CVE-2026-56763: Hono Prototype Pollution Vulnerability

CVE-2026-56763 Overview

CVE-2026-56763 is a prototype pollution vulnerability in the Hono web framework versions before 4.12.7. The flaw exists in the parseBody function when the dot option is enabled. Hono fails to filter __proto__ keys from form field names, letting attackers craft input that creates objects with __proto__ properties. When downstream code merges these parsed results into standard JavaScript objects using unsafe merge patterns, the pollution propagates to Object.prototype. The weakness is tracked as CWE-1321: Improperly Controlled Modification of Object Prototype Attributes.

Critical Impact
Attackers can pollute Object.prototype through crafted form submissions, altering application behavior, bypassing security checks, or enabling denial of service depending on how the parsed body is consumed.

Affected Products

Hono web framework versions before 4.12.7
Applications using parseBody with the dot option enabled
Downstream services that merge parsed body data using unsafe merge patterns

Discovery Timeline

2026-07-11 - CVE-2026-56763 published to NVD
2026-07-14 - Last updated in NVD database

Technical Details for CVE-2026-56763

Vulnerability Analysis

Hono is a lightweight web framework for edge runtimes such as Cloudflare Workers, Deno, and Bun. Its parseBody helper converts multipart and URL-encoded form submissions into JavaScript objects. When developers enable the dot option, Hono interprets dot-delimited field names as nested object paths, converting user.name=alice into { user: { name: 'alice' } }.

The parser does not reject __proto__ as a reserved key during this expansion. An attacker can submit a field named __proto__.polluted and Hono will construct an object containing a __proto__ property. On its own, this creates an own-property named __proto__ rather than modifying the prototype chain. The exploitation becomes viable when application code merges the parsed object into another object using recursive or shallow-unsafe merge routines that walk __proto__ as if it were a regular key.

Root Cause

The root cause is missing key filtering in the dot-notation expansion logic of parseBody. Prototype-related keys including __proto__, constructor, and prototype are not blocked before the parser assigns them into the result object. This class of flaw is catalogued as CWE-1321.

Attack Vector

Exploitation requires a network-reachable Hono endpoint that calls parseBody with { dot: true } and passes the result to an unsafe merge function. The attacker sends a POST request whose form fields include __proto__.<property>=<value>. When the application merges the parsed body into a shared object, the payload writes onto Object.prototype, affecting every object in the process. Impact ranges from authentication bypass and property injection to denial of service, depending on how downstream code reads the polluted properties. The attack complexity is elevated because success depends on a vulnerable downstream merge, not the framework alone.

No verified exploitation code examples are published. See the GitHub Security Advisory GHSA-v8w9-8mx6-g223 and the VulnCheck Advisory on Prototype Pollution for advisory-level detail.

Detection Methods for CVE-2026-56763

Indicators of Compromise

HTTP POST requests containing form fields with __proto__, constructor, or prototype substrings in field names
Multipart or URL-encoded bodies containing dot-notation keys such as __proto__.isAdmin or constructor.prototype.polluted
Unexpected application behavior after processing user-submitted forms, including new properties appearing on unrelated objects

Detection Strategies

Inventory Hono deployments and identify routes that invoke parseBody with the dot option enabled
Deploy web application firewall rules that block request bodies containing __proto__, constructor.prototype, or prototype. patterns in field names
Audit application code for merge utilities such as Object.assign chains, lodash.merge, or custom recursive mergers used on parsed request bodies

Monitoring Recommendations

Log full form field names at ingress and alert on prototype-related tokens
Instrument runtime checks that detect writes to Object.prototype in Node.js and edge runtime environments
Correlate anomalous authorization outcomes with recent form submissions containing dot-notation payloads

How to Mitigate CVE-2026-56763

Immediate Actions Required

Upgrade Hono to version 4.12.7 or later across all deployments
Disable the dot option in parseBody calls where nested parsing is not required
Replace unsafe merge utilities with prototype-safe alternatives that reject __proto__ and constructor keys

Patch Information

The Hono maintainers addressed the flaw in version 4.12.7 by filtering prototype-related keys during dot-notation expansion in parseBody. Details are published in the GitHub Security Advisory GHSA-v8w9-8mx6-g223.

Workarounds

Set dot: false or omit the dot option when invoking parseBody
Sanitize parsed body objects by deleting __proto__, constructor, and prototype keys before any merge operation
Use Object.create(null) for target objects in merge routines to eliminate a prototype chain
Apply a WAF signature that rejects request bodies containing prototype-related field names

bash

# Upgrade Hono to the patched release
npm install hono@^4.12.7

# Verify installed version
npm ls hono