Reject unsafe file extensions

Signed-off-by: eternal-flame-AD <yume@yumechi.jp>

README.md

@@ -24,6 +24,14 @@ Currently to do:
 - [X] Sandboxing the image rendering
 - [X] Prometheus-format metrics
 
+## Spec Compliance
+
+This project is designed to match the upstream [specification](https://github.com/misskey-dev/media-proxy/blob/master/SPECIFICATION.md), however a few deviations are made:
+
+- We will not honor remote `Content-Disposition` headers but instead reply with the actual filename in the request URL.
+- Remote `Content-Type` headers will only be used as a hint rather than authoritative, and resniffing is unconditionally performed.
+- SVG rasterization is planned to be removed from the proxy in favor of sanitization and CSP enforcement.
+
 ## Demo
 
 ### Avatar resizing

src/lib.rs

@@ -992,6 +992,18 @@ impl<C: UpstreamClient + 'static, S: Sandboxing + Send + Sync + 'static> App<C,
     where
         <<C as UpstreamClient>::Response as HTTPResponse>::BodyStream: Unpin,
     {
+        if let Some(ext) = filename.split('.').last() {
+            if [
+                "exe", "com", "dll", "sys", "bat", "cmd", "sh", "bash", "zsh", "fish", "ps1",
+                "psm1", "elf", "so", "dylib", "dmg", "scr", "url", "app", "jar", "apk", "msi",
+                "deb", "rpm", "rpm", "pkg",
+            ]
+            .iter()
+            .any(|x| x.eq_ignore_ascii_case(ext))
+            {
+                return Err(ErrorResponse::unsafe_media());
+            }
+        }
         Self::proxy_impl(method, Some(&filename), State(state), Query(query), info).await
     }
 }