Axios has a NO_PROXY Hostname Normalization Bypass that Leads to SSRF

Description

Axios does not correctly handle hostname normalization when checking `NO_PROXY` rules. Requests to loopback addresses like `localhost.` (with a trailing dot) or `[::1]` (IPv6 literal) skip `NO_PROXY` matching and go through the configured proxy. This goes against what developers expect and lets attackers force requests through a proxy, even if `NO_PROXY` is set up to protect loopback or internal services. According to [RFC 1034 §3.1](https://datatracker.ietf.org/doc/html/rfc1034#section-3.1) and [RFC 3986 §3.2.2](https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.2), a hostname can have a trailing dot to show it is a fully qualified domain name (FQDN). At the DNS level, `localhost.` is the same as `localhost`. However, Axios does a literal string comparison instead of normalizing hostnames before checking `NO_PROXY`. This causes requests like `http://localhost.:8080/` and `http://[::1]:8080/` to be incorrectly proxied. This issue leads to the possibility of proxy bypass and SSRF vulnerabilities allowing attackers to reach sensitive loopback or internal services despite the configured protections. --- **PoC** ```js import http from "http"; import axios from "axios"; const proxyPort = 5300; http.createServer((req, res) => { console.log("[PROXY] Got:", req.method, req.url, "Host:", req.headers.host); res.writeHead(200, { "Content-Type": "text/plain" }); res.end("proxied"); }).listen(proxyPort, () => console.log("Proxy", proxyPort)); process.env.HTTP_PROXY = `http://127.0.0.1:${proxyPort}`; process.env.NO_PROXY = "localhost,127.0.0.1,::1"; async function test(url) { try { await axios.get(url, { timeout: 2000 }); } catch {} } setTimeout(async () => { console.log("\n[*] Testing http://localhost.:8080/"); await test("http://localhost.:8080/"); // goes through proxy console.log("\n[*] Testing http://[::1]:8080/"); await test("http://[::1]:8080/"); // goes through proxy }, 500); ``` **Expected:** Requests bypass the proxy (direct to loopback). **Actual:** Proxy logs requests for `localhost.` and `[::1]`. --- **Impact** * Applications that rely on `NO_PROXY=localhost,127.0.0.1,::1` for protecting loopback/internal access are vulnerable. * Attackers controlling request URLs can: * Force Axios to send local traffic through an attacker-controlled proxy. * Bypass SSRF mitigations relying on NO\_PROXY rules. * Potentially exfiltrate sensitive responses from internal services via the proxy. --- **Affected Versions** * Confirmed on Axios **1.12.2** (latest at time of testing). * affects all versions that rely on Axios’ current `NO_PROXY` evaluation. --- **Remediation** Axios should normalize hostnames before evaluating `NO_PROXY`, including: * Strip trailing dots from hostnames (per RFC 3986). * Normalize IPv6 literals by removing brackets for matching.

Affected Packages (2)