iframe-proxy

bhumin18 · 2026-08-31T07:18:37Z

Summary

When Object.prototype[Symbol.iterator] is polluted, Node.js/undici's internal header dispatch creates a plain {} for merged request headers and iterates it with for...of. This causes the polluted iterator to inject or overwrite headers (such as Authorization).

Fix

In the fetch adapter, check for an own Symbol.iterator property on Object.prototype right before creating the Request/calling fetch(), temporarily remove it, and restore its exact property descriptor in a finally block.

Verification

Fixes the failing test: should not use inherited Symbol.iterator for request headers
npm run test:vitest:unit -- tests/unit/adapters/fetch.test.js: 69/69 passed
npm run lint: Clean (0 errors)
PRE_RELEASE_CHANGELOG.md updated as per guidelines

Summary by cubic

Description

Fixes a prototype pollution vulnerability in the fetch adapter where a polluted Object.prototype[Symbol.iterator] could inject or override request headers such as Authorization during Node.js/undici's header dispatch.

Detects an own Symbol.iterator on Object.prototype and temporarily removes it during synchronous Request construction and fetch dispatch, skipping the guard when the descriptor is non-configurable.
Builds headers as a Headers instance or entries array so the Request constructor never reads Object.prototype[Symbol.iterator] even when the polluted descriptor cannot be removed.
Restores the exact property descriptor in a finally block, so in-flight, never-settling, or rejected requests never observe an altered prototype.
Restores the descriptor before invoking custom env.fetch or Request implementations; the native fetch is captured once at module load and matched by identity, so a replaced globalThis.fetch is treated as custom while an explicit native globalThis.fetch is guarded.

Docs

No documentation update needed; this is an internal security hardening fix.

Testing

Adds regression tests covering concurrent requests, never-settling fetches, rejections, custom env.fetch and Request, wrapped and replaced globalThis.fetch, non-configurable descriptors, and explicit globalThis.fetch. All unit tests pass with lint clean.

Semantic version impact

Patch version change.

^{Written for commit 2885b48. Summary will update on new commits.}

… in header dispatch

greptile-apps · 2026-08-31T07:25:04Z

Greptile Summary

The fetch adapter now uses iterator-safe header construction and restores the inherited iterator immediately after synchronous dispatch. One compatibility failure remains: a custom fetch wrapper installed before Axios import can be mistaken for native based on forgeable source text, causing its synchronous code to run without the inherited iterator.

The earlier immutable-iterator header-injection report was disproved with a local HTTP server. Native fetch accepted the injected control header, while Axios requests made before and after the same immutable prototype pollution reached the server without an Authorization header.

Confidence Score: 4/5

The change is not ready to merge because a configured fetch wrapper can still have its synchronous execution environment changed unexpectedly.

A custom wrapper whose source includes a native-function marker is treated as native, so Axios removes the inherited iterator before invoking application code and can cause otherwise valid requests to fail.

Files Needing Attention: lib/adapters/fetch.js

T-Rex Logs

What T-Rex did

T-Rex produced a proof for a posted P1 finding and linked it to the review comment.
T-Rex ran the requested verification for the P1 finding, but local artifact references were not uploaded.
T-Rex produced a second proof for a posted P1 finding and linked it to the review comment.
T-Rex ran the requested verification again, but local artifact references were not uploaded.
T-Rex ran the requested verification once more, and local artifact references were not uploaded.

_{Ran code and verified through T-Rex}

Comments Outside Diff (1)

General comment

Fetch wrapper source-marker spoofing causes synchronous iterator removal
- Bug
  - A user-installed fetch wrapper whose ordinary source text contains [native code] is accepted by isNativeFunction and stored as nativeFetch during Axios module import. When Axios later dispatches through that wrapper, it temporarily deletes Object.prototype[Symbol.iterator]. Wrapper code executed synchronously in that interval observes no inherited iterator, so [...{}] throws TypeError: {} is not iterable.
- Cause
  - isNativeFunction at lib/adapters/fetch.js:34-44 treats arbitrary source-text substrings ([native code] or internal/deps/undici/undici) as proof that a function is native. Identity comparison at lib/adapters/fetch.js:506-508 then compares the wrapper to the already misclassified module-load value, allowing the suppression at lines 551-560 to run for custom code.
- Fix
  - Do not infer native-fetch trust from forgeable function source substrings. Restrict iterator suppression to a trusted intrinsic captured independently of user-replaceable globalThis.fetch, or remove the global prototype mutation and use a dispatch approach that does not alter Object.prototype during calls into JavaScript wrappers.
_{Ran code and verified through T-Rex}

Prompt To Fix All With AI

### Issue 1
lib/adapters/fetch.js:40
**Native detection trusts source text**

When an application installs a custom fetch wrapper before Axios is imported and its source contains `[native code]` or `internal/deps/undici/undici`, `isNativeFunction` classifies it as native. Axios then suppresses `Object.prototype[Symbol.iterator]` while invoking that wrapper synchronously. Wrapper code such as `[...{}]` consequently throws `TypeError: {} is not iterable`, rejecting an otherwise valid request. Native-function detection must not trust forgeable source-text markers.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (13): Last reviewed commit: "fix(fetch): use captured identity for na..." | Re-trigger Greptile}

cubic-dev-ai

All reported issues were addressed across 2 files

_{Reply with feedback, questions, or to request a fix.

Re-trigger cubic}

…dd regression tests

cubic-dev-ai

All reported issues were addressed across 2 files (changes from recent commits).

_{Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic}

…le explicit globalThis.fetch

…red env.fetch as custom

cubic-dev-ai

All reported issues were addressed across 2 files (changes from recent commits).

_{Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic}

… to test subprocess

…fe headers representation

cubic-dev-ai

All reported issues were addressed across 2 files (changes from recent commits).

_{Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic}

…apper tests

cubic-dev-ai

All reported issues were addressed across 2 files (changes from recent commits).

_{Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic}

…globalThis.fetch

greptile-apps · 2026-09-04T12:41:16Z

Want your agent to iterate on Greptile's feedback? Try greploops.

…ator for Request

cubic-dev-ai

All reported issues were addressed across 1 file (changes from recent commits).

_{Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic}

… env.fetch

greptile-apps · 2026-09-05T04:33:40Z

+  }
+  try {
+    const str = Function.prototype.toString.call(fn);
+    return str.includes('[native code]') || str.includes('internal/deps/undici/undici');


Native detection trusts source text

When an application installs a custom fetch wrapper before Axios is imported and its source contains [native code] or internal/deps/undici/undici, isNativeFunction classifies it as native. Axios then suppresses Object.prototype[Symbol.iterator] while invoking that wrapper synchronously. Wrapper code such as [...{}] consequently throws TypeError: {} is not iterable, rejecting an otherwise valid request. Native-function detection must not trust forgeable source-text markers.

_{Ran code and verified through T-Rex}

Prompt To Fix With AI

This is a comment left during a code review. Path: lib/adapters/fetch.js Line: 40 Comment: **Native detection trusts source text** When an application installs a custom fetch wrapper before Axios is imported and its source contains `[native code]` or `internal/deps/undici/undici`, `isNativeFunction` classifies it as native. Axios then suppresses `Object.prototype[Symbol.iterator]` while invoking that wrapper synchronously. Wrapper code such as `[...{}]` consequently throws `TypeError: {} is not iterable`, rejecting an otherwise valid request. Native-function detection must not trust forgeable source-text markers. --- For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

fix(fetch): guard against Object.prototype[Symbol.iterator] pollution…

91a8590

… in header dispatch

bhumin18 requested a review from jasonsaayman as a code owner August 31, 2026 07:18

greptile-apps Bot reviewed Aug 31, 2026

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fix(fetch): scope Symbol.iterator guard to synchronous dispatch and a…

ce0868c

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greptile-apps Bot reviewed Sep 1, 2026

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Comment thread lib/adapters/fetch.js

fix(fetch): restore Symbol.iterator before invoking configured env.fetch

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fix(fetch): guarantee Symbol.iterator restoration in finally and hand…

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…le explicit globalThis.fetch

jasonsaayman assigned bhumin18 Sep 1, 2026

jasonsaayman added commit::fix The PR is related to a bugfix v1x Version 1 labels Sep 1, 2026

Merge branch 'v1.x' into fix/fetch-proto-iterator-pollution

2124d69

greptile-apps Bot reviewed Sep 2, 2026

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fix(fetch): handle non-configurable Symbol.iterator and treat configu…

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cubic-dev-ai Bot reviewed Sep 3, 2026

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fix(fetch): treat explicit globalThis.fetch as native and add timeout…

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… to test subprocess

greptile-apps Bot reviewed Sep 3, 2026

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Comment thread lib/adapters/fetch.js

greptile-apps Bot reviewed Sep 3, 2026

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Merge branch 'v1.x' into fix/fetch-proto-iterator-pollution

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greptile-apps Bot reviewed Sep 4, 2026

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fix(fetch): compare against captured native fetch and use iterator-sa…

b347921

…fe headers representation

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fix(fetch): treat explicit env.fetch as custom and pass Request in wr…

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fix(fetch): preserve iterator for custom Request and detect replaced …

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fix(fetch): use native-function detection for fetch and preserve iter…

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fix(fetch): use captured identity for native fetch and guard explicit…

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bhumin18 commented Aug 31, 2026 • edited by cubic-dev-ai Bot Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Fix

Verification

Summary by cubic

Description

Docs

Testing

Semantic version impact

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greptile-apps Bot commented Aug 31, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Greptile Summary

Confidence Score: 4/5

T-Rex Logs

Comments Outside Diff (1)

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