CVE-2026-55091
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk40th percentile - higher than 40% of all known CVEs
Summary
In flat-to-nested prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent fields as keys in temp and pendingChildOf objects. When parent or id is __proto__, prototype pollution occurs, potentially causing application-logic corruption, denial of service, or further exploitation.
Risk Assessment
An application processing untrusted records may suffer prototype pollution, leading to logic corruption, crashes, or escalation when combined with other gadgets.
Recommendation
Upgrade flat-to-nested to version 1.1.2, which safely handles __proto__, constructor, and prototype keys.
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Original NVD description (English source)
flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is __proto__, temp[parent] can resolve to Object.prototype, and initPush() can write attacker-controlled data to the global children prototype property while existing prototype methods remain intact. Any application that passes attacker-influenced flat records to convert() can therefore expose unrelated objects to polluted inherited state, causing application-logic corruption or denial of service and potentially enabling greater impact when a downstream prototype-pollution gadget is present. The constructor and prototype strings are also unsafe inherited-key values in the same lookup design. This issue is fixed in version 1.1.2.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

