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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

re2 provides Node.js bindings for Google's RE2 regular expression engine. Prior to 1.25.2, re2 validates lastIndex against the UTF-8 byte length of a subject but uses it as a UTF-16 code-unit offset in exec, test, match, replace, and split, allowing an attacker-influenced lastIndex on a non-ASCII subject to trigger an out-of-bounds heap read and an uncatchable process crash, with limited heap information disclosure in some cases. This issue is fixed in 1.25.2.

Source published

30 Jul 2026 19:59:55 UTC

Source modified

31 Jul 2026 23:07:18 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 5.7 CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H

re2 provides Node.js bindings for Google's RE2 regular expression engine. Prior to 1.25.2, re2 validates lastIndex against the UTF-8 byte length of a subject but uses it as a UTF-16 code-unit offset in exec, test, match, replace, and split, allowing an attacker-influenced lastIndex on a non-ASCII subject to trigger an out-of-bounds heap read and an uncatchable process crash, with limited heap information disclosure in some cases. This issue is fixed in 1.25.2.

Source published

30 Jul 2026 20:18:15 UTC

Source modified

23 Oct 2026 22:10:00 UTC

Observed here

25 Oct 2026 15:18:51 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 5.7 CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H

## 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 <= 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-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 < size; --n) { size_t s = getUtf8CharSize(data[from]); from += s; if (s == 4 && n >= 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 <= <utf16Length>`, 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.

Source published

31 Jul 2026 16:53:16 UTC

Source modified

31 Jul 2026 17:00:20 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H

Publisher rating · MODERATE