GHSA-ff84-5f28-78qj re2: Out-of-bounds heap read in `exec`/`test`/`match` via attacker-influenced `lastIndex` on a non-ASCII subject → uncatchable process crash (DoS)

• npm re2: < 1.25.2 → fixed in 1.25.2

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 count • `lib/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 0; --n) { size_t s = getUtf8CharSize(data[from]); // = 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 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-8 sizes — so it is noted for completeness, not as a dependable primitive. This is distinct from GHSA-8hcv-x26h-mcgp (the global `replace()` output-amplification abort), which was fixed in 1.25.1. This `lastIndex` out-of-bounds read is a separate defect and remains present in 1.25.1. Suggested fix Two independent hardenings; either closes the crash, both is safest: 1. Bound the walk so it can never read past the buffer: ```cpp inline size_t getUtf16PositionByCounter(const char *data, size_t size, size_t from, size_t n) { for (; n > 0 && from = 2) --n; } return from > size ? size : from; } ``` (thread `size` through the two call sites in `StrVal::setIndex`). 2. Validate `lastIndex` against the true UTF-16 length, not the UTF-8 byte length — e.g. store `s->Length()` (UTF-16 units) as the value compared in `isValidIndex = newIndex `, so an out-of-range `lastIndex` takes the existing `!isValidIndex` early-return path. Resolution Fixed in re2 1.25.2. `lastIndex` is now validated against the subject's UTF-16 length instead of its UTF-8 byte length (`lib/addon.cc`), so an out-of-range `lastIndex` is rejected before the buffer is walked. As defense in depth, the code-unit walk (`getUtf16PositionByCounter` in `lib/wrapped_re2.h`) is now bounded by the buffer size and can no longer read past the end. Remediation: upgrade to `[email protected]` or later. Workaround (if you cannot upgrade): do not assign `lastIndex` from untrusted input, or clamp it to the subject's string length (`str.length`) before calling `exec`/`test`/`match`/`replace`/`split` on a non-ASCII subject.