity headers (Authorization, X-Role, X-User-ID, X-Tenant-ID) forwarded from upstream API gateways. In this scenario, the vulnerable component (Axios in Service A) and the impacted component (Service B, which acts on the injected identity) are under different security authorities. The injected headers cross a trust boundary, meaning the impact extends beyond the security scope of the vulnerable component, the CVSS v3.1 definition of a Scope Change. We conservatively score S:U here, but maintainers should evaluate which one applies better here.

Recommended Fix

Add an explicit own-property check in lib/adapters/http.js:

- } else if (utils.isFormData(data) && utils.isFunction(data.getHeaders)) {
-     headers.set(data.getHeaders());
+ } else if (utils.isFormData(data) && utils.isFunction(data.getHeaders) &&
+            Object.prototype.hasOwnProperty.call(data, 'getHeaders')) {
+     headers.set(data.getHeaders());