Executive brief
Hono is a popular web framework for JavaScript used to build HTTP APIs and web applications. When dot-notation parsing is enabled in form field handling, an attacker can craft requests that cause excessive memory allocation, potentially crashing the server and causing service downtime. This affects only applications that explicitly enable dot-notation parsing, not the default configuration.
Technical details
The vulnerability exists in Hono's parseBody() method, which converts dot-separated form field names into nested JavaScript objects without enforcing limits on nesting depth or the total number of intermediate objects created. Each dot-separated segment in a field name creates an intermediate object, and empty segments are preserved, allowing a single field name to encode multiple nesting levels per byte. An attacker can exploit this through two attack patterns: a single deeply dotted field name or many shallowly dotted fields in one request body. Because the amplification occurs after the body has been accepted, normal request size limits do not prevent the attack. The vulnerability is triggered by sending concurrent requests to endpoints with dot-notation parsing enabled, exhausting the JavaScript heap and terminating the process. The issue was patched in version 4.13.5 by adding bounds on nesting depth and the number of objects created.
Affected products
- Honojs Hono < 4.13.5
Timeline
- 2026-08-26: disclosed
- 2026-09-08: patched: Version 4.13.5 released with fix
- 2026-09-08: advisory: Published to GitHub Advisory Database