Medium · CVSS 5.9CVE-2026-107299GHSA-26WQ-P25C-J6FV

msgpack5: Reserved byte can cause unbounded stream bufferingCVE-2026-107299

Answer in brief

CVE-2026-107299 records a Medium severity (CVSS 5.9) vulnerability in msgpack5: Reserved byte can cause unbounded stream buffering. The current sources do not mark it as known exploited. The current feed maps mcollina/msgpack5 (generic), msgpack5 (npm). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 8, 2026Updated Oct 8, 2026Source checked Oct 8, 2026First seen by HOL Oct 8, 2026Material review Oct 8, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 5.9
Exploitation: Not marked as known exploited
Affected software: 2 mapped packages or products
Fix availability: Available

Why this deserves its current priority

CVSS is 5.9. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps mcollina/msgpack5 (generic), msgpack5 (npm). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
mcollina/msgpack5generic	<6.1.0	6.1.0
msgpack5npm	<6.1.0	6.1.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for mcollina/msgpack5, msgpack5.
Review the reported fix. Update mcollina/msgpack5 to 6.1.0; msgpack5 to 6.1.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Oct 8, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 8, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 8, 2026

Sources and claim methodology

Upstream source description

msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, the streaming decoder treats the reserved MessagePack byte 0xc1 as incomplete input instead of invalid input. When 0xc1 begins a stream, subsequent data remains buffered while the decoder waits for bytes that cannot make the value valid, allowing a remote peer to exhaust memory. This issue is fixed in version 6.1.0.

Quoted source text, attributed separately from HOL analysis.