# CVE-2026-107299: msgpack5 unbounded stream buffering via reserved byte

Source: https://junglewise.ai/threats/cve-2026-107299-msgpack5-reserved-byte-can-cause-unbounded-stream-buffering  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-08. Updated: 2026-10-08  
License: CC BY 4.0 with a link back

CVE-2026-107299 · Severity: medium · CVSS 5.9 · Published 2026-10-08

msgpack5 is a JavaScript library for encoding and decoding MessagePack binary data, commonly used in web applications and microservices. When a malformed MessagePack message containing the reserved byte 0xc1 is sent to msgpack5, the decoder incorrectly waits for additional bytes that never arrive, causing it to accumulate data in memory indefinitely. An attacker can trigger memory exhaustion and crash the application by sending crafted inputs.

Technologies: [Mcollina Msgpack5](https://junglewise.ai/threats/technologies/mcollina-msgpack5), [msgpack5 (npm)](https://junglewise.ai/threats/technologies/msgpack5). Vendors: [Mcollina](https://junglewise.ai/threats/vendors/mcollina), [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-107299 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-107299 being exploited in the wild.

**How severe is CVE-2026-107299?** Medium severity, with a CVSS score of 5.9, published 8 October 2026.

**Which products and versions are affected?** Mcollina Msgpack5 < 6.1.0; msgpack5 (npm) < 6.1.0, SEMVER:introduced=0,fixed=6.1.0.

**Is there a fix for CVE-2026-107299?** Yes. msgpack5 (npm): upgrade to 6.1.0 or later. Until then: Reject MessagePack byte 0xc1 before passing input to msgpack5, and enforce maximum limits on stream buffer size and decoding time to prevent unbounded resource consumption. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-107299 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-107299?** Check whether you run Mcollina Msgpack5 or 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
  ```


Reject MessagePack byte 0xc1 before passing input to msgpack5, and enforce maximum limits on stream buffer size and decoding time to prevent unbounded resource consumption.

## Technical details

The vulnerability lies in improper handling of syntactically invalid MessagePack structures (CWE-228). When the reserved byte 0xc1 appears at the start of a decoder stream, the decoder treats it as an incomplete value rather than an invalid one, buffering all subsequent input in memory while waiting for completion bytes that can never make it valid. An attacker with network access to a service using msgpack5's streaming decoder can exhaust memory without requiring authentication or user interaction.

## Affected products

- mcollina msgpack5 < 6.1.0

## Identifiers

- CVE-2026-107299
- GHSA-26wq-p25c-j6fv

## Timeline

- 2026-09-26: disclosed
- 2026-10-08: advisory

## References

- [https://api.github.com/users/SWIFI-AI](https://api.github.com/users/SWIFI-AI)

## 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 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)
- [msgpack5 quadratic parsing in streaming decoder](https://junglewise.ai/threats/cve-2026-107297-msgpack5-quadratic-parsing-in-the-streaming-decoder)