diff --git a/src/plumb/ninep.cpp b/src/plumb/ninep.cpp
index abbbcda..163ed1a 100644
--- a/src/plumb/ninep.cpp
+++ b/src/plumb/ninep.cpp
@@ -184,6 +184,15 @@ bool NineP::recvMsg(uint8_t *type, uint16_t *tag, QByteArray *body)
         m_error = QStringLiteral("9P message too short (%1)").arg(size);
         return false;
     }
+    // The negotiated msize is the hard ceiling for any single 9P message. A
+    // larger size[4] means a desynced or corrupt stream; trusting it would make
+    // the b.resize()/readExactly() below allocate and write a bogus (possibly
+    // multi-gigabyte, or negative-when-cast-to-int) buffer, corrupting the heap.
+    // Reject it instead of crashing.
+    if (size > m_msize) {
+        m_error = QStringLiteral("9P message too large (%1 > msize %2)").arg(size).arg(m_msize);
+        return false;
+    }
     *type = uint8_t(hdr[4]);
     *tag = uint16_t(uint8_t(hdr[5]) | (uint16_t(uint8_t(hdr[6])) << 8));
     const int bodyLen = int(size) - 7;