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Qualifier Challenge - NRFIN_00035

Original Versions

Known Vulnerabilities

  • CWE-128 - Wrap-around Error
  • CWE-129 - Improper Validation of Array Index
  • CWE-190 - Integer Overflow or Wraparound
  • CWE-20 - Improper Input Validation
  • CWE-788 - Access of Memory Location After End of Buffer
  • CWE-805 - Buffer Access with Incorrect Length Value
  • CWE-824 - Access of Uninitialized Pointer
  • CWEs are listed as indicated by the challenge author.


  • ForAllSecure: 4.0
  • Lekkertech: 4.0
  • Shellphish: 3.7
  • Disekt: 0.96
  • CodeJitsu: 0.93
  • CSDS: 0.85
  • DeepRed: 0.27
  • TECHx: 0.0
  • FuzzBOMB: 0.0
  • TrailofBits: 0.0
  • The maximum score for each challenge in CQE is 4, following the CQE Scoring Document.

Passed consensus evaluation

  • CodeJitsu - CB1
  • CSDS - CB1
  • DeepRed - CB1
  • Disekt - CB1
  • ForAllSecure - CB1
  • Lekkertech - CB1
  • Shellphish - CB1
  • TrailofBits - CB1

Proved a POV in reference challenge

Defense against reference POVs

  • CodeJitsu: 100.0% - CB1
  • Disekt: 100.0% - CB1
  • ForAllSecure: 100.0% - CB1
  • Lekkertech: 100.0% - CB1
  • Shellphish: 100.0% - CB1
  • CSDS: 100.0% - CB1
  • TrailofBits: 100.0% - CB1
  • DeepRed: 100.0% - CB1

No submission

  • Eighth Place Team
  • Eleventh Place Team
  • Fifth Place Team - Finalist
  • First Place Team - Finalist
  • Fourth Place Team - Finalist
  • Ninth Place Team
  • Second Place Team - Finalist
  • Seventh Place Team - Finalist
  • Sixth Place Team - Finalist
  • Tenth Place Team
  • Third Place Team - Finalist
  • Thirteenth Place Team
  • Twelfth Place Team

All Submissions

Author Information

"Nick Davis"

DARPA performer group

Narf Industries (NRFIN)


H20 Flow Inc brings you the hottest technology in programmable logic controllers (PLCs) used in the water distribution industry. We know how important it is for your water distrubition PLCs to do exactly what you tell them to do and nothing more. H2FLOW is our latest generation water flow valve controller.

Feature List

The H2FLOW CB is a simple flow controller that manages the valves for the fill lines of several water tanks, based on commands provided by the intelligent PLC (the remote user).

Each water tank has a defined capacity. Each tank also has a minimum fill level. If the fill level drops below the minimum fill level, an emergency circuit will activate the backup fill pump until the level returns to the minimum fill level. If the fill level drops below 5 units of volume, a critical fill level alarm is raised.

Each water tank has a random rate that water is flowing out of it (drain rate). This simulates periodic water usage by an end user or device.

There are a fixed number of tanks, their ID numbers are sequential starting from 0, the capacities are defined by the TANK_CAP value, and initial fill level is defined by the INIT_LEVEL value.

To simulate time and water flowing in and out of tanks, H2FLOW uses a sleep timer. During the sleep timer, the service is simulating execution of all received commands and the filling/emptying of tanks.

Between sleep timers, the service will iterate through the tanks. For each tank, it will send the remote user the tank status and then accept a command to apply to the valves for that tank.

During each iteration, the drain and fill functions will remove water from tanks based on a random outflow rate and add water to tanks based on valve position and a fixed inflow rate. Finally, each tank will be tested for under fill.

H2FLOW has a very simple protocol. It accepts the following commands: - OPEN_VALVE : open fill valve - CLOSE_VALVE : close fill valve - NO_CHANGE : no valve changes - SET_IN_SERVICE : for a tank that is OUT_OF_SERVICE, set the fill rate and allow tank to supply water to users - SET_OUT_OF_SERVICE : close input valve, set fill_rate and drain_rate to 0 - SET_END_OF_LIFE : same as SET_OUT_OF_SERVICE, but is permanant


Two things need to happen to trigger the POV.

First, a tank needs to be filled beyond capacity. Due to the tank level being an unsigned char, when it reach 255, the next unit of H2O added will cause the value to overflow to 0. The function tank.c:add_water() does not check the bounds on t->level to prevent overflow.

Second, after both the combined fill and drain operation, the level of the tank must be in the range [0-5]. This will ensure the test for critically low level at operation.c:88 returns TRUE. Note, the update_water_levels function (drain and fill operations) is run before the low level tests are run. Thus, it is required for the drain + fill to cause a net int overflow. Also, it is possible for the int overflow to overflow to a level greater than 5, resulting in a value that is not in the range [0-5], and thus will not trigger the critical low level alert.

When the syslog function is called in operation.c:93, the tank number variable access has a coding mistake where the array index is the TANK_QTY, instead of idx, which causes an out of bounds access and a null pointer dereference. This will trigger SIGSEGV.

Generic class of vulnerability

Null pointer dereference

Improper checking of loop invariant

Improper array bounds checking

Integer Overflow

Access memory beyond buffer

CWE classification

Improper Input Validation CWE-20

Wrap-around Error CWE-128

Improper Validation of Array Index CWE-129

Integer Overflow or Wraparound CWE-190

Access of Memory Location After End of Buffer CWE-788

Buffer Access with Incorrect Length Value CWE-805

Access of Uninitialized Pointer CWE-824


  • The protocol is pretty simple, but the first hurdle is to know that the tanks all start in the OUT_OF_SERVICE status. So, continued communication with the CB requires the first commands to be SET_IN_SERVICE to place tanks in service. After that, some of the other commands will work. A dumb random fuzzer will have problems with this because the order of commands used on a given tank matters.

  • To make the POV harder to exploit, the sleep() function is run once per iteration through the tanks. This makes the POV harder to exploit, because sending random commands will eventually trigger the poller timeout before it will have enough time to overflow the tank level and trigger the POV. The sleep value was chosen specifically to make so it is only possible to overflow the tank level if sending the first command as SET_IN_SERVICE followed by OPEN_VALVE for every subsequent iteration. Other sequences have a high probability of ending due to timeout before they can fill the tank. To get N iterations in 15 seconds, want a sleep of 15/N. In testing we found the following results: with fill_rate 10, it takes 39 iterations -> sleep 15/39 = .385s with fill_rate 11, it takes 33 iterations -> sleep 15/33 = .454s will fill_rate 12, it takes 26 iterations -> sleep 15/26 = .577s with fill_rate 15, it takes 17 iterations -> sleep 15/17 = .882s

  • Additionally, to make the POV harder to reach by fuzzing, we added commands that place the tank either out-of-service or in an end-of-life state. When the tank is in the end-of-life state it can no longer be brought back into service. When the tank is in the out-of-service state, it requires the SET_IN_SERVICE command to bring it back into service.

  • One more thing that makes this more challenging is the lack of for-release pollers. This series of challenges should help to show distinctions between how CRS's use fuzzing.

Curated by Lunge Technology, LLC. Questions or comments? Send us email