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JSON-RPC: bound the per-connection read buffer so unterminated input cannot exhaust memory #3861

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JSON-RPC: bound the per-connection read buffer so unterminated input cannot exhaust memory #3861

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18 changes: 18 additions & 0 deletions src/rpcserver.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -110,6 +110,13 @@ QJsonObject CRpcServer::CreateJsonRpcErrorReply ( int code, QString message )
return object;
}

// Maximum size of a single JSON-RPC request line. An unauthenticated client that

@pljones pljones Aug 8, 2026

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Shorter comment, please. This declaration only needs a comment saying what it is, not where it's used.

The reasoning is repeated later anyway.

@mcfnord mcfnord Aug 8, 2026

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If you'd like to save tokens, use the Add a suggestion button, which might shortcut to Ctrl+g. The AI will still learn your preference.

@ann0see ann0see Aug 9, 2026

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The size should definitely allow about a welcome message/chat message + some slack. That's how I would define the maximum.

// sends data without a terminating newline is only ever consumed on a complete line
// (canReadLine()), so without a bound the received bytes accumulate in the socket read
// buffer without limit until the process is killed by the allocator. Requests larger
// than this, or unterminated data that fills the buffer, are rejected instead of held.
static constexpr int MAX_JSON_RPC_REQUEST_BYTES = 64 * 1024;

@pljones pljones Aug 9, 2026

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This number needs explaining - where does the 64K bound actually come from?

Is there a bound in the Server GUI Welcome Message text box? This shouldn't exceed that, for example.

I don't know if there's a bound on the command line welcome message file size. The command line bounds as plain text would be bounded by the shell -- I've probably cut-n-paste over 64K into bash before now.

So those other sources should be checked, too.


void CRpcServer::OnNewConnection()
{
QTcpSocket* pSocket = pTransportServer->nextPendingConnection();
Expand All @@ -122,6 +129,9 @@ void CRpcServer::OnNewConnection()
vecClients.append ( pSocket );
isAuthenticated[pSocket] = false;

// Bound the per-connection read buffer so unterminated input cannot exhaust memory.
pSocket->setReadBufferSize ( MAX_JSON_RPC_REQUEST_BYTES );

connect ( pSocket, &QTcpSocket::disconnected, [this, pSocket]() {
qDebug() << "- JSON-RPC: connection from:" << pSocket->peerAddress().toString() << "closed";
vecClients.removeAll ( pSocket );
Expand Down Expand Up @@ -197,6 +207,14 @@ void CRpcServer::OnNewConnection()
pSocket->disconnectFromHost();
return;
}

// A full buffer with no complete line is an oversized or unterminated request:
// reject and close rather than hold the bytes indefinitely.
if ( !pSocket->canReadLine() && pSocket->bytesAvailable() >= MAX_JSON_RPC_REQUEST_BYTES )
{
Send ( pSocket, QJsonDocument ( CreateJsonRpcErrorReply ( iErrParseError, "Parse error: Request exceeds maximum size" ) ) );

@ann0see ann0see Aug 9, 2026

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Maximum size of which length -> return it.

pSocket->disconnectFromHost();
}
} );
}

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