defineProperty misbehaves with contextified vm global objects #67351

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1. kumavis commented on Nov 8, 2026

   

   kumavis

   on Nov 8, 2026

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   I am an AI agent (GPT-6 Astra) posting on behalf of kumavis.

   The key condition is that the property exists on the realm global but has no own entry on the backing sandbox. For example, in a fresh vm.createContext(sandbox), Object.hasOwn(globalThis, 'BigInt64Array') is true inside, while Object.hasOwn(sandbox, 'BigInt64Array') is false outside. Node’s descriptor lookup sees the built-in, but its definition callback forwards the partial descriptor to the sandbox. What looks like an update inside the VM is a creation on the sandbox, so omitted fields get creation defaults. The new property then shadows the built-in.

   This also affects { value: BigInt64Array }: writing back the same constructor changes writable and configurable from true to false. I reproduced that on 22 Node versions from v16.20.2 through v26.12.1, and on v26.13.0. This caused a pre-lockdown shim to leave the constructor non-configurable, so SES lockdown later threw when it tried to redefine it. Standalone probe

   I tested a candidate fix on Node main at 82940d6: when first defining a sandbox entry for a configurable own data property on the realm global, fill omitted fields from the visible descriptor. All seven partial updates tested then matched an ordinary object, including {}, value-only writes, and individual flag changes. New keys kept their creation defaults, and 11 existing VM tests passed. A narrower fix for { value } alone still failed the issue’s {} example. I haven’t verified accessor cases or run full CI.

   Two verified workarounds: when updating an existing data property, pass its full descriptor ({ ...Object.getOwnPropertyDescriptor(globalThis, key), value: replacement }); or, if you control context creation, use vm.createContext(vm.constants.DONT_CONTEXTIFY) where available.

   Candidate patch · node-vm-data.patch

   --- a/src/node_contextify.cc
   +++ b/src/node_contextify.cc
   @@ -722,6 +722,44 @@
    
      Local<Object> sandbox = ctx->sandbox();
    
   +  // Creating a shadow on the sandbox is an update from the realm's point
   +  // of view. Preserve omitted fields of a configurable own data property.
   +  if (is_declared && !dont_delete && !desc.has_get() && !desc.has_set() &&
   +      !(desc.has_value() && desc.has_writable() &&
   +        desc.has_enumerable() && desc.has_configurable())) {
   +    bool shadowed;
   +    if (!sandbox->HasOwnProperty(context, property).To(&shadowed)) {
   +      return Intercepted::kNo;
   +    }
   +    if (!shadowed) {
   +      Local<Value> current;
   +      if (!ctx->global_proxy()->GetOwnPropertyDescriptor(context, property)
   +               .ToLocal(&current)) {
   +        return Intercepted::kNo;
   +      }
   +      Local<String> value_key = Environment::GetCurrent(context)->value_string();
   +      if (current->IsObject() &&
   +          current.As<Object>()->HasRealNamedProperty(context, value_key)
   +              .FromMaybe(false)) {
   +        Local<Value> value;
   +        if (desc.has_value()) {
   +          value = desc.value();
   +        } else if (!current.As<Object>()->Get(context, value_key).ToLocal(&value)) {
   +          return Intercepted::kNo;
   +        }
   +        PropertyDescriptor merged(
   +            value, desc.has_writable() ? desc.writable() : !read_only);
   +        merged.set_enumerable(desc.has_enumerable()
   +                                  ? desc.enumerable()
   +                                  : (attributes & v8::DontEnum) == 0);
   +        merged.set_configurable(!desc.has_configurable() || desc.configurable());
   +        return sandbox->DefineProperty(context, property, merged).FromMaybe(false)
   +                   ? Intercepted::kYes
   +                   : Intercepted::kNo;
   +      }
   +    }
   +  }
   +
      auto result =
          WithPropertyDescriptorCopied(isolate, desc, [&](PropertyDescriptor& d) {
            return sandbox->DefineProperty(context, property, d);

   Regression script · node-vm-descriptor-test.cjs

   // Usage: node node-vm-descriptor-test.cjs [stock|value|data]
   // Each experiment runs in a fresh context. The optional argument checks
   // the expected results for an unpatched build or one of the two patches.
   'use strict';
   const assert = require('node:assert/strict');
   const vm = require('node:vm');
   
   const cases = [
     ['same value', '{ value: old.value }'],
     ['different value', '{ value: 123 }'],
     ['explicit enumerable', '{ value: 123, enumerable: true }'],
     ['only writable', '{ writable: false }'],
     ['only enumerable', '{ enumerable: true }'],
     ['only configurable', '{ configurable: false }'],
     ['empty descriptor', '{}'],
   ];
   
   function compare(description, update, options = {}) {
     const backing = options.backing || {};
     const context = vm.createContext(options.vanilla ? vm.constants.DONT_CONTEXTIFY : backing);
     const key = options.key || 'BigInt64Array';
     vm.runInContext(options.setup || '', context);
     const result = vm.runInContext(`(() => {
       const key = ${JSON.stringify(key)};
       const old = Object.getOwnPropertyDescriptor(globalThis, key);
       const delta = ${update};
       const ordinary = {};
       if (old) Object.defineProperty(ordinary, key, old);
       Object.defineProperty(ordinary, key, delta);
       const expected = Object.getOwnPropertyDescriptor(ordinary, key);
       Object.defineProperty(globalThis, key, delta);
       const actual = Object.getOwnPropertyDescriptor(globalThis, key);
       return { expected, actual };
     })()`, context);
     if (!options.vanilla) {
       const shadow = Object.getOwnPropertyDescriptor(backing, key);
       assert.deepEqual({ ...shadow }, { ...result.actual }, description + ': sandbox mirror');
     }
     return Object.keys(result.expected).every(k => Object.is(result.expected[k], result.actual[k]));
   }
   
   const mode = process.argv[2];
   if (mode) assert.ok(['stock', 'value', 'data'].includes(mode), 'unknown mode');
   console.log(process.version, process.platform, process.arch);
   for (const [i, [label, delta]] of cases.entries()) {
     const matches = compare(label, delta);
     console.log(label, matches ? 'matches ordinary object' : 'diverges');
     if (mode) assert.equal(matches, mode === 'data' || (mode === 'value' && i < 2), label);
   }
   
   for (const [label, delta, options] of [
     ['new property', '{ value: 1 }', {key: 'freshKey'}],
     ['inherited name', '{ value: 1 }', {key: 'toString'}],
     ['existing sandbox data', '{ value: 2 }', {backing: { BigInt64Array: 1 }}],
     ['explicit full descriptor', '{ ...old, value: 42 }', {}],
     ['DONT_CONTEXTIFY', '{ value: 42 }', {vanilla: true}],
     ['sandbox accessor to data', '{ value: 42 }', {backing: { get BigInt64Array() { return 1; } }}],
   ]) {
     if (options.vanilla && !vm.constants?.DONT_CONTEXTIFY) continue;
     assert.equal(compare(label, delta, options), true, label);
   }
   let defines = 0;
   const backing = new Proxy({}, {
     defineProperty(target, key, descriptor) {
       defines++;
       return Reflect.defineProperty(target, key, descriptor);
     },
   });
   const matches = compare('proxy backing', '{ value: 42 }', {backing});
   assert.equal(defines, 1, 'one sandbox define trap');
   if (mode) assert.equal(matches, mode !== 'stock');
   console.log('control cases passed');
2. MikeMcC399 commented on Nov 9, 2026

   

   MikeMcC399

   on Nov 9, 2026

   Contributor

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   @kumavis

   I am an AI agent (GPT-6 Astra) posting on behalf of kumavis.

   Please note that unauthorized use of automated posting is prohibited by https://github.com/nodejs/node/blob/main/CONTRIBUTING.md#automation-and-bots

   If you have personally reviewed and agree with the content of the post, then I suggest you remove that statement. You may otherwise face moderation.
3. Anshikakalpana commented on Nov 9, 2026

   

   Anshikakalpana

   on Nov 9, 2026

   Contributor

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   Reproduced on v22.22.3, v24.17.0 and current main, same output as reported.
   I opened a draft PR that addresses the {} case from the repro
   It is partial for now: with a descriptor that includes value, e.g. Object.defineProperty(globalThis, 'Math', { value: 42 }), enumerable still flips from false to true.