Description

node-re2: String.prototype.replace(re2, template) aborts the Node process (uncatchable ToLocalChecked on empty MaybeLocal) when the result exceeds V8's max string length

Description

WrappedRE2::Replace builds the replacement result and hands it to V8 with .ToLocalChecked() without checking for the empty MaybeLocal that V8 returns when the string/buffer exceeds its maximum length:

lib/replace.cc (v1.24.1):

// L553 — Buffer return path
info.GetReturnValue().Set(Nan::CopyBuffer(result.data(), result.size()).ToLocalChecked());
// L556 — String return path
info.GetReturnValue().Set(Nan::New(result).ToLocalChecked());

When a global replace uses an output-amplifying template — $' (text after the match) or $` (text before the match) — the result grows to O(input²). For an input of ~40,000+ identical single-char matches the result exceeds V8's String::kMaxLength (~536,870,888 chars on 64-bit). Nan::New(result) then returns an empty MaybeLocal, and the unchecked .ToLocalChecked() calls v8::Utils::ReportApiFailure → FATAL ERROR: v8::ToLocalChecked Empty MaybeLocal → abort() (SIGABRT).

This is an uncatchable crash: it is not a JavaScript exception, so a surrounding try/catch cannot stop it — the entire Node process (or worker) dies.

The built-in regex engine handles the identical case correctly by throwing a catchable RangeError: Invalid string length. node-re2 diverges from that contract and aborts instead.

Proof of concept

npm i re2
node poc.js

const RE2 = require('re2');

// Built-in engine: same case -> CATCHABLE RangeError (correct)
try { 'a'.repeat(50000).replace(/a/g, "$'"); }
catch (e) { console.log('native:', e.constructor.name, e.message); } // RangeError: Invalid string length

// re2: ABORTS the whole process (uncatchable; try/catch does not help)
'a'.repeat(50000).replace(new RE2('a', 'g'), "$'");...
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Observed (Node v24, clean npm i re2 → [email protected]): native branch prints RangeError: Invalid string length; the re2 branch aborts with FATAL ERROR: v8::ToLocalChecked Empty MaybeLocal, stack top WrappedRE2::Replace, process exit code 134.

Threshold matches the mechanism precisely: input of 30,000 chars completes; 40,000 aborts (30000²/2 ≈ 4.5e8 < 5.37e8 max; 40000²/2 ≈ 8e8 > max). $&/constant templates and non-global replaces do not amplify and do not crash.

Impact

A remote, unauthenticated denial of service against any service that runs String.prototype.replace / the re2 [Symbol.replace] path where either the replacement template (containing $' or $`) or the input size is attacker-influenced. Because the failure is a native abort(), it cannot be contained by try/catch or domains — one request takes down the whole process/worker. This is especially impactful for re2's core audience, who adopt it specifically to process untrusted patterns/inputs safely.

Suggested fix

Check the MaybeLocal before ToLocalChecked on both return paths (and the intermediate group-string builds), and throw a catchable RangeError to match the built-in engine:

auto maybe = Nan::New(result);
if (maybe.IsEmpty()) { Nan::ThrowRangeError("Invalid string length"); return; }
info.GetReturnValue().Set(maybe.ToLocalChecked());

(Apply equivalently to the Nan::CopyBuffer(...) buffer path at L553 and to the per-group Nan::New(data, size).ToLocalChecked() sites used by the replacer-function path.)

Resolution

Resolved in re2 1.25.1. WrappedRE2::Replace now checks the returned MaybeLocal on every result path and throws a catchable RangeError: Invalid string length (matching the built-in engine) instead of aborting the process with an uncatchable SIGABRT. No API changes --- upgrade to re2 >= 1.25.1 via a plain npm upgrade to receive the fix.

Mitigation

Aliases

References