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.
References
Summary
On Windows hosts, an encoded backslash (
%5C) in the request path decodes to\, which the Windows path resolver treats as a separator.serve-staticthen resolves a single URL segment such asadmin\secret.txtinto 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.txtis one segment and middleware on/admin/*does not run. Theserve-staticguard 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.
References