[Server] Parse the rightmost address from forwarded header lists (#7586)

X-Forwarded-For carries a comma-separated list, and constructing a
QHostAddress from the raw header value returns a null address for it, so a
configured X-Forwarded-For or a multi-hop header never resolved and websocket
clients stayed on the proxy's address.

- Add parseForwardedAddress, returning the rightmost usable entry of a
  forwarded header list
- Use it for the websocket handshake instead of parsing the raw value
- Exempt loopback from per-address auth rate limits so a same-host proxy with a
  customized trusted_sources list cannot lock every user out
- Add forwarded_address_test

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/src/forwarded_address.cpp

@@ -0,0 +1,21 @@
+#include "forwarded_address.h"
+
+#include <QString>
+
+QHostAddress parseForwardedAddress(const QByteArray &headerValue)
+{
+    const QList<QByteArray> entries = headerValue.split(',');
+    for (auto it = entries.crbegin(); it != entries.crend(); ++it) {
+        const QByteArray entry = it->trimmed();
+        if (entry.isEmpty()) {
+            continue;
+        }
+
+        const QHostAddress parsed(QString::fromLatin1(entry));
+        if (!parsed.isNull()) {
+            return parsed;
+        }
+    }
+
+    return {};
+}

servatrice/src/forwarded_address.h

@@ -0,0 +1,18 @@
+#ifndef FORWARDED_ADDRESS_H
+#define FORWARDED_ADDRESS_H
+
+#include <QByteArray>
+#include <QHostAddress>
+
+/**
+ * @brief Parses the real client address out of a forwarded header value (X-Real-IP, X-Forwarded-For).
+ *
+ * A comma-separated list resolves to its rightmost usable entry. That entry was appended by the
+ * proxy closest to the server and is the only one a direct client cannot choose, since the earlier
+ * entries are passed through verbatim from whatever the client sent.
+ *
+ * @return the parsed address, or a null QHostAddress when the value holds none.
+ */
+QHostAddress parseForwardedAddress(const QByteArray &headerValue);
+
+#endif

servatrice/src/servatrice.cpp

@@ -1551,8 +1551,11 @@ bool Servatrice::isRateLimitExempt(const QString &ipAddress) const
 {
     // Mirrors the max_users_per_address bypass in initSession: trusted sources
     // (e.g. 127.0.0.1 for Webatrice behind local nginx, where every websocket
-    // client shares one address) are not subject to per-address limits.
-    return isTrustedSource(getTrustedSources(), ipAddress);
+    // client shares one address) are not subject to per-address limits. Loopback
+    // is always exempt, so a same-host proxy whose forwarded address could not be
+    // resolved cannot lock every user out together when trusted_sources is
+    // customized.
+    return QHostAddress(ipAddress).isLoopback() || isTrustedSource(getTrustedSources(), ipAddress);
 }
 
 void Servatrice::clearFailedLogins(const QString &ipAddress, const QString &userName)

servatrice/src/serversocketinterface.cpp

@@ -22,6 +22,7 @@
 
 #include "deck_tag_serialization.h"
 #include "email_parser.h"
+#include "forwarded_address.h"
 #include "libcockatrice/protocol/pb/admin_commands.pb.h"
 #include "libcockatrice/protocol/pb/developer_commands.pb.h"
 #include "libcockatrice/protocol/pb/moderator_commands.pb.h"
@@ -4560,8 +4561,7 @@ void WebsocketServerSocketInterface::initConnection(void *_socket)
         // Only trust the forwarded header if the connection itself comes from a trusted proxy,
         // otherwise a client could spoof its address to bypass bans and rate limits.
         if (isTrustedSource(servatrice->getTrustedSources(), socket->peerAddress())) {
-            QString header(socket->request().rawHeader(websocketIPHeader));
-            QHostAddress parsed(header);
+            const QHostAddress parsed = parseForwardedAddress(socket->request().rawHeader(websocketIPHeader));
             if (!parsed.isNull()) {
                 address = parsed;
             }