CVE-2026-55091

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.

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.

Summary

convert() builds the nested tree by using each flat record's id and parent field values directly as object keys, with no guard against __proto__ / constructor / prototype. A record whose parent is the string "__proto__" makes temp[parent] resolve to Object.prototype, and the following initPush(...) writes attacker-controlled data onto the global prototype. Any application that passes attacker-influenced records to convert() is affected, and the base prototype methods stay intact so the pollution is stealthy.

Details

In index.js, convert() (FlatToNested.prototype.convert):

• temp = {} (line 45) and pendingChildOf = {} (line 46) are plain objects, so they inherit from Object.prototype.
• For each record, parent = flatEl[this.config.parent] (line 51) is taken verbatim from input.
• Line 57: if (temp[parent] !== undefined) — when parent === "__proto__", temp["__proto__"] resolves via the prototype chain to Object.prototype, which is !== undefined, so the branch is taken.
• Line 59: initPush(this.config.children, temp[parent], flatEl) → effectively initPush("children", Object.prototype, flatEl).
• initPush (lines 4-9): Object.prototype["children"] = [] then Object.prototype["children"].push(flatEl) — attacker-controlled data is written onto the global Object.prototype.

There is no sanitization of id / parent anywhere; they flow straight into temp[id], temp[parent], and pendingChildOf[parent] as dynamic keys.

PoC

const FlatToNested = require('flat-to-nested');

new FlatToNested().convert([
  { id: 1, parent: '__proto__', polluted: 'PWNED' }
]);

console.log(({}).children); // => [ { id: 1, polluted: 'PWNED' } ]
A freshly-created, unrelated object {} now carries an attacker-controlled children property. ({}).toString === Object.prototype.toString remains true, so existing methods are untouched (stealthy). If the consumer configures a custom children key, that arbitrary prototype property is polluted instead.

Impact

Prototype pollution (CWE-1321). Any service that builds a tree from attacker-influenced flat records (the package's core purpose — e.g. records derived from a DB/REST/user input) can have Object.prototype polluted. Consequences range from application-logic corruption and denial of service to serving as a gadget toward privilege escalation or RCE depending on downstream sinks. No special privileges or user interaction required; the malicious value is ordinary input data.

Suggested fix

Use prototype-less lookup tables so inherited keys like proto cannot be reached: var temp = Object.create(null); var pendingChildOf = Object.create(null); (Optionally also reject id/parent values equal to proto, constructor, or prototype.) Verified: with Object.create(null) for both temp and pendingChildOf, the PoC no longer pollutes Object.prototype and normal nesting output is unchanged. A patch with a regression test is ready.

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.

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.

Summary

convert() builds the nested tree by using each flat record's id and parent field values directly as object keys, with no guard against __proto__ / constructor / prototype. A record whose parent is the string "__proto__" makes temp[parent] resolve to Object.prototype, and the following initPush(...) writes attacker-controlled data onto the global prototype. Any application that passes attacker-influenced records to convert() is affected, and the base prototype methods stay intact so the pollution is stealthy.

Details

In index.js, convert() (FlatToNested.prototype.convert):

• temp = {} (line 45) and pendingChildOf = {} (line 46) are plain objects, so they inherit from Object.prototype.
• For each record, parent = flatEl[this.config.parent] (line 51) is taken verbatim from input.
• Line 57: if (temp[parent] !== undefined) — when parent === "__proto__", temp["__proto__"] resolves via the prototype chain to Object.prototype, which is !== undefined, so the branch is taken.
• Line 59: initPush(this.config.children, temp[parent], flatEl) → effectively initPush("children", Object.prototype, flatEl).
• initPush (lines 4-9): Object.prototype["children"] = [] then Object.prototype["children"].push(flatEl) — attacker-controlled data is written onto the global Object.prototype.

There is no sanitization of id / parent anywhere; they flow straight into temp[id], temp[parent], and pendingChildOf[parent] as dynamic keys.

PoC

const FlatToNested = require('flat-to-nested');

new FlatToNested().convert([
  { id: 1, parent: '__proto__', polluted: 'PWNED' }
]);

console.log(({}).children); // => [ { id: 1, polluted: 'PWNED' } ]
A freshly-created, unrelated object {} now carries an attacker-controlled children property. ({}).toString === Object.prototype.toString remains true, so existing methods are untouched (stealthy). If the consumer configures a custom children key, that arbitrary prototype property is polluted instead.

Impact

Prototype pollution (CWE-1321). Any service that builds a tree from attacker-influenced flat records (the package's core purpose — e.g. records derived from a DB/REST/user input) can have Object.prototype polluted. Consequences range from application-logic corruption and denial of service to serving as a gadget toward privilege escalation or RCE depending on downstream sinks. No special privileges or user interaction required; the malicious value is ordinary input data.

Suggested fix

Use prototype-less lookup tables so inherited keys like proto cannot be reached: var temp = Object.create(null); var pendingChildOf = Object.create(null); (Optionally also reject id/parent values equal to proto, constructor, or prototype.) Verified: with Object.create(null) for both temp and pendingChildOf, the PoC no longer pollutes Object.prototype and normal nesting output is unchanged. A patch with a regression test is ready.