diff --git a/src/components/net/http_loader.rs b/src/components/net/http_loader.rs
index b1f6f6f8c78..55467fa573d 100644
--- a/src/components/net/http_loader.rs
+++ b/src/components/net/http_loader.rs
@@ -106,6 +106,20 @@ fn load(load_data: LoadData, start_chan: Sender<LoadResponse>) {
         if 3 == (response.status.code() / 100) {
             match response.headers.location {
                 Some(new_url) => {
+                    // CORS (http://fetch.spec.whatwg.org/#http-fetch, status section, point 9, 10)
+                    match load_data.cors {
+                        Some(ref c) => {
+                            if c.preflight {
+                                // The preflight lied
+                                send_error(url, "Preflight fetch inconsistent with main fetch".to_string(), start_chan);
+                                return;
+                            } else {
+                                // XXXManishearth There are some CORS-related steps here,
+                                // but they don't seem necessary until credentials are implemented
+                            }
+                        }
+                        _ => {}
+                    }
                     info!("redirecting to {:s}", new_url.serialize());
                     url = new_url;
                     continue;
diff --git a/src/components/net/resource_task.rs b/src/components/net/resource_task.rs
index 15ede7b28e3..8dd2f5d37db 100644
--- a/src/components/net/resource_task.rs
+++ b/src/components/net/resource_task.rs
@@ -32,7 +32,8 @@ pub struct LoadData {
     pub url: Url,
     pub method: Method,
     pub headers: RequestHeaderCollection,
-    pub data: Option<Vec<u8>>
+    pub data: Option<Vec<u8>>,
+    pub cors: Option<ResourceCORSData>
 }
 
 impl LoadData {
@@ -41,11 +42,20 @@ impl LoadData {
             url: url,
             method: Get,
             headers: RequestHeaderCollection::new(),
-            data: None
+            data: None,
+            cors: None
         }
     }
 }
 
+#[deriving(Clone)]
+pub struct ResourceCORSData {
+    /// CORS Preflight flag
+    pub preflight: bool,
+    /// Origin of CORS Request
+    pub origin: Url
+}
+
 /// Metadata about a loaded resource, such as is obtained from HTTP headers.
 pub struct Metadata {
     /// Final URL after redirects.
diff --git a/src/components/script/cors.rs b/src/components/script/cors.rs
new file mode 100644
index 00000000000..8cfca743808
--- /dev/null
+++ b/src/components/script/cors.rs
@@ -0,0 +1,418 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! A partial implementation of CORS
+//! For now this library is XHR-specific.
+//! For stuff involving <img>, <iframe>, <form>, etc please check what the request mode should be
+//! and compare with the fetch spec
+//! This library will eventually become the core of the Fetch crate
+//! with CORSRequest being expanded into FetchRequest (etc)
+
+use std::ascii::{StrAsciiExt, OwnedStrAsciiExt};
+use std::from_str::FromStr;
+use std::io::BufReader;
+use std::str::StrSlice;
+use time;
+use time::{now, Timespec};
+
+use ResponseHeaderCollection = http::headers::response::HeaderCollection;
+use RequestHeaderCollection = http::headers::request::HeaderCollection;
+use RequestHeader = http::headers::request::Header;
+
+use http::client::{RequestWriter, NetworkStream};
+use http::headers::{HeaderConvertible, HeaderEnum, HeaderValueByteIterator};
+use http::headers::content_type::MediaType;
+use http::headers::request::{Accept, AcceptLanguage, ContentLanguage, ContentType};
+use http::method::{Method, Get, Head, Post, Options};
+
+use url::{RelativeSchemeData, Url, UrlParser};
+
+#[deriving(Clone)]
+pub struct CORSRequest {
+    pub origin: Url,
+    pub destination: Url,
+    pub mode: RequestMode,
+    pub method: Method,
+    pub headers: RequestHeaderCollection,
+    /// CORS preflight flag (http://fetch.spec.whatwg.org/#concept-http-fetch)
+    /// Indicates that a CORS preflight request and/or cache check is to be performed
+    pub preflight_flag: bool
+}
+
+/// http://fetch.spec.whatwg.org/#concept-request-mode
+/// This only covers some of the request modes. The
+/// `same-origin` and `no CORS` modes are unnecessary for XHR.
+#[deriving(PartialEq, Clone)]
+pub enum RequestMode {
+    CORSMode, // CORS
+    ForcedPreflightMode // CORS-with-forced-preflight
+}
+
+impl CORSRequest {
+    /// Creates a CORS request if necessary. Will return an error when fetching is forbidden
+    pub fn maybe_new(referer: Url, destination: Url, mode: RequestMode,
+                     method: Method, headers: RequestHeaderCollection) -> Result<Option<CORSRequest>, ()> {
+        if referer.scheme == destination.scheme &&
+           referer.host() == destination.host() &&
+           referer.port() == destination.port() {
+            return Ok(None); // Not cross-origin, proceed with a normal fetch
+        }
+        match destination.scheme.as_slice() {
+            // Todo: If the request's same origin data url flag is set (which isn't the case for XHR)
+            // we can fetch a data URL normally. about:blank can also be fetched by XHR
+            "http" | "https" => {
+                let mut req = CORSRequest::new(referer, destination, mode, method, headers);
+                req.preflight_flag = (!is_simple_method(&req.method) || mode == ForcedPreflightMode);
+                if req.headers.iter().all(|h| is_simple_header(&h)) {
+                    req.preflight_flag = true;
+                }
+                Ok(Some(req))
+            },
+            _ => Err(()),
+        }
+    }
+
+    fn new(mut referer: Url, destination: Url, mode: RequestMode, method: Method,
+           headers: RequestHeaderCollection) -> CORSRequest {
+        match referer.scheme_data {
+            RelativeSchemeData(ref mut data) => data.path = vec!(),
+            _ => {}
+        };
+        referer.fragment = None;
+        referer.query = None;
+        CORSRequest {
+            origin: referer,
+            destination: destination,
+            mode: mode,
+            method: method,
+            headers: headers,
+            preflight_flag: false
+        }
+    }
+
+    /// http://fetch.spec.whatwg.org/#concept-http-fetch
+    /// This method assumes that the CORS flag is set
+    /// This does not perform the full HTTP fetch, rather it handles part of the CORS filtering
+    /// if self.mode is ForcedPreflightMode, then the CORS-with-forced-preflight
+    /// fetch flag is set as well
+    pub fn http_fetch(&self) -> CORSResponse {
+        let response = CORSResponse::new();
+        // Step 2: Handle service workers (unimplemented)
+        // Step 3
+        // Substep 1: Service workers (unimplemented )
+        // Substep 2
+        let cache = &mut CORSCache(vec!()); // XXXManishearth Should come from user agent
+        if self.preflight_flag &&
+           !cache.match_method(self, &self.method) &&
+           !self.headers.iter().all(|h| is_simple_header(&h) && cache.match_header(self, h.header_name().as_slice())) {
+            if !is_simple_method(&self.method) || self.mode == ForcedPreflightMode {
+                return self.preflight_fetch();
+                // Everything after this is part of XHR::fetch()
+                // Expect the organization of code to improve once we have a fetch crate
+            }
+        }
+        response
+    }
+
+    /// http://fetch.spec.whatwg.org/#cors-preflight-fetch
+    fn preflight_fetch(&self) -> CORSResponse {
+        let error = CORSResponse::new_error();
+        let mut cors_response = CORSResponse::new();
+
+        let mut preflight = self.clone(); // Step 1
+        preflight.method = Options; // Step 2
+        preflight.headers = RequestHeaderCollection::new(); // Step 3
+        // Step 4
+        preflight.insert_string_header("Access-Control-Request-Method".to_string(), self.method.http_value());
+
+        // Step 5 - 7
+        let mut header_names = vec!();
+        for header in self.headers.iter() {
+            header_names.push(header.header_name().into_ascii_lower());
+        }
+        header_names.sort();
+        let header_list = header_names.connect(", "); // 0x2C 0x20
+        preflight.insert_string_header("Access-Control-Request-Headers".to_string(), header_list);
+
+        // Step 8 unnecessary, we don't use the request body
+        // Step 9, 10 unnecessary, we're writing our own fetch code
+
+        // Step 11
+        let preflight_request = RequestWriter::<NetworkStream>::new(preflight.method, preflight.destination);
+        let mut writer = match preflight_request {
+            Ok(w) => box w,
+            Err(_) => return error
+        };
+
+        let host = writer.headers.host.clone();
+        writer.headers = box preflight.headers.clone();
+        writer.headers.host = host;
+        let response = match writer.read_response() {
+            Ok(r) => r,
+            Err(_) => return error
+        };
+
+        // Step 12
+        match response.status.code() {
+         200 .. 299 => {}
+         _ => return error
+        }
+        cors_response.headers = *response.headers.clone();
+        // Substeps 1-3 (parsing rules: http://fetch.spec.whatwg.org/#http-new-header-syntax)
+        fn find_header(headers: &ResponseHeaderCollection, name: &str) -> Option<String> {
+            headers.iter().find(|h| h.header_name().as_slice()
+                                                   .eq_ignore_ascii_case(name))
+                                    .map(|h| h.header_value())
+        }
+        let methods_string = match find_header(response.headers, "Access-Control-Allow-Methods") {
+            Some(s) => s,
+            _ => return error
+        };
+        let methods = methods_string.as_slice().split(',');
+        let headers_string = match find_header(response.headers, "Access-Control-Allow-Headers") {
+            Some(s) => s,
+            _ => return error
+        };
+        let headers = headers_string.as_slice().split(0x2Cu8 as char);
+        // The ABNF # rule will consider consecutive delimeters as a single delimeter
+        let mut methods: Vec<String> = methods.filter(|s| s.len() > 0).map(|s| s.to_string()).collect();
+        let headers: Vec<String> = headers.filter(|s| s.len() > 0).map(|s| s.to_string()).collect();
+        // Substep 4
+        if methods.len() == 0 || preflight.mode == ForcedPreflightMode {
+            methods = vec!(self.method.http_value());
+        }
+        // Substep 5
+        if !is_simple_method(&self.method) &&
+           !methods.iter().any(|ref m| self.method.http_value().as_slice().eq_ignore_ascii_case(m.as_slice())) {
+           return error;
+        }
+        // Substep 6
+        for h in self.headers.iter() {
+            if is_simple_header(&h) {
+                continue;
+            }
+            if !headers.iter().any(|ref h2| h.header_name().as_slice().eq_ignore_ascii_case(h2.as_slice())) {
+                return error;
+            }
+        }
+        // Substep 7, 8
+        let max_age: uint = find_header(response.headers, "Access-Control-Max-Age")
+                                .and_then(|h| FromStr::from_str(h.as_slice())).unwrap_or(0);
+        // Substep 9: Impose restrictions on max-age, if any (unimplemented)
+        // Substeps 10-12: Add a cache (partially implemented, XXXManishearth)
+        // This cache should come from the user agent, creating a new one here to check
+        // for compile time errors
+        let cache = &mut CORSCache(vec!());
+        for m in methods.iter() {
+            let maybe_method: Option<Method> = FromStr::from_str(m.as_slice());
+            maybe_method.map(|ref m| {
+                let cache_match = cache.match_method_and_update(self, m, max_age);
+                if !cache_match {
+                    cache.insert(CORSCacheEntry::new(self.origin.clone(), self.destination.clone(),
+                                                     max_age, false, MethodData(m.clone())));
+                }
+            });
+        }
+        for h in headers.iter() {
+            let cache_match = cache.match_header_and_update(self, h.as_slice(), max_age);
+            if !cache_match {
+                cache.insert(CORSCacheEntry::new(self.origin.clone(), self.destination.clone(),
+                                                 max_age, false, HeaderData(h.to_string())));
+            }
+        }
+        cors_response
+    }
+
+    fn insert_string_header(&mut self, name: String, value: String) {
+        let value_bytes = value.into_bytes();
+        let mut reader = BufReader::new(value_bytes.as_slice());
+        let maybe_header: Option<RequestHeader> = HeaderEnum::value_from_stream(
+                                                                String::from_str(name.as_slice()),
+                                                                &mut HeaderValueByteIterator::new(&mut reader));
+        self.headers.insert(maybe_header.unwrap());
+    }
+}
+
+
+pub struct CORSResponse {
+    pub network_error: bool,
+    pub headers: ResponseHeaderCollection
+}
+
+impl CORSResponse {
+    fn new() -> CORSResponse {
+        CORSResponse {
+            network_error: false,
+            headers: ResponseHeaderCollection::new()
+        }
+    }
+
+    fn new_error() -> CORSResponse {
+         CORSResponse {
+            network_error: true,
+            headers: ResponseHeaderCollection::new()
+        }
+    }
+}
+
+// CORS Cache stuff
+
+/// A CORS cache object. Anchor it somewhere to the user agent.
+#[deriving(Clone)]
+pub struct CORSCache(Vec<CORSCacheEntry>);
+
+/// Union type for CORS cache entries
+/// Each entry might pertain to a header or method
+#[deriving(Clone)]
+pub enum HeaderOrMethod {
+    HeaderData(String),
+    MethodData(Method)
+}
+
+impl HeaderOrMethod {
+    fn match_header(&self, header_name: &str) -> bool {
+        match *self {
+            HeaderData(ref s) => s.as_slice().eq_ignore_ascii_case(header_name),
+            _ => false
+        }
+    }
+
+    fn match_method(&self, method: &Method) -> bool {
+        match *self {
+            MethodData(ref m) => m == method,
+            _ => false
+        }
+    }
+}
+
+// An entry in the CORS cache
+#[deriving(Clone)]
+pub struct CORSCacheEntry {
+    pub origin: Url,
+    pub url: Url,
+    pub max_age: uint,
+    pub credentials: bool,
+    pub header_or_method: HeaderOrMethod,
+    created: Timespec
+}
+
+impl CORSCacheEntry {
+    fn new (origin:Url, url: Url, max_age: uint, credentials: bool, header_or_method: HeaderOrMethod) -> CORSCacheEntry {
+        CORSCacheEntry {
+            origin: origin,
+            url: url,
+            max_age: max_age,
+            credentials: credentials,
+            header_or_method: header_or_method,
+            created: time::now().to_timespec()
+        }
+    }
+}
+
+impl CORSCache {
+    /// http://fetch.spec.whatwg.org/#concept-cache-clear
+    fn clear (&mut self, request: &CORSRequest) {
+        let CORSCache(buf) = self.clone();
+        let new_buf: Vec<CORSCacheEntry> = buf.move_iter().filter(|e| e.origin == request.origin && request.destination == e.url).collect();
+        *self = CORSCache(new_buf);
+    }
+
+    // Remove old entries
+    fn cleanup(&mut self) {
+        let CORSCache(buf) = self.clone();
+        let now = time::now().to_timespec();
+        let new_buf: Vec<CORSCacheEntry> = buf.move_iter().filter(|e| now.sec > e.created.sec + e.max_age as i64).collect();
+        *self = CORSCache(new_buf);
+    }
+
+    /// http://fetch.spec.whatwg.org/#concept-cache-match-header
+    fn find_entry_by_header<'a>(&'a mut self, request: &CORSRequest, header_name: &str) -> Option<&'a mut CORSCacheEntry> {
+        self.cleanup();
+        let CORSCache(ref mut buf) = *self;
+        // Credentials are not yet implemented here
+        let entry = buf.mut_iter().find(|e| e.origin.scheme == request.origin.scheme &&
+                            e.origin.host() == request.origin.host() &&
+                            e.origin.port() == request.origin.port() &&
+                            e.url == request.destination &&
+                            e.header_or_method.match_header(header_name));
+        entry
+    }
+
+    fn match_header(&mut self, request: &CORSRequest, header_name: &str) -> bool {
+        self.find_entry_by_header(request, header_name).is_some()
+    }
+
+    fn match_header_and_update(&mut self, request: &CORSRequest, header_name: &str, new_max_age: uint) -> bool {
+        self.find_entry_by_header(request, header_name).map(|e| e.max_age = new_max_age).is_some()
+    }
+
+    fn find_entry_by_method<'a>(&'a mut self, request: &CORSRequest, method: &Method) -> Option<&'a mut CORSCacheEntry> {
+        // we can take the method from CORSRequest itself
+        self.cleanup();
+        let CORSCache(ref mut buf) = *self;
+        // Credentials are not yet implemented here
+        let entry = buf.mut_iter().find(|e| e.origin.scheme == request.origin.scheme &&
+                            e.origin.host() == request.origin.host() &&
+                            e.origin.port() == request.origin.port() &&
+                            e.url == request.destination &&
+                            e.header_or_method.match_method(method));
+        entry
+    }
+
+    /// http://fetch.spec.whatwg.org/#concept-cache-match-method
+    fn match_method(&mut self, request: &CORSRequest, method: &Method) -> bool {
+        self.find_entry_by_method(request, method).is_some()
+    }
+
+    fn match_method_and_update(&mut self, request: &CORSRequest, method: &Method, new_max_age: uint) -> bool {
+        self.find_entry_by_method(request, method).map(|e| e.max_age = new_max_age).is_some()
+    }
+
+    fn insert(&mut self, entry: CORSCacheEntry) {
+        self.cleanup();
+        let CORSCache(ref mut buf) = *self;
+        buf.push(entry);
+    }
+}
+
+fn is_simple_header(h: &RequestHeader) -> bool {
+    match *h {
+        Accept(_) | AcceptLanguage(_) | ContentLanguage(_) => true,
+        ContentType(MediaType {type_: ref t, subtype: ref s, ..}) => match (t.as_slice(), s.as_slice()) {
+            ("text", "plain") | ("application", "x-www-form-urlencoded") | ("multipart", "form-data") => true,
+            _ => false
+        },
+        _ => false
+    }
+}
+
+fn is_simple_method(m: &Method) -> bool {
+    match *m {
+        Get | Head | Post => true,
+        _ => false
+    }
+}
+
+/// Perform a CORS check on a header list and CORS request
+/// http://fetch.spec.whatwg.org/#cors-check
+pub fn allow_cross_origin_request(req: &CORSRequest, headers: &ResponseHeaderCollection) -> bool {
+    let allow_cross_origin_request =  headers.iter().find(|h| h.header_name()
+                                                               .as_slice()
+                                                               .eq_ignore_ascii_case("Access-Control-Allow-Origin"));
+    match allow_cross_origin_request {
+        Some(h) => {
+            let origin_str = h.header_value();
+            if origin_str == "*".to_string() {
+                return true; // Not always true, depends on credentials mode
+            }
+            match UrlParser::new().parse(origin_str.as_slice()) {
+                Ok(parsed) => parsed.scheme == req.origin.scheme &&
+                              parsed.host() == req.origin.host() &&
+                              parsed.port() == req.origin.port(),
+                _ => false
+            }
+        },
+        None => false
+    }
+}
diff --git a/src/components/script/dom/xmlhttprequest.rs b/src/components/script/dom/xmlhttprequest.rs
index d6813ed253b..de67dca73f1 100644
--- a/src/components/script/dom/xmlhttprequest.rs
+++ b/src/components/script/dom/xmlhttprequest.rs
@@ -42,7 +42,8 @@ use js::jsval::{JSVal, NullValue, UndefinedValue};
 use libc;
 use libc::c_void;
 
-use net::resource_task::{ResourceTask, Load, LoadData, Payload, Done};
+use net::resource_task::{ResourceTask, ResourceCORSData, Load, LoadData, Payload, Done};
+use cors::{allow_cross_origin_request, CORSRequest, CORSMode, ForcedPreflightMode};
 use script_task::{ScriptChan, XHRProgressMsg};
 use servo_util::str::DOMString;
 use servo_util::task::spawn_named;
@@ -180,7 +181,8 @@ impl XMLHttpRequest {
     }
 
     fn fetch(fetch_type: &SyncOrAsync, resource_task: ResourceTask,
-             load_data: LoadData, terminate_receiver: Receiver<Error>) -> ErrorResult {
+             mut load_data: LoadData, terminate_receiver: Receiver<Error>,
+             cors_request: Result<Option<CORSRequest>,()>) -> ErrorResult {
         fn notify_partial_progress(fetch_type: &SyncOrAsync, msg: XHRProgress) {
             match *fetch_type {
                 Sync(ref xhr) => {
@@ -193,6 +195,22 @@ impl XMLHttpRequest {
             }
         }
 
+        match cors_request {
+            Err(_) => return Err(Network), // Happens in case of cross-origin non-http URIs
+            Ok(Some(ref req)) => {
+                let response = req.http_fetch();
+                if response.network_error {
+                    return Err(Network)
+                } else {
+                    load_data.cors = Some(ResourceCORSData {
+                        preflight: req.preflight_flag,
+                        origin: req.origin.clone()
+                    })
+                }
+            },
+            _ => {}
+        }
+
         // Step 10, 13
         let (start_chan, start_port) = channel();
         resource_task.send(Load(load_data, start_chan));
@@ -201,6 +219,17 @@ impl XMLHttpRequest {
             Ok(e) => return Err(e),
             _ => {}
         }
+        match cors_request {
+            Ok(Some(ref req)) => {
+                    match response.metadata.headers {
+                        Some(ref h) if allow_cross_origin_request(req, h) => {},
+                        _ => return Err(Network)
+                    }
+            },
+            _ => {}
+        }
+        // XXXManishearth Clear cache entries in case of a network error
+
         notify_partial_progress(fetch_type, HeadersReceivedMsg(
             response.metadata.headers.clone(), response.metadata.status.clone()));
         let mut buf = vec!();
@@ -514,23 +543,48 @@ impl<'a> XMLHttpRequestMethods<'a> for JSRef<'a, XMLHttpRequest> {
             request_headers.borrow_mut().accept = Some(String::from_str("*/*"))
         }
 
-        // XXXManishearth this is to be replaced with Origin for CORS (with no path)
-        let referer_url = self.global.root().root_ref().get_url();
-        self.request_headers.deref().borrow_mut().referer =
-            Some(referer_url.serialize_no_fragment());
-
         load_data.headers = (*self.request_headers.deref().borrow()).clone();
         load_data.method = (*self.request_method.deref().borrow()).clone();
         let (terminate_sender, terminate_receiver) = channel();
         *self.terminate_sender.deref().borrow_mut() = Some(terminate_sender);
+
+        // CORS stuff
+        let referer_url = self.global.root().root_ref().get_url();
+        let mode = if self.upload_events.deref().get() {
+            ForcedPreflightMode
+        } else {
+            CORSMode
+        };
+        let cors_request = CORSRequest::maybe_new(referer_url.clone(), load_data.url.clone(), mode,
+                                                  load_data.method.clone(), load_data.headers.clone());
+        match cors_request {
+            Ok(None) => {
+                let mut buf = String::new();
+                buf.push_str(referer_url.scheme.as_slice());
+                buf.push_str("://".as_slice());
+                referer_url.serialize_host().map(|ref h| buf.push_str(h.as_slice()));
+                referer_url.port().as_ref().map(|&p| {
+                    buf.push_str(":".as_slice());
+                    buf.push_str(p);
+                });
+                referer_url.serialize_path().map(|ref h| buf.push_str(h.as_slice()));
+                self.request_headers.deref().borrow_mut().referer = Some(buf);
+            },
+            Ok(Some(ref req)) => self.insert_trusted_header("origin".to_string(),
+                                                            format!("{}", req.origin)),
+            _ => {}
+        }
+
         if self.sync.deref().get() {
-            return XMLHttpRequest::fetch(&mut Sync(self), resource_task, load_data, terminate_receiver);
+            return XMLHttpRequest::fetch(&mut Sync(self), resource_task, load_data,
+                                         terminate_receiver, cors_request);
         } else {
             let builder = TaskBuilder::new().named("XHRTask");
             self.fetch_time.deref().set(time::now().to_timespec().sec);
             let script_chan = global.root_ref().script_chan().clone();
             builder.spawn(proc() {
-                let _ = XMLHttpRequest::fetch(&mut Async(addr.unwrap(), script_chan), resource_task, load_data, terminate_receiver);
+                let _ = XMLHttpRequest::fetch(&mut Async(addr.unwrap(), script_chan),
+                                              resource_task, load_data, terminate_receiver, cors_request);
             });
             let timeout = self.timeout.deref().get();
             if timeout > 0 {
diff --git a/src/components/script/script.rs b/src/components/script/script.rs
index 9354ccaab15..7da28383687 100644
--- a/src/components/script/script.rs
+++ b/src/components/script/script.rs
@@ -42,6 +42,8 @@ extern crate sync;
 extern crate servo_msg = "msg";
 extern crate url = "url_";
 
+
+pub mod cors;
 pub mod dom {
     pub mod bindings {
         pub mod global;