Axios: Null Byte Injection via Reverse-Encoding in AxiosURLSearchParams

CISA KEV lists vulnerabilities known to be exploited. FIRST EPSS estimates the likelihood of exploitation in the next 30 days. An unavailable score does not mean zero risk.

No exploitation or prediction data is available here for these CVEs. This does not mean they are safe or unlikely to be exploited.

Axios: Null Byte Injection via Reverse-Encoding in AxiosURLSearchParams

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. Primary impact is limited because the standard axios request flow is not affected. This vulnerability is fixed in 1.15.1 and 0.31.1.

Source published

24 Apr 2026 17:40:31 UTC

Source modified

27 Apr 2026 13:48:06 UTC

Saved here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

Source: cve-program · CVE Program Terms of Use. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, the encode() function in lib/helpers/AxiosURLSearchParams.js contains a character mapping (charMap) at line 21 that reverses the safe percent-enco…

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. Primary impact is limited because the standard axios request flow is not affected. This vulnerability is fixed in 1.15.1 and 0.31.1.

Source published

Apr 24, 2026, 5:40:31 PM (timezone not stated)

Source modified

Apr 27, 2026, 1:48:06 PM (timezone not stated)

Saved here

15 Nov 2026 13:03:15 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

Source: euvd · ENISA EUVD · source terms and attribution apply. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

EUVD CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, the encode() function in lib/helpers/AxiosURLSearchParams.js contains a character mapping (charMap) at line 21 that reverses the safe percent-enco…

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. Primary impact is limited because the standard axios request flow is not affected. This vulnerability is fixed in 1.15.1 and 0.31.1.

Source published

24 Apr 2026 18:16:30 UTC

Source modified

11 Nov 2026 23:10:00 UTC

Saved here

12 Nov 2026 16:19:25 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

Source: nvd · NIST NVD usage terms; underlying source attribution retained. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 3.1 · [email protected] · Secondary · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

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](https://cwe.mitre.org/data/definitions/626.html) - [CWE-116: Improper Encoding or Escaping of Output](https://cwe.mitre.org/data/definitions/116.html) - [OWASP: Embedding Null Code](https://owasp.org/www-community/attacks/Embedding_Null_Code) - [Axios GitHub Repository](https://github.com/axios/axios) ## 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 |

Source published

5 May 2026 00:18:03 UTC

Source modified

10 Sep 2026 03:50:48 UTC

Saved here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Publisher rating · LOW