Unrated

Prototype Pollution in flat-to-nested convert()

IdentifiersCVE-2026-55091CWE-1321

CVE-2026-55091 is a prototype pollution vulnerability in the npm package flat-to-nested. According to the provided advisory context, the issue is in the convert() functionality, which unsafely uses attacker-controlled id and parent values as object keys. Because these keys are inserted into ordinary JavaScript objects without blocking special property names, an attacker can supply values such as 'proto' and thereby modify Object.prototype. The vulnerable behavior stems from using user-influenced identifiers as keys in temporary lookup structures, enabling pollution of the global object prototype chain.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows an attacker to pollute Object.prototype, which can corrupt application logic across the process. Depending on how the application and its dependencies subsequently use inherited properties, this may result in denial of service, authorization or logic bypass, privilege escalation, or potentially remote code execution if suitable exploitation gadgets exist downstream.

Mitigation

If you can’t patch tonight, do this now.

Do not pass attacker-influenced records into convert() until a fixed version is deployed. Sanitize or reject untrusted id and parent values, especially 'proto', 'constructor', and 'prototype'. Where local modification is feasible, replace ordinary object-based lookup tables with prototype-less objects created via Object.create(null) to prevent prototype chain mutation.

Remediation

Patch, then assume compromise.

Upgrade flat-to-nested to version 1.1.2 or later. If an immediate upgrade is not possible, patch the affected code so that temporary lookup maps such as temp and pendingChildOf are created with Object.create(null) instead of normal objects, and add strict validation or filtering to reject dangerous key names including 'proto', 'constructor', and 'prototype'.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

ACTIVITY FEED

Recent activity

No public activity tracked yet. Mallory keeps watching.

0 SOURCESView more in app

No public activity observed for this vulnerability.

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Exposure mapping

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Threat actor evidence

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Associated malware

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Detection signatures

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Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity