diff --git a/src/FOSSA-Comms.cpp b/src/FOSSA-Comms.cpp
index b87215b..8dda65f 100644
--- a/src/FOSSA-Comms.cpp
+++ b/src/FOSSA-Comms.cpp
@@ -184,6 +184,14 @@ int16_t FCP_Get_OptData(char* callsign, uint8_t* frame, uint8_t frameLen, uint8_
       uint8_t optDataLen = *framePtr;
       framePtr += 1;
 
+      // check that the declared length matches the number of bytes actually
+      // present in the frame, otherwise a malformed length byte would cause
+      // the memcpy below to overrun the caller's optData buffer (same check
+      // as FCP_Get_OptData_Length)
+      if(optDataLen != (uint8_t)(frameLen - strlen(callsign) - 2)) {
+        return(ERR_LENGTH_MISMATCH);
+      }
+
       // get optional data
       memcpy(optData, framePtr, optDataLen);
       framePtr += optDataLen;