From d5a52d9b3edaae6c273b732456d98e6b28ed7b31 Mon Sep 17 00:00:00 2001
From: Gavin Andresen <gavinandresen@gmail.com>
Date: Sun, 26 Aug 2012 17:08:18 -0400
Subject: [PATCH] Alert system DoS prevention

This fixes two alert system vulnerabilities found by
Sergio Lerner; you could send peers unlimited numbers
of invalid alert message to try to either fill up their
debug.log with messages and/or keep their CPU busy
checking signatures.

Fixed by disconnecting/banning peers if they send 10 or more
bad (invalid/expired/cancelled) alerts.
---
 src/main.cpp | 25 +++++++++++++++++++------
 src/main.h   |  2 +-
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index f71cfe7c..e9bd6103 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -2997,14 +2997,27 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         CAlert alert;
         vRecv >> alert;
 
-        if (alert.ProcessAlert())
+        uint256 alertHash = alert.GetHash();
+        if (pfrom->setKnown.count(alertHash) == 0)
         {
-            // Relay
-            pfrom->setKnown.insert(alert.GetHash());
+            if (alert.ProcessAlert())
             {
-                LOCK(cs_vNodes);
-                BOOST_FOREACH(CNode* pnode, vNodes)
-                    alert.RelayTo(pnode);
+                // Relay
+                pfrom->setKnown.insert(alertHash);
+                {
+                    LOCK(cs_vNodes);
+                    BOOST_FOREACH(CNode* pnode, vNodes)
+                        alert.RelayTo(pnode);
+                }
+            }
+            else {
+                // Small DoS penalty so peers that send us lots of
+                // duplicate/expired/invalid-signature/whatever alerts
+                // eventually get banned.
+                // This isn't a Misbehaving(100) (immediate ban) because the
+                // peer might be an older or different implementation with
+                // a different signature key, etc.
+                pfrom->Misbehaving(10);
             }
         }
     }
diff --git a/src/main.h b/src/main.h
index cbc48e05..dc9f9ee7 100644
--- a/src/main.h
+++ b/src/main.h
@@ -1535,7 +1535,7 @@ public:
 
     uint256 GetHash() const
     {
-        return SerializeHash(*this);
+        return Hash(this->vchMsg.begin(), this->vchMsg.end());
     }
 
     bool IsInEffect() const