CVE-2026-47676
ADVISORY - githubSummary
Summary
app.mount() strips the mount prefix from the incoming request path using the raw URL pathname, while route matching is performed against the percent-decoded path. This inconsistency causes the prefix to be stripped at the wrong position when the path contains percent-encoded multi-byte characters, resulting in the mounted sub-application receiving an incorrect path.
Details
When app.mount(prefix, subApp) is called, Hono calculates the number of characters to strip based on the decoded mount prefix length, but then applies that slice to the raw URL pathname. When the URL contains percent-encoded characters that expand to fewer characters when decoded (such as encoded non-ASCII characters), the two representations have different lengths, so the prefix is stripped at the wrong byte offset.
As a result, the sub-application receives a path that does not correspond to the intended sub-path — it may receive a partial or garbled path instead of the expected value after the mount prefix is removed.
This issue arises when an application uses app.mount() with paths that contain percent-encoded characters, particularly when the mount prefix itself or the request path contains encoded non-ASCII characters.
Impact
A mounted sub-application may receive an incorrectly stripped path, causing requests to be routed to unintended handlers within the sub-application.
This may lead to:
- Middleware or route handlers in the sub-application being bypassed or incorrectly matched due to the malformed path
- Requests reaching sub-application routes that the developer did not intend to be accessible via the mounted path
This issue affects applications that use app.mount() where the request URL may contain percent-encoded characters in the mount prefix or subsequent path segments.
GitHub
CVSS SCORE
5.3medium| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| hono | npm | - | - | <4.12.21 | 4.12.21 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
There is no loss of trust or accuracy within the impacted component.
There is no impact to availability within the impacted component.