Add security checks when fetching objects (#95)

* Add security checks when fetching objects

* static

* as ref

* update comment

* fix header

* Update comment

src/error.rs

@@ -1,13 +1,11 @@
 //! Error messages returned by this library
 
-use std::string::FromUtf8Error;
-
+use crate::fetch::webfinger::WebFingerError;
 use http_signature_normalization_reqwest::SignError;
 use openssl::error::ErrorStack;
+use std::string::FromUtf8Error;
 use url::Url;
 
-use crate::fetch::webfinger::WebFingerError;
-
 /// Error messages returned by this library
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
@@ -62,6 +60,14 @@ pub enum Error {
     /// Signing errors
     #[error(transparent)]
     SignError(#[from] SignError),
+    /// Attempted to fetch object which doesn't have valid ActivityPub Content-Type
+    #[error(
+        "Attempted to fetch object from {0} which doesn't have valid ActivityPub Content-Type"
+    )]
+    FetchInvalidContentType(Url),
+    /// Attempted to fetch object but the response's id field doesn't match
+    #[error("Attempted to fetch object from {0} but the response's id field doesn't match")]
+    FetchWrongId(Url),
     /// Other generic errors
     #[error("{0}")]
     Other(String),

src/fetch/mod.rs

@@ -5,12 +5,13 @@
 use crate::{
     config::Data,
     error::{Error, Error::ParseFetchedObject},
+    extract_id,
     http_signatures::sign_request,
     reqwest_shim::ResponseExt,
     FEDERATION_CONTENT_TYPE,
 };
 use bytes::Bytes;
-use http::StatusCode;
+use http::{HeaderValue, StatusCode};
 use serde::de::DeserializeOwned;
 use std::sync::atomic::Ordering;
 use tracing::info;
@@ -29,6 +30,8 @@ pub struct FetchObjectResponse<Kind> {
     pub object: Kind,
     /// Contains the final URL (different from request URL in case of redirect)
     pub url: Url,
+    content_type: Option<HeaderValue>,
+    object_id: Option<Url>,
 }
 
 /// Fetch a remote object over HTTP and convert to `Kind`.
@@ -42,12 +45,32 @@ pub struct FetchObjectResponse<Kind> {
 /// [Error::RequestLimit]. This prevents denial of service attacks where an attack triggers
 /// infinite, recursive fetching of data.
 ///
-/// The `Accept` header will be set to the content of [`FEDERATION_CONTENT_TYPE`].
+/// The `Accept` header will be set to the content of [`FEDERATION_CONTENT_TYPE`]. When parsing the
+/// response it ensures that it has a valid `Content-Type` header as defined by ActivityPub, to
+/// prevent security vulnerabilities like [this one](https://github.com/mastodon/mastodon/security/advisories/GHSA-jhrq-qvrm-qr36).
+/// Additionally it checks that the `id` field is identical to the fetch URL (after redirects).
 pub async fn fetch_object_http<T: Clone, Kind: DeserializeOwned>(
     url: &Url,
     data: &Data<T>,
 ) -> Result<FetchObjectResponse<Kind>, Error> {
-    fetch_object_http_with_accept(url, data, FEDERATION_CONTENT_TYPE).await
+    static CONTENT_TYPE: HeaderValue = HeaderValue::from_static(FEDERATION_CONTENT_TYPE);
+    static ALT_CONTENT_TYPE: HeaderValue = HeaderValue::from_static(
+        r#"application/ld+json; profile="https://www.w3.org/ns/activitystreams""#,
+    );
+    let res = fetch_object_http_with_accept(url, data, &CONTENT_TYPE).await?;
+
+    // Ensure correct content-type to prevent vulnerabilities.
+    if res.content_type.as_ref() != Some(&CONTENT_TYPE)
+        && res.content_type.as_ref() != Some(&ALT_CONTENT_TYPE)
+    {
+        return Err(Error::FetchInvalidContentType(res.url));
+    }
+
+    // Ensure id field matches final url
+    if res.object_id.as_ref() != Some(&res.url) {
+        return Err(Error::FetchWrongId(res.url));
+    }
+    Ok(res)
 }
 
 /// Fetch a remote object over HTTP and convert to `Kind`. This function works exactly as
@@ -55,7 +78,7 @@ pub async fn fetch_object_http<T: Clone, Kind: DeserializeOwned>(
 async fn fetch_object_http_with_accept<T: Clone, Kind: DeserializeOwned>(
     url: &Url,
     data: &Data<T>,
-    content_type: &str,
+    content_type: &HeaderValue,
 ) -> Result<FetchObjectResponse<Kind>, Error> {
     let config = &data.config;
     // dont fetch local objects this way
@@ -93,9 +116,17 @@ async fn fetch_object_http_with_accept<T: Clone, Kind: DeserializeOwned>(
     }
 
     let url = res.url().clone();
+    let content_type = res.headers().get("Content-Type").cloned();
     let text = res.bytes_limited().await?;
+    let object_id = extract_id(&text).ok();
+
     match serde_json::from_slice(&text) {
-        Ok(object) => Ok(FetchObjectResponse { object, url }),
+        Ok(object) => Ok(FetchObjectResponse {
+            object,
+            url,
+            content_type,
+            object_id,
+        }),
         Err(e) => Err(ParseFetchedObject(
             e,
             url,

src/fetch/webfinger.rs

@@ -5,6 +5,7 @@ use crate::{
     traits::{Actor, Object},
     FEDERATION_CONTENT_TYPE,
 };
+use http::HeaderValue;
 use itertools::Itertools;
 use once_cell::sync::Lazy;
 use regex::Regex;
@@ -33,6 +34,9 @@ impl WebFingerError {
     }
 }
 
+/// The content-type for webfinger responses.
+pub static WEBFINGER_CONTENT_TYPE: HeaderValue = HeaderValue::from_static("application/jrd+json");
+
 /// Takes an identifier of the form `name@example.com`, and returns an object of `Kind`.
 ///
 /// For this the identifier is first resolved via webfinger protocol to an Activitypub ID. This ID
@@ -58,7 +62,7 @@ where
     let res: Webfinger = fetch_object_http_with_accept(
         &Url::parse(&fetch_url).map_err(Error::UrlParse)?,
         data,
-        "application/jrd+json",
+        &WEBFINGER_CONTENT_TYPE,
     )
     .await?
     .object;

src/lib.rs

@@ -35,7 +35,7 @@ use serde::{de::DeserializeOwned, Deserialize};
 use url::Url;
 
 /// Mime type for Activitypub data, used for `Accept` and `Content-Type` HTTP headers
-pub static FEDERATION_CONTENT_TYPE: &str = "application/activity+json";
+pub const FEDERATION_CONTENT_TYPE: &str = "application/activity+json";
 
 /// Deserialize incoming inbox activity to the given type, perform basic
 /// validation and extract the actor.
@@ -53,12 +53,8 @@ where
 {
     let activity: Activity = serde_json::from_slice(body).map_err(|e| {
         // Attempt to include activity id in error message
-        #[derive(Deserialize)]
-        struct Id {
-            id: Url,
-        }
-        let id = serde_json::from_slice::<Id>(body).ok();
-        Error::ParseReceivedActivity(e, id.map(|i| i.id))
+        let id = extract_id(body).ok();
+        Error::ParseReceivedActivity(e, id)
     })?;
     data.config.verify_url_and_domain(&activity).await?;
     let actor = ObjectId::<ActorT>::from(activity.actor().clone())
@@ -66,3 +62,12 @@ where
         .await?;
     Ok((activity, actor))
 }
+
+/// Attempt to parse id field from serialized json
+fn extract_id(data: &[u8]) -> serde_json::Result<Url> {
+    #[derive(Deserialize)]
+    struct Id {
+        id: Url,
+    }
+    Ok(serde_json::from_slice::<Id>(data)?.id)
+}