CVE-2026-54286: Hono: Path traversal in `serve-static` on Windows via encoded backslash (`%5C`)
Summary
On Windows hosts, an encoded backslash (%5C) in the request path decodes to \, which the Windows path resolver treats as a separator. serve-static then resolves a single URL segment such as admin\secret.txt into a nested file under the root and serves it, letting an attacker read static files meant to be protected behind prefix-mounted middleware. Directory escape (..) remains blocked.
Details
The router splits paths only on /, so /admin%5Csecret.txt is one segment and middleware on /admin/ does not run. The serve-static guard rejects ./.. and consecutive separators but lets a lone \ through; on Windows the file resolver re-splits it into the protected subtree.
This affects Windows hosts serving static files via the Node, Bun, or Deno adapters that guard a static subtree with prefix-mounted middleware.
Impact
An unauthenticated attacker can read static files under a middleware-guarded prefix on Windows hosts. The read stays within the configured root; escape outside the root is not possible.
Other sources
Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.12.25, on Windows hosts, an encoded backslash (%5C) in the request path decodes to \, which the Windows path resolver treats as a separator. serve-static then resolves a single URL segment such as admin\secret.txt into a nested file under the root and serves it, letting an attacker read static files meant to be protected behind prefix-mounted middleware. This vulnerability is fixed in 4.12.25.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/honoto a version that resolves this vulnerability.Fixed in 4.12.25 - Upgrade
Upgrade
honoto a version that resolves this vulnerability.Fixed in 4.12.25
Event History
Frequently Asked Questions
What is the severity of CVE-2026-54286?
The severity of CVE-2026-54286 is medium, with a score of 5.9.
What kind of vulnerability is CVE-2026-54286?
CVE-2026-54286 is a path traversal vulnerability affecting Windows hosts.
How does CVE-2026-54286 impact affected systems?
CVE-2026-54286 allows attackers to read arbitrary files by exploiting the backslash encoding in request paths.
Which software is affected by CVE-2026-54286?
CVE-2026-54286 affects the npm package 'hono'.
How can I mitigate CVE-2026-54286?
To mitigate CVE-2026-54286, ensure proper validation and sanitization of input paths to prevent path traversal.