[TS codegen] Escape JSDoc terminator in doc comments to prevent comment closure and import‑time code execution

[Ky0toFu]

Summary

The TypeScript generator in flatc writes schema documentation comments directly into a JSDoc block without escaping the terminator. A crafted line like */…/* in a .fbs doc comment prematurely closes the block and injects top‑level JavaScript into the generated module, which then executes on import/bundling.

This is a first‑party codegen bug in this repository (not a third‑party dependency).

Affected code

• File: src/idl_gen_ts.cpp
• Function: GenDocComment
• Behavior: concatenates " *" + line + "\n" into a /** … */ JSDoc block without escaping */.

// Generate a documentation comment, if available.
static void GenDocComment(const std::vector<std::string>& dc,
                          std::string* code_ptr,
                          const char* indent = nullptr) {
  if (dc.empty()) { return; }
  std::string& code = *code_ptr;
  if (indent) code += indent;
  code += "/**\n";
  for (auto it = dc.begin(); it != dc.end(); ++it) {
    if (indent) code += indent;
    code += " *" + *it + "\n";  // no escaping of "*/"
  }
  if (indent) code += indent;
  code += " */\n";
}

Minimal reproduction (safe payload)

1. malicious.fbs

namespace Poc;

/// */console.log('INJECTED_FROM_DOC_COMMENT')/*
table A { x:int; }

root_type A;

2. Generate TypeScript

flatc --ts -o out malicious.fbs

3. Observe injection in generated module

/**
 * */console.log('INJECTED_FROM_DOC_COMMENT')/*
 */
export class A {

4. Any TS→JS path will execute the payload at import‑time (example)

npx --yes esbuild out/poc/A.ts --format=cjs --bundle --outfile=out/bundle.cjs --external:flatbuffers
node out/bundle.cjs  # prints: INJECTED_FROM_DOC_COMMENT

Expected vs actual

• Expected: doc comments should never be able to close comments and introduce executable code into generated sources.
• Actual: unescaped */ terminates JSDoc and injects top‑level JS that runs on import/bundling.

Proposed fix (minimal and safe)

Escape */ in each doc comment line before emission to the JSDoc block.

// inside the for-loop over doc lines
auto safe = *it;
for (size_t pos = 0; (pos = safe.find("*/", pos)) != std::string::npos; ) {
  safe.replace(pos, 2, "*\\/");
  pos += 3; // advance past the replacement
}
code += " *" + safe + "\n";

Alternative (even safer)

Render documentation as line comments in TypeScript (// …) so early termination by input is impossible by construction.

Notes

• This issue is opened to coordinate a code‑hardening fix in the TS generator.
• A full end‑to‑end PoC (including generated output and import‑time execution) is available upon request and trivial to reproduce with the steps above.

Environment (used for reproduction)

• Node.js ≥ 18 (tested on v22.17.1)
• flatc 25.9.23 (Windows release)