CVE-2026-39410: Hono has a non-breaking space prefix bypass in cookie name handling in getCookie()

Summary

A discrepancy between browser cookie parsing and parse() handling allows cookie prefix protections to be bypassed.

Cookie names that are treated as distinct by the browser may be normalized to the same key by parse(), allowing attacker-controlled cookies to override legitimate ones.

Details

Browsers follow RFC 6265bis and only trim SP (0x20) and HTAB (0x09) from cookie names. Other characters, such as the non-breaking space (U+00A0), are preserved as part of the cookie name.

For example, the browser treats the following cookies as distinct:

"dummy-cookie" "\u00a0dummy-cookie"

However, parse() previously used JavaScript's trim(), which removes a broader set of characters including U+00A0. As a result, both names are normalized to:

"dummy-cookie"

This mismatch allows attacker-controlled cookies with a U+00A0 prefix to shadow or override legitimate cookies when accessed via getCookie().

Impact

An attacker who can set cookies (e.g., via a man-in-the-middle on a non-secure page or other injection vector) can bypass cookie prefix protections and override sensitive cookies.

This may lead to:

Bypassing Secure- and Host- prefix protections Overriding cookies that rely on the Secure attribute Session fixation or session hijacking depending on application usage

This issue affects applications that rely on getCookie() for security-sensitive cookie handling.