GHSA-j4r3-hg7j-8chg

## Summary

`re2` infers a character's byte length from its UTF-8 lead byte alone, with no bound on the bytes actually remaining in the input. `Buffer` arguments reach the native layer verbatim — only strings are re-encoded into well-formed UTF-8 — so a `Buffer` whose last byte is a multi-byte lead promises continuation bytes that are not there, and the result builders read up to 3 bytes past the end of the buffer. In `replace()` and `split()` those bytes are copied into the returned `Buffer`, disclosing adjacent heap memory to JavaScript. The trigger is deterministic and requires no special heap grooming.

Only `Buffer` input is affected. String input was never at risk: re-encoding guarantees every multi-byte sequence is complete.

## Root cause

`getUtf8CharSize` maps a lead byte to a length of 1–4 and never sees the input size:

```cpp // lib/wrapped_re2.h inline size_t getUtf8CharSize(char ch) { return ((0xE5000000 >> ((ch >> 3) & 0x1E)) & 3) + 1; } ```

Callers then read that many bytes. In the zero-width branch of `replace()`, the guard proves only that at least *one* byte remains:

```cpp // lib/replace.cc else if ((size_t)offset < size) { auto sym_size = getUtf8CharSize(data[offset]); // may claim up to 4 bytes result.append(data + offset, sym_size); // reads data[offset .. offset + 3] byteIndex = offset + sym_size; } ```

`offset < size` permits `offset == size - 1`, so a lead byte of `0xF0` makes `append` read `data[size]`, `data[size + 1]` and `data[size + 2]`.

Seven read sites shared the defect:

| Site | Argument | Disclosed to JS | |---|---|---| | `lib/replace.cc` (zero-width branch) | subject | yes | | `lib/replace.cc` (callback replacer) | subject | yes | | `lib/replace.cc` (replacement scan) | replacement | yes | | `lib/split.cc` | subject | yes | | `lib/pattern.cc` `translateRegExp` (x2) | pattern | no | | `lib/pattern.cc` `escapeRegExp` | pattern | no |

Three further callers were **not** vulnerable, because they use the result only to advance an index and never dereference past the end: `getUtf16PositionByCounter` in `lib/wrapped_re2.h` (clamps its return to the buffer size), `lib/match.cc` (the value feeds `RE2::Match`, which rejects `startpos > endpos`), and the `getMaxSubmatch` scan in `lib/replace.cc` (an overshoot just ends the loop).

## Proof of concept

Each call returns more bytes than were supplied; the trailing bytes are heap contents and vary between runs.

```js const RE2 = require('re2'); const hex = buf => [...buf].map(b => b.toString(16).padStart(2, '0')).join(' ');

// subject: 2 bytes in, 5 bytes out console.log(hex(new RE2('', 'g').replace(Buffer.from([0x41, 0xf0]), ''))); // 41 f0 61 7b eb <- last 3 bytes are adjacent heap memory

// replacement argument console.log(hex(new RE2('A', 'g').replace(Buffer.from('A'), Buffer.from([0x42, 0xf0])))); // 42 f0 41 26 d6

// split console.log(new RE2('', 'g').split(Buffer.from([0x41, 0xf0])).map(hex)); // [ '41', 'f0 e2 e4 df' ] ```

`0xC2` (2-byte lead) and `0xE2` (3-byte lead) over-read 1 and 2 bytes respectively; `0xF0` over-reads 3.

For the pattern path the over-read occurs in `translateRegExp` / `escapeRegExp`, which run before RE2 validates the pattern, but RE2 then rejects the malformed input, so the bytes are discarded rather than returned:

```js new RE2(Buffer.from([0xf0])); // SyntaxError: invalid UTF-8 — read already happened ```

## Impact

**Information disclosure (`replace`, `split`).** Up to 3 bytes of heap memory adjacent to the input buffer are returned to JavaScript per call. The read is repeatable, so an attacker who controls `Buffer` input and observes output can sample heap memory incrementally. What lands there depends on allocator layout and is not directly steerable, but it may include fragments of other buffers.

**Out-of-bounds read (pattern compilation).** No disclosure path, since the malformed pattern is rejected — but the read is still undefined behavior and can fault if the buffer ends on a page boundary.

Applications that pass only strings, or only well-formed UTF-8 buffers, are unaffected. The exposure matters most where `re2` is used as intended: running patterns or subjects derived from untrusted input.

## Suggested fix

Clamp the inferred character size to the bytes that actually remain, at every site whose result indexes the buffer:

```cpp inline size_t getUtf8CharSize(char ch, size_t remaining) { size_t size = getUtf8CharSize(ch); return size < remaining ? size : remaining; } ```

This is O(1) and changes no algorithm's complexity. A truncated tail then round-trips as the bytes it really holds, which preserves the documented contract that `Buffer` input is passed through verbatim. Rejecting malformed UTF-8 in `Buffer` input would also close the hole, but is a breaking API change.

## Resolution

Fixed in `re2@1.26.1`.

All seven read sites now clamp the character size to the remaining input, so a `Buffer` ending in a truncated multi-byte character round-trips as its own bytes instead of reading past the end. Regression tests cover the subject, replacement and pattern positions for 2-, 3- and 4-byte leads, including partially truncated sequences.

**Remediation:** upgrade to `re2@1.26.1` or later.

**Workaround** (if you cannot upgrade): pass strings rather than `Buffer`s, or validate that `Buffer` input is well-formed UTF-8 before calling `replace`, `split`, or the `RE2` constructor — for example `Buffer.compare(Buffer.from(buf.toString('utf8')), buf) === 0`.

Reported by [@OvOhao](https://github.com/OvOhao) in [#272](https://github.com/uhop/node-re2/issues/272).