# CVE-2026-107296: msgpack5: Decoding negative int64 values mutates the input buffer

Source: https://junglewise.ai/threats/cve-2026-107296-msgpack5-decoding-negative-int64-values-mutates-the-input-buffer  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-08  
License: CC BY 4.0 with a link back

CVE-2026-107296 · Severity: low · CVSS 3.7 · Published 2026-10-08

### Impact

Decoding a negative signed 64-bit integer modifies the corresponding bytes in the caller-provided input buffer. Applications that retain or reuse encoded input for integrity checks, logging, or subsequent processing may observe silently corrupted data.

Positive integers and other MessagePack value types are not affected.

### Patches

The decoder now computes signed 64-bit values without writing to the input buffer.

### Workarounds

Copy untrusted MessagePack input before decoding it, or do not retain or reuse input buffers after decoding.

Technologies: [msgpack5 (npm)](https://junglewise.ai/threats/technologies/msgpack5). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-107296 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-107296 being exploited in the wild.

**How severe is CVE-2026-107296?** Low severity, with a CVSS score of 3.7, published 8 October 2026.

**Which products and versions are affected?** msgpack5 (npm) < 6.1.0, SEMVER:introduced=0,fixed=6.1.0.

**Is there a fix for CVE-2026-107296?** Yes. msgpack5 (npm): upgrade to 6.1.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2026-107296?** Check whether you run msgpack5 (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- msgpack5 (npm): upgrade to 6.1.0 or later

  ```sh
  npm install msgpack5@6.1.0
  ```


## Affected products

- msgpack5 (npm) < 6.1.0, SEMVER:introduced=0,fixed=6.1.0

## Identifiers

- CVE-2026-107296
- GHSA-qw35-55vc-rhgj

## Related threats

- [msgpack5 out-of-bounds read in truncated map32 header](https://junglewise.ai/threats/cve-2026-107302-msgpack5-truncated-map32-headers-throw-an-unexpected-error)
- [msgpack5 uncontrolled recursion in streaming decoder](https://junglewise.ai/threats/cve-2026-107300-msgpack5-many-buffered-values-can-exhaust-the-streaming-decoder)
- [msgpack5 unbounded stream buffering via reserved byte](https://junglewise.ai/threats/cve-2026-107299-msgpack5-reserved-byte-can-cause-unbounded-stream-buffering)
- [msgpack5 prototype pollution via partial options](https://junglewise.ai/threats/cve-2026-107301-msgpack5-partial-options-disable-prototype-protection)
- [msgpack5 uncontrolled recursion in decoder](https://junglewise.ai/threats/cve-2026-107298-msgpack5-deeply-nested-input-can-exhaust-the-decoder-stack)