re2: Global String.prototype.match with an empty-matchable pattern never advances → infinite loop with unbounded native memory growth (DoS)
Summary
String.prototype.match with a global RE2 collects all matches in a native loop that advances the cursor by the match length. A zero-width (empty) match has length 0, so the cursor never advances: the same empty match is found forever and appended to an ever-growing native vector. Any pattern that can match the empty string (a*, b?, x{0,3}, (a)|, (?:), …) therefore causes an infinite loop with unbounded memory growth. The call is synchronous native code, so it blocks the entire event loop and cannot be interrupted by try/catch, AbortController, --max-old-space-size, or timers — the process must be killed externally. This diverges from the built-in engine, where 'xxxx'.match(/a*/g) returns a finite array.
Root cause
```cpp
// lib/match.cc:44 — global branch of WrappedRE2::Match
while (re2->regexp.Match(str, byteIndex, str.size, anchor, &match, 1)) {
groups.push_back(match);
byteIndex = match.data() - str.data + match.size(); // += 0 for a zero-width match
}
```
When match.size() == 0, byteIndex is unchanged, so the next iteration matches the same empty position again; groups grows without bound. The other iteration paths already guard this: lib/split.cc:50-55 advances by getUtf8CharSize on an empty match, and exec advances lastIndex. Only this global Match loop is missing the guard.
Proof of concept
```js
const RE2 = require('re2');
'x'.match(new RE2('a*', 'g')); // never returns; grows memory until OOM
// also: 'b?', 'x{0,3}', '(a)|', 'c*d*', '(?:)'; empty subject '' triggers it too
```
Compare with the built-in engine, which terminates:
```js
'xxxx'.match(/a*/g); // -> ["", "", "", "", ""]
```
Measured on a clean npm install [email protected] (latest), stock prebuilt binary: resident memory grew ~550 MB → 2.3 GB in ~3 seconds at 100% CPU, and the process had to be SIGKILLed externally.
Impact
Denial of service. Reachable remotely and without authentication wherever an application runs a global RE2 through String.prototype.match and either the pattern or the subject is attacker-influenced — e.g. a user-supplied regular expression, or a fixed empty-matchable pattern applied to user input. Because the loop blocks the event loop and exhausts memory in seconds, a single request can wedge a worker and, via memory exhaustion, affect the whole host.
Suggested fix
Mirror the empty-match handling already present in split.cc: when the match is zero-width, advance the cursor by one code point.
```cpp
// lib/match.cc, inside the global while-loop
groups.push_back(match);
size_t off = match.data() - str.data;
if (match.size()) {
byteIndex = off + match.size();
} else {
byteIndex = off + (off < str.size ? getUtf8CharSize(str.data[off]) : 1);
}
```
Resolution
Fixed in re2 1.25.2.
The global match loop in lib/match.cc now advances the cursor by one Unicode
code point when a match is zero-width, so a pattern that can match the empty
string terminates with a finite result identical to the built-in engine
('xxxx'.match(/a*/g) returns five empty strings). This mirrors the guard
already present in split.
Remediation: upgrade to [email protected] or later.
Workaround (if you cannot upgrade): do not run a global RE2 through
String.prototype.match when the pattern is attacker-influenced or can match
the empty string. Iterate with matchAll/exec, or use the non-global form;
both already advanced the cursor correctly.
