CVE-2026-107301 mcollina CVE debrief

Who should care

Defenders responsible for npm packages, particularly those using msgpack5, should assess exposure and verify patched versions. Roles responsible for npm package management should review the CVE record and NVD entry for details on patches, workarounds, and potential impacts. They should also verify msgpack5 inventory and check for patched versions (6.1.0 or later). Additionally, defenders should explicitly set protoAction: 'error' when constructing msgpack5

Why it matters

Defenders should care about CVE-2026-107301 because it affects msgpack5, potentially leading to unexpected behavior. Roles responsible for npm package management should assess exposure and verify patched versions.

• Potential unexpected behavior in downstream code due to prototype changes
• Verification of msgpack5 inventory and patched versions is required

Technical summary

Passing an empty or partial options object to msgpack5 disables the default protoAction: 'error' protection, allowing a map with a __proto__ key to replace the decoded object's prototype. This affects msgpack5, potentially leading to unexpected behavior in downstream code. The vulnerability has a CVSS score of 6.5 and is classified as MEDIUM severity. The CVE record provides details on patches, workarounds, and potential impacts. Defenders should assess exposure and verify patched versions, particularly for npm packages using msgpack5.

Defensive priority

Defenders should prioritize verifying msgpack5 inventory and checking for patched versions.

Recommended defensive actions

• Verify msgpack5 inventory and check for patched versions (6.1.0 or later)
• Explicitly set protoAction: 'error' when constructing msgpack5 instances
• Validate decoded values before use
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

Evidence from osv_dev indicates that msgpack5's prototype protection can be disabled with partial options, potentially leading to unexpected behavior. The vulnerability allows a map with a __proto__ key to replace the decoded object's prototype. Defenders should verify msgpack5 inventory and check for patched versions (6.1.0 or later). They should also explicitly set protoAction: 'error' when constructing msgpack5 instances and validate decoded values before use. The CVE record and NVD entry provide additional context on the severity,

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.