Axios: Excessive recursion in formDataToJSON can cause denial of service

## Summary Axios versions `0.28.0` and later contain uncontrolled recursion in `formDataToJSON`, the helper behind the public `axios.formToJSON()` / named `formToJSON` API and the default request transform used when FormData is sent with an `application/json` content type. Applications are affected when they pass attacker-controlled `FormData` field names into this functionality. A field name with thousands of nested bracket segments can exhaust the JavaScript call stack and throw `RangeError: Maximum call stack size exceeded`, causing request failure and, in applications that do not handle the exception or rejected promise, possible process termination. ## Impact The impact is denial of service against applications that process untrusted `FormData` field names through axios' FormData-to-JSON conversion. The vulnerable path is not reached by merely installing axios, by normal multipart `FormData` pass-through, or by ordinary axios requests that do not request JSON serialisation of `FormData`. In the default axios request, the error is produced before network I/O and returned as a rejected Promise. Direct use of `formToJSON()` throws synchronously. Server-side applications are the primary risk when remote users can submit arbitrary form field names, and the application converts those fields with `formToJSON()` or sends them through axios as JSON. ## Affected Functionality Affected APIs and paths: - `axios.formToJSON(formData)` - `import { formToJSON } from "axios"` - `lib/helpers/formDataToJSON.js` - axios default `transformRequest` when `data` is `FormData` and `Content-Type` contains `application/json` Unaffected or lower-risk paths: - Normal multipart `FormData` requests without `JSON Content-Type` - `toFormData()` object-to-FormData serialisation, which already has a `maxDepth` guard - Axios versions before 0.28.0, where this helper and public API were not present ## Technical Details `lib/helpers/formDataToJSON.js` parses a form field name into path segments with `parsePropPath()`. For a key such as `a[x][x][x]`, each bracketed segment becomes another path element. `formDataToJSON()` then calls the nested `buildPath(path, value, target, index)` function. `buildPath()` recursively calls itself once for each path segment and does not enforce a maximum depth: `const result = buildPath(path, value, target[name], index);` A key containing thousands of bracket segments, therefore, creates thousands of recursive calls. At sufficient depth, V8 throws `RangeError: Maximum call stack size exceeded`. Axios already applies a depth guard to the inverse serializer in `lib/helpers/toFormData.js`, where `maxDepth` defaults to 100 and exceeding it throws `AxiosError` with code `ERR_FORM_DATA_DEPTH_EXCEEDED`. `formDataToJSON()` does not currently have equivalent protection. ## Proof of Concept of Attack ```js import { formToJSON } from "axios"; const fd = new FormData(); fd.append("a" + "[x]".repeat(15000), "value"); try { formToJSON(fd); console.log("not vulnerable"); } catch (err) { console.log(`${err.constructor.name}: ${err.message}`); } ``` Expected vulnerable result: RangeError: Maximum call stack size exceeded The axios request transform path can also be reached before network I/O: ```js import axios from "axios"; const fd = new FormData(); fd.append("a" + "[x]".repeat(15000), "value"); await axios .post("http://127.0.0.1:1/", fd, { headers: { "Content-Type": "application/json" } }) .catch((err) => console.log(`${err.constructor.name}: ${err.message}`)); ``` Expected vulnerable result: RangeError: Maximum call stack size exceeded ## Workarounds Applications can avoid the vulnerable path by not converting attacker-controlled `FormData` to JSON with axios. If conversion is required before a fixed axios release is available, validate `FormData` field names before calling `formToJSON()` or before sending `FormData` with `Content-Type: application/json`. Reject keys whose parsed nesting depth exceeds the application's expected schema. For axios requests carrying untrusted `FormData`, avoid setting `Content-Type: application/json`; leaving the data as multipart FormData bypasses `formDataToJSON()`. Catching the resulting error can prevent process termination, but it does not remove the uncontrolled-recursion behaviour and should not be treated as the primary mitigation. <details> <summary>Original Report</summary> # Axios SSRF via Incomplete Loopback Detection ## CWE-918 | CVSS 7.5 (HIGH) | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L --- ## 1. Classification | CWE | CVSS Score | Severity | Type | |-----|-----------|----------|------| | CWE-918 | 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L) | HIGH | Server-Side Request Forgery | ## 2. Description ### Summary The `shouldBypassProxy()` function in Axios fails to recognise `0.0.0.0`, `::`, and `::ffff:0.0.0.0` as loopback addresses. When `NO_PROXY=localhost` is configured, requests to these addresses are incorrectly forwarded through the proxy instead of being sent directly, enabling an SSRF attack against internal services reachable via the proxy's loopback interface. ### Root Cause **File:** `lib/helpers/shouldBypassProxy.js` **`isIPv4Loopback` (lines 3-8):** Only checks for `127.x.x.x` addresses by inspecting `parts[0] !== '127'`. The `0.0.0.0` address has `parts[0] === '0'`, so it falls through as non-loopback, even though on Linux `0.0.0.0` routes to the loopback interface. **`isIPv6Loopback` (lines 10-38):** Only checks `host === '::1'`. The `::` address (unspecified IPv6) also routes to the loopback, but is not recognised. **Attack Flow:** ``` isIPv4Loopback (line 3) — fails for 0.0.0.0 → isLoopback (line 44) — wraps both checks, returns false → shouldBypassProxy (line 127) — PUBLIC API, exported default → lib/adapters/http.js (line 190) — Node.js HTTP adapter ``` ### Attack Vector - **Access Vector:** Network (AV:N) - **Access Complexity:** Low (AC:L) — attacker only needs control of a URL - **Privileges Required:** None (PR:N) - **User Interaction:** None (UI:N) ## 3. Proof of Concept ### Phase 1: Logic Verification ```javascript import shouldBypassProxy from 'axios/lib/helpers/shouldBypassProxy.js'; // Normal loopback — correctly returns true (bypasses proxy) shouldBypassProxy('http://127.0.0.1:9999/'); // → true // Vulnerable — returns false (goes through proxy!) shouldBypassProxy('http://0.0.0.0:9999/'); // → false ← SSRF shouldBypassProxy('http://[::]:9999/'); // → false ← SSRF shouldBypassProxy('http://[::ffff:0.0.0.0]:9999/'); // → false ← SSRF ``` ### Phase 2: Docker E2E Reproduction A full 3-container Docker reproduction was created and tested: - **Proxy container:** Simple HTTP forward proxy on port 8888 - **Internal container:** Internal service on port 9999 (simulates sensitive internal resource) - **Attacker container:** Runs the test script with Axios source mounted **Reproduction steps:** ```bash cd /tmp/deep-e2e docker compose up -d docker compose exec attacker node test-ssrf.js ``` **Results:** - Test 1: `127.0.0.1 + NO_PROXY=localhost` → BYPASS (correct) - Test 2: `0.0.0.0 + NO_PROXY=localhost` → VIA_PROXY (SSRF) - Test 3: `[::] + NO_PROXY=localhost` → VIA_PROXY (SSRF) - Test 4: `[::ffff:0.0.0.0] + NO_PROXY=localhost` → VIA_PROXY (SSRF) ### Phase 3: Actual Axios Client The real Axios HTTP client (v1.16.1, source tree) was tested through proxy configuration: - Axios with `proxy: { host: 'proxy', port: 8888 }` - Setting `NO_PROXY=localhost` and requesting `http://0.0.0.0:9999/` - Result: Axios forwarded the request through the proxy instead of bypassing it ## 4. Impact ### Attack Scenario 1. Attacker has control over a URL that an Axios client will request (direct input, redirect target, open redirect chain) 2. The Axios client is configured with a proxy (e.g., corporate proxy) and `NO_PROXY=localhost` to protect internal services 3. Attacker supplies `http://0.0.0.0:8080/admin` as the target URL 4. Axios sends the request through the proxy 5. The proxy resolves `0.0.0.0` → the proxy's own loopback → reaches the internal admin service on port 8080 ### Potential Consequences - **Information disclosure (C:L):** Internal service responses become accessible - **Integrity impact (I:L):** Attacker can trigger actions on internal services (if proxy supports PUT/POST/DELETE) - **Availability impact (A:L):** Limited — depends on internal service behavior ### Likelihood - **High** — proxy bypass is a common pattern in microservice architectures - **Medium** — requires attacker control of a URL (not always available) ## 5. Remediation ### Code Fix **File:** `lib/helpers/shouldBypassProxy.js` ```javascript function isIPv4Loopback(host) { if (host === '0.0.0.0') return true; // ADD THIS LINE const parts = host.split('.'); if (parts.length !== 4) return false; if (parts[0] !== '127') return false; return parts.every(p => /^\d+$/.test(p) && Number(p) >= 0 && Number(p) <= 255); } function isIPv6Loopback(host) { if (host === '::1' || host === '::') return true; // ADD '::' // ... rest of implementation } ``` ### Workarounds - Add `0.0.0.0` and `::` to the `NO_PROXY` environment variable explicitly - Use `127.0.0.1` instead of `0.0.0.0` in all internal service URLs - Implement URL validation to reject `0.0.0.0` and `::` before passing to Axios

Quoted source text, attributed separately from HOL analysis.