Summary
msgpack5: Reserved byte can cause unbounded stream buffering
Workarounds
Reject 0xc1 before streaming input to msgpack5, and enforce stream byte and time limits.
Impact
The reserved MessagePack byte 0xc1 is incorrectly treated as incomplete input. When it appears at the start of a decoder stream, all subsequent data is retained while the decoder waits for bytes that can never make the value valid, allowing memory exhaustion.
CVE-2026-107299 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.1.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The decoder now rejects 0xc1 as invalid input. Streaming decoders release buffered input and stop after unrecoverable decoding errors.
Frequently Asked Questions
- What is CVE-2026-107299? CVE-2026-107299 is a medium-severity security vulnerability in msgpack5 (npm), affecting versions < 6.1.0. It is fixed in 6.1.0.
- How severe is CVE-2026-107299? CVE-2026-107299 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of msgpack5 are affected by CVE-2026-107299? msgpack5 (npm) versions < 6.1.0 is affected.
- Is there a fix for CVE-2026-107299? Yes. CVE-2026-107299 is fixed in 6.1.0. Upgrade to this version or later.
- Is CVE-2026-107299 exploitable, and should I be worried? Whether CVE-2026-107299 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-107299 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-107299? Upgrade
msgpack5to 6.1.0 or later.