1 files changed, 21 insertions, 1 deletions

diff --git a/src/compactblock.cpp b/src/compactblock.cpp
index 3999ab7..1ca6118 100644
--- a/src/compactblock.cpp
+++ b/src/compactblock.cpp
@@ -48,8 +48,16 @@ bool BuildCompactBlock(const CBlock& block, CCmpctBlock& out)
     out.nBits          = block.nBits;
     out.nNonce         = block.nNonce;
 
+    // Upstream ignored the RAND_bytes return value here. On failure nonceBytes
+    // is left uninitialised and an indeterminate shortIdNonce goes out on the
+    // wire -- which is precisely the value this file relies on being
+    // unpredictable to stop short IDs being precomputed or ground against.
+    // Fail closed instead, matching every other RAND_bytes call site in this
+    // tree (bip32.cpp, db.cpp): the caller falls back to full-block relay,
+    // which is always correct.
     unsigned char nonceBytes[sizeof(uint64)];
-    RAND_bytes(nonceBytes, sizeof(nonceBytes));
+    if (RAND_bytes(nonceBytes, sizeof(nonceBytes)) != 1)
+        return false;
     memcpy(&out.shortIdNonce, nonceBytes, sizeof(uint64));
 
     out.vPrefilledIndex.clear();
@@ -75,6 +83,18 @@ bool TryReconstructBlock(const CCmpctBlock& cmpct, CBlock& outBlock, std::vector
     if (cmpct.vPrefilledIndex.size() != cmpct.vPrefilledTx.size() || nTotal == 0)
         return false; // malformed
 
+    // nTotal comes straight off the wire once this is wired into the P2P
+    // layer, and serialize.h only bounds a vector length by MAX_SIZE
+    // (0x02000000), so an unbounded assign() below would let one 60-odd-byte
+    // message ask this node for ~33M default-constructed CTransactions. The
+    // smallest a transaction can serialise to is 10 bytes (nVersion, empty
+    // vin, empty vout, nLockTime), so no block within MAX_BLOCK_SIZE can hold
+    // more than MAX_BLOCK_SIZE/10 of them; anything above that is not a block
+    // we could ever accept. Rejecting here just falls back to full-block
+    // relay, which is always correct.
+    if (nTotal > MAX_BLOCK_SIZE / 10)
+        return false; // implausible tx count -- refuse to size a vector from it
+
     outBlock.SetNull();
     outBlock.nVersion      = cmpct.nVersion;
     outBlock.hashPrevBlock  = cmpct.hashPrevBlock;