Summary
Echo's router and static file handler disagree on URL path decoding. The router matches routes using the raw encoded path (preserving %2F as-is), while StaticDirectoryHandler unescapes %2F to / before resolving filesystem paths. This allows an attacker to bypass route-level access controls and read static files without authorization.
Details
Root cause 1 — router.go lines 798-802:
The router uses req.URL.RawPath for route matching when useEscapedPathForRouting is false (the default). This means /admin%2Fsecret.txt is treated as a single path segment and does NOT match the /admin/* route pattern.
if !r.useEscapedPathForRouting && req.URL.RawPath != "" {
path = req.URL.RawPath
}
Root cause 2 — echo.go lines 559-568:
StaticDirectoryHandler calls url.PathUnescape() on the path parameter before opening files. This converts %2F back to /, resolving admin/secret.txt on disk.
if !disablePathUnescaping {
tmpPath, err := url.PathUnescape(p)
p = tmpPath
}
name := filepath.ToSlash(filepath.Clean(strings.TrimPrefix(p, "/")))
PoC (Screenshot)
Sample:
<img width="1291" height="970" alt="image" src="https://github.com/user-attachments/assets/0bc58059-3e6d-4678-ab25-a5c79b006738" />
403:
<img width="526" height="194" alt="image" src="https://github.com/user-attachments/assets/2f55ffdd-87b2-4a1b-8a13-130ebad0f257" />
Bypass with encoded slash:
<img width="592" height="203" alt="image" src="https://github.com/user-attachments/assets/1191cd39-ae8f-4d7e-8fb1-cb9cf31f484f" />
Impact
Unauthorized static file disclosure. Applications that protect route prefixes with authentication middleware while also serving static files from a broader root are vulnerable. An attacker only needs to encode the slash (/ → %2F) in the URL to bypass all route-level protection.
Common affected pattern:
adminGroup := e.Group("/admin", authMiddleware)
e.StaticFS("/", os.DirFS("public"))
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