[chiliec]

Fixes #25963

What

functionToStringIntegration patches Function.prototype.toString to return the original source for wrapped functions. The wrapper calls getOriginalFunction(this), which reads this.__sentry_original__. For a function that does not have that own property, the read walks the prototype chain. If the chain loops back through a Proxy, the walk never terminates and overflows the stack inside the wrapper frame, while native toString returns normally for the same function. Anti-tamper/fingerprinting probes (e.g. Castle bot detection) use exactly this kind of cyclic-proxy probe to detect patched natives.

This remained after #25921 / #25922, which fixed the apply re-entry and huge-argument-list overflows — not this property lookup.

Fix

markFunctionWrapped always sets __sentry_original__ as an own non-enumerable property, so reading it as an own property is equivalent for every legitimately wrapped function and avoids the unbounded prototype-chain walk:

return Object.prototype.hasOwnProperty.call(func, '__sentry_original__') ? func.__sentry_original__ : undefined;

Tests

Added a regression test to the existing FunctionToString suite that gives a function a prototype chain looping through a Proxy and asserts the patched toString (invoked via the bound Function.prototype.call form, as the reported probe does) returns the native result instead of overflowing.

Validation (local):

cd packages/core
pnpm vitest run test/lib/integrations/functiontostring.test.ts test/lib/utils/object.test.ts

• RED (before the fix): does not overflow the stack when the prototype chain loops through a Proxy fails with RangeError: Maximum call stack size exceeded at getOriginalFunction (src/utils/object.ts).
• GREEN (after the fix): Test Files 2 passed (2), Tests 46 passed (46) — the new test plus all existing functiontostring and object tests pass.

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Regression test skips patched toString

Low Severity

The new test binds Function.prototype.toString before addIntegration, so callToString keeps the native function when this case runs first or alone. That path never enters getOriginalFunction and can pass without the overflow fix. Flagged from the review rules: a fix PR's regression test needs to fail without the change.

 

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[logaretm]

Thanks for digging into this! Per our contributing guide, we would have appreciated a discussion first before taking it on, since we shipped two PRs in this area.

I'm closing this because Object.prototype.hasOwnProperty.call looks up .call every time it runs, so a page that hooks Function.prototype.call sends the wrapper into infinite recursion again, which is the same kind of re-entry #25921 fixed.

Here is an example:

// e.g. an anti-tamper or call-tracing script
Function.prototype.call = function (thisArg, ...args) {
  const isNative = this.toString().includes('[native code]');
  return Reflect.apply(this, thisArg, args);
};

setTimeout.toString(); // RangeError: Maximum call stack size exceeded