ISA99 ICSSCADA Practice Questions (Page 10)

Questions 46–47

1. 46expert · hard

   A refinery is conducting an ISA99 risk assessment for its crude unit. The crude unit has a DCS that is connected to a remote access server for vendor support. The remote access server is in a demilitarized zone (DMZ) and requires two-factor authentication. The risk assessment identifies that a successful attack on the DCS could cause a fire, but the likelihood is considered low because of the existing controls. The target security level (SL-T) for the crude unit zone is SL 2. The current security level (SL-C) is assessed as SL 1 because the DCS does not have application whitelisting. The team is considering two countermeasures: (A) implement application whitelisting on the DCS, or (B) remove the remote access server and require vendors to be on-site. Which countermeasure is most aligned with the ISA99 risk assessment methodology?

   • ARemove the remote access server because it eliminates the primary attack vector and reduces the likelihood of a successful attack.
   • BRe-evaluate the SL-T because the current controls already reduce the likelihood, so SL 2 may not be required.
   • CImplement both countermeasures because the risk is high due to the potential for a fire.
   • DImplement application whitelisting on the DCS because it directly raises the SL-C to SL 2.
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2. 47application · medium

   A water treatment plant is implementing ISA99 guidance. They have a limited budget and need to prioritize security improvements. The plant has a single control network with no segmentation, and remote access is provided via a VPN. Which improvement should be prioritized according to ISA99's risk-based approach?

   • ADisable the VPN to eliminate remote access risk.
   • BImplement a complex password policy for all users.
   • CReplace all legacy PLCs with modern ones that support encryption.
   • DSegment the control network into zones and conduits to limit the spread of an attack.
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