fix: reject forged subdomains in the media proxy (#9)

* fix: make memcache concurrency-safe and stamp the version at link time

memcache was a crash waiting for traffic. Readers touched tempFS without
holding mx, while a per-entry goroutine deleted from it under the lock: a
concurrent map read and map write, which the runtime treats as a fatal error
that recover cannot catch. The option ships in config.example.json and was the
one cache key SETUP.md never documented, so it read like a free win to enable.

Put every map and field access behind the mutex, and age the whole map from one
janitor instead of a goroutine per cached file, each of which looped forever
holding its entry alive. mx is now a plain Mutex: every operation here mutates
something, and the old code took an RLock to write. Document the option, and
cover it with tests that run the readers, writers and janitor concurrently.

Split the disk/origin fetch out of DownloadAndSendMedia while there, so the
error path returns instead of falling through to write an empty body after the
error page.

Release.Version was hardcoded to 1.3.2, so images tagged v1.3.6 reported 1.3.2
from --help and /api/instance, and --help linked to the wrong release. Take it
from a main.version string the release workflow links in from the git tag.

* fix: reject forged subdomains in the media proxy

DownloadAndSendMedia built its upstream URL by concatenation, pasting the
subdomain segment of the request path straight into the host position. That
segment reaches the handler already percent-decoded, so it can carry "@", "#",
"?" and "/" — the characters that end a host. A request for

    /media/file/[email protected]:8080%2F/f/x.jpg

built a URL whose host parsed as 127.0.0.1:8080, with images-wixmp-x demoted to
userinfo, letting any caller aim the instance's fetcher at any address it could
reach, including services behind the firewall.

Validate the label against ^[a-zA-Z0-9-]+$ and refuse anything else with a 400.
Rejecting rather than escaping is what closes this: the label is the host, and
url.URL passes a host through verbatim, so building the URL structurally is not
sufficient on its own. DeviantArt's own media URLs use a hex-and-dash label, and
ParseMedia already splits on the first dot, so a legitimate label cannot contain
one.

Build the URL from url.URL fields as well, which escapes the path, and encode
the token argument, which reached the request unescaped.

Reported by CodeQL as go/request-forgery (CWE-918).

1 files changed, 145 insertions, 66 deletions

diff --git a/app/cache.go b/app/cache.go
index 91c9847..e4822db 100755
--- a/app/cache.go
+++ b/app/cache.go
@@ -6,7 +6,9 @@ import (
 	"crypto/sha1" //nolint:gosec // G505: SHA-1 is a cache-key hash here, not a security primitive
 	"encoding/hex"
 	"io"
+	"net/url"
 	"os"
+	"regexp"
 	"strings"
 	"sync"
 	"syscall"
@@ -18,90 +20,139 @@ type file struct {
 	Content []byte
 }
 
+// tempFS is the in-memory media cache, guarded by mx. A plain Mutex rather than
+// an RWMutex on purpose: every operation here mutates something (a read bumps
+// Score), and the previous code took an RLock to write, which is not exclusive.
 var tempFS = make(map[[20]byte]*file)
-var mx = &sync.RWMutex{}
+var mx sync.Mutex
+
+// memGet returns the cached body for key and raises its score so that popular
+// entries outlive the janitor, or nil when the entry is absent or still empty.
+func memGet(key [20]byte) []byte {
+	mx.Lock()
+	defer mx.Unlock()
+
+	f := tempFS[key]
+	if f == nil || f.Content == nil {
+		return nil
+	}
+	f.Score += 2
+	return f.Content
+}
+
+// memPut caches body under key. An empty body is not cached, so a failed fetch
+// cannot poison the cache with a zero-length image.
+func memPut(key [20]byte, body []byte) {
+	if len(body) == 0 {
+		return
+	}
+
+	mx.Lock()
+	defer mx.Unlock()
+	tempFS[key] = &file{Content: body}
+}
+
+// InitMemCacheJanitor ages the in-memory cache forever, dropping entries whose
+// score has run out. Run it in its own goroutine, once, and only when memcache
+// is enabled.
+//
+// One loop ages the whole map. The previous design started a goroutine per
+// cached file, each looping until its own entry was evicted, and each touching
+// the map without holding mx — a concurrent map read and write, which the Go
+// runtime treats as a fatal error that recover cannot catch.
+func InitMemCacheJanitor() {
+	for {
+		time.Sleep(1 * time.Minute)
+		ageMemCache()
+	}
+}
+
+// ageMemCache runs one round of aging: every entry loses a point, and entries
+// that are already out of points are dropped. An entry starts at zero, so a body
+// nothing asks for again is gone within a round.
+func ageMemCache() {
+	mx.Lock()
+	defer mx.Unlock()
+
+	for k, f := range tempFS {
+		if f.Score <= 0 {
+			delete(tempFS, k)
+			continue
+		}
+		f.Score--
+	}
+}
+
+// mediaSubdomain matches the one hostname label wixmp media URLs vary: a hex
+// string, sometimes with dashes. Anything outside that set is rejected rather
+// than escaped, because this label is what selects the host to fetch from.
+var mediaSubdomain = regexp.MustCompile(`^[a-zA-Z0-9-]+$`)
+
+// buildMediaURL returns the wixmp CDN URL for one media item, reporting false
+// when subdomain is not a bare hostname label.
+//
+// subdomain and path arrive already percent-decoded from the request path, so
+// they can carry the characters that end a host. Concatenated into a URL string,
+// a subdomain of "[email protected]#" reparses as host attacker.example, with
+// "images-wixmp-x" demoted to userinfo and the intended host to a fragment —
+// pointing the fetch at whatever the caller names, including addresses reachable
+// only from the instance itself.
+func buildMediaURL(subdomain, path, token string) (string, bool) {
+	if !mediaSubdomain.MatchString(subdomain) {
+		return "", false
+	}
+
+	// Fields rather than concatenation: String escapes the path, so a decoded
+	// "#" or "?" in it stays part of the path instead of ending it. The host is
+	// checked above rather than escaped, because url.URL passes it through
+	// verbatim.
+	u := url.URL{
+		Scheme: "https",
+		Host:   "images-wixmp-" + subdomain + ".wixmp.com",
+		Path:   "/" + path,
+	}
+	if token != "" {
+		u.RawQuery = url.Values{"token": {token}}.Encode()
+	}
+	return u.String(), true
+}
 
 // DownloadAndSendMedia proxies one image from DeviantArt's wixmp CDN to the
 // client, serving it from the on-disk or in-memory cache when enabled. It
 // responds 403 when proxying is turned off for this instance.
 func (s skunkyart) DownloadAndSendMedia(subdomain, path string) {
-	var url strings.Builder
-	url.WriteString("https://images-wixmp-")
-	url.WriteString(subdomain)
-	url.WriteString(".wixmp.com/")
-	url.WriteString(path)
-	if t := s.Args.Get("token"); t != "" {
-		url.WriteString("?token=")
-		url.WriteString(t)
+	mediaURL, ok := buildMediaURL(subdomain, path, s.Args.Get("token"))
+	if !ok {
+		s.ReturnHTTPError(400)
+		return
 	}
 
 	var response []byte
 
 	switch {
 	case CFG.Cache.Enabled:
-		fileName := sha1.Sum([]byte(subdomain + path)) //nolint:gosec // G401: cache-key hash, not a security primitive
-		filePath := CFG.Cache.Path + "/" + hex.EncodeToString(fileName[:])
-
-		c := func() {
-			// filePath is built from a SHA-1 of the request, not from user input,
-			// so it cannot escape the cache directory.
-			file, err := os.Open(filePath) //nolint:gosec // G304: path is a hash, not user-controlled
-			if err != nil {
-				dwnld := Download(url.String())
-				if dwnld.Status == 200 && strings.HasPrefix(dwnld.Headers.Get("Content-Type"), "image") {
-					response = dwnld.Body
-					try(os.WriteFile(filePath, response, 0600))
-				} else {
-					s.ReturnHTTPError(dwnld.Status)
-					return
-				}
-			} else {
-				defer func() { try(file.Close()) }()
-				file, e := io.ReadAll(file)
-				try(e)
-				response = file
-			}
-		}
+		key := sha1.Sum([]byte(subdomain + path)) //nolint:gosec // G401: cache-key hash, not a security primitive
+		filePath := CFG.Cache.Path + "/" + hex.EncodeToString(key[:])
 
 		if CFG.Cache.MemCache {
-			mx.Lock()
-			if tempFS[fileName] == nil {
-				tempFS[fileName] = &file{}
-			}
-			mx.Unlock()
-
-			if tempFS[fileName].Content != nil {
-				response = tempFS[fileName].Content
-				tempFS[fileName].Score += 2
+			if cached := memGet(key); cached != nil {
+				response = cached
 				break
-			} else {
-				c()
-				go func() {
-					defer restore()
-
-					mx.RLock()
-					tempFS[fileName].Content = response
-					mx.RUnlock()
-
-					for {
-						time.Sleep(1 * time.Minute)
-
-						mx.Lock()
-						if tempFS[fileName].Score <= 0 {
-							delete(tempFS, fileName)
-							mx.Unlock()
-							return
-						}
-						tempFS[fileName].Score--
-						mx.Unlock()
-					}
-				}()
 			}
-		} else {
-			c()
+		}
+
+		body, ok := s.loadOrFetchMedia(filePath, mediaURL)
+		if !ok {
+			// loadOrFetchMedia has already written the error response.
+			return
+		}
+		response = body
+
+		if CFG.Cache.MemCache {
+			memPut(key, response)
 		}
 	case CFG.Proxy:
-		dwnld := Download(url.String())
+		dwnld := Download(mediaURL)
 		if dwnld.Status != 200 {
 			s.ReturnHTTPError(dwnld.Status)
 			return
@@ -115,6 +166,34 @@ func (s skunkyart) DownloadAndSendMedia(subdomain, path string) {
 	_, _ = s.Writer.Write(response)
 }
 
+// loadOrFetchMedia returns the media body for filePath, preferring the on-disk
+// cache and falling back to fetching mediaURL, which it then writes back to the
+// cache. It reports false when it has already written an error response, so the
+// caller must not write anything further.
+func (s skunkyart) loadOrFetchMedia(filePath, mediaURL string) ([]byte, bool) {
+	// filePath is built from a SHA-1 of the request, not from user input, so it
+	// cannot escape the cache directory.
+	if f, err := os.Open(filePath); err == nil { //nolint:gosec // G304: path is a hash, not user-controlled
+		defer func() { try(f.Close()) }()
+
+		if body, err := io.ReadAll(f); err == nil {
+			return body, true
+		} else {
+			// An unreadable cache entry is not fatal; re-fetch it instead.
+			try(err)
+		}
+	}
+
+	dwnld := Download(mediaURL)
+	if dwnld.Status != 200 || !strings.HasPrefix(dwnld.Headers.Get("Content-Type"), "image") {
+		s.ReturnHTTPError(dwnld.Status)
+		return nil, false
+	}
+
+	try(os.WriteFile(filePath, dwnld.Body, 0600))
+	return dwnld.Body, true
+}
+
 // InitCacheSystem runs the cache rotation loop forever, evicting files past
 // their lifetime and emptying the cache when it outgrows max-size. Run it in its
 // own goroutine.