CVE-2026-42040: Axios: Null Byte Injection via Reverse-Encoding in AxiosURLSearchParams

Vulnerability Disclosure: Null Byte Injection via Reverse-Encoding in AxiosURLSearchParams

Summary

The encode() function in lib/helpers/AxiosURLSearchParams.js contains a character mapping (charMap) at line 21 that reverses the safe percent-encoding of null bytes. After encodeURIComponent('\x00') correctly produces the safe sequence %00, the charMap entry '%00': '\x00' converts it back to a raw null byte.

This is a clear encoding defect: every other charMap entry encodes in the safe direction (literal → percent-encoded), while this single entry decodes in the opposite (dangerous) direction.

Severity: Low (CVSS 3.7) Affected Versions: All versions containing this charMap entry Vulnerable Component: lib/helpers/AxiosURLSearchParams.js:21

CWE

- CWE-626: Null Byte Interaction Error (Poison Null Byte) - CWE-116: Improper Encoding or Escaping of Output

CVSS 3.1

Score: 3.7 (Low)

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | Attacker controls input parameters remotely | | Attack Complexity | High | Standard axios request flow (buildURL) uses its own encode function which does NOT have this bug. Only triggered via direct AxiosURLSearchParams.toString() without an encoder, or via custom paramsSerializer delegation | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | Impact limited to HTTP request URL | | Confidentiality | None | No confidentiality impact | | Integrity | Low | Null byte in URL can cause truncation in C-based backends, but requires a vulnerable downstream parser | | Availability | None | No availability impact |

Vulnerable Code

File: lib/helpers/AxiosURLSearchParams.js, lines 13-26

javascript function encode(str) { const charMap = { '!': '%21', // literal → encoded (SAFE direction) "'": '%27', // literal → encoded (SAFE direction) '(': '%28', // literal → encoded (SAFE direction) ')': '%29', // literal → encoded (SAFE direction) '~': '%7E', // literal → encoded (SAFE direction) '%20': '+', // standard transformation (SAFE) '%00': '\x00', // LINE 21: encoded → raw null byte (UNSAFE direction!) }; return encodeURIComponent(str).replace(/[!'()~]|%20|%00/g, function replacer(match) { return charMap[match]; }); }

Why the Standard Flow Is NOT Affected

javascript // buildURL.js:36 — uses its OWN encode function (lines 14-20), not AxiosURLSearchParams's const encode = (options && options.encode) || encode; // buildURL's encode

// buildURL.js:53 — passes buildURL's encode to AxiosURLSearchParams new AxiosURLSearchParams(params, options).toString(encode); // external encoder used

// AxiosURLSearchParams.js:48 — when encoder is provided, internal encode is NOT used const encode = encoder ? function(value) { return encoder.call(this, value, encode); } : encode; // ^^^^^^ // internal encode passed as 2nd arg but only used if // the external encoder explicitly delegates to it

Proof of Concept

javascript import AxiosURLSearchParams from './lib/helpers/AxiosURLSearchParams.js'; import buildURL from './lib/helpers/buildURL.js';

// Test 1: Direct AxiosURLSearchParams (VULNERABLE path) const params = new AxiosURLSearchParams({ file: 'test\x00.txt' }); const result = params.toString(); // NO encoder → uses internal encode with charMap console.log('Direct toString():', JSON.stringify(result)); // Output: "file=test\u0000.txt" (contains raw null byte) console.log('Hex:', Buffer.from(result).toString('hex')); // Output: 66696c653d74657374002e747874 (00 = null byte)

// Test 2: Via buildURL (NOT vulnerable — standard axios flow) const url = buildURL('http://example.com/api', { file: 'test\x00.txt' }); console.log('Via buildURL:', url); // Output: http://example.com/api?file=test%00.txt (%00 preserved safely)

Verified PoC Output

Direct toString(): "file=test\u0000.txt" Contains raw null byte: true Hex: 66696c653d74657374002e747874

Via buildURL: http://example.com/api?file=test%00.txt Contains raw null byte: false Contains safe %00: true

Impact Analysis

Primary impact is limited because the standard axios request flow is not affected. However:

- Direct API users: Applications using AxiosURLSearchParams directly for custom serialization are affected - Custom paramsSerializer: A paramsSerializer.encode that delegates to the internal encoder triggers the bug - Code defect signal: The directional inconsistency in charMap is a clear coding error with no legitimate use case

If null bytes reach a downstream C-based parser, impacts include URL truncation, WAF bypass, and log injection.

Recommended Fix

Remove the %00 entry from charMap and update the regex:

javascript function encode(str) { const charMap = { '!': '%21', "'": '%27', '(': '%28', ')': '%29', '~': '%7E', '%20': '+', // REMOVED: '%00': '\x00' }; return encodeURIComponent(str).replace(/[!'()~]|%20/g, function replacer(match) { // ^^^^ removed |%00 return charMap[match]; }); }

Resources

- CWE-626: Null Byte Interaction Error - CWE-116: Improper Encoding or Escaping of Output - OWASP: Embedding Null Code - Axios GitHub Repository

Timeline

| Date | Event | |---|---| | 2026-04-15 | Vulnerability discovered during source code audit | | 2026-04-16 | Report revised: documented standard-flow limitation, corrected CVSS | | TBD | Report submitted to vendor via GitHub Security Advisory |