8.21 - Software Hazard Causes

1. Introduction

1.1 Software Hazard Causes

When a device or system can lead to injury, death, the destruction or loss of vital equipment, or damage to the environment, system safety is paramount.  The system safety discipline focuses on  “hazards” and the prevention of hazardous situations. 

A hazard is the presence of a potential risk situation that can result in or contribute to a mishap. To ensure the system being developed is as safe as possible, it is important to begin identifying potential hazards as early as possible in the development. Thus, the software and system safety personnel generally look at the hazardous events that could happen and what could potentially cause them.

Every hazard has at least one cause, which in turn can lead to several effects (e.g., damage, illness, failure). A hazard cause may be a defect in hardware or software, a human operator error, or an unexpected input or event which results in a hazard. The table below provides several potential software causes to consider in the project when developing the list of hazards and their potential causes.

Hazard control is a method for preventing the hazard, reducing the likelihood of the hazard occurring, or the reduction of the impact of that hazard.  Hazard controls use software (e.g. detection of the stuck valve and automatic response to open secondary valve), hardware (e.g. pressure relief valve), operator procedures, or a combination of methods to avert the hazard.  For every hazard cause, there must be at least one control method, usually a design feature (hardware and/or software) or a procedural step.

1.2 Table of Software Causes

Potential Software Causes to Consider When Identifying Software Causes in Hazard Analysis


Software Cause Areas to Consider Potential Software Causes
Data errors
  1. Asynchronous communications
  2. Single or double event upset/bit flip or hardware induced error
  3. Communication to/from an unexpected system on the network
  4. An out-of-range input value, a value above or below the range
  5. Start-up or hardware initiation data errors
  6. Data from an antenna gets corrupted
  7. Failure of software interface to memory
  8. Failure of flight software to suppress outputs from a failed component
  9. Failure of software to monitor bus controller rates to ensure communication with all remote terminals on the bus schedule's avionics buses
  10. Ground or onboard database error
  11. Interface error
  12. Latent data
  13. Communication bus overload
  14. Missing or failed integrity checks on inputs, failure to check the validity of input/output data
  15. Excessive network traffic/babbling node - keeps the network so busy it inhibits communication from other nodes
  16. Sensors or actuators stuck at some value
  17. Wrong software state for the input
Commanding errors1. Command buffer error or overflow
2. Corrupted software load
3. Error in real-time command build or sequence build
4. Failure to command during hazardous operations
5. Failure to perform prerequisite checks before the execution of safety-critical software commands
6. Ground or onboard database error for the command structure
7. Error in command data introduced by command server error
8. Incorrect operator input commands
9. Wrong command or a miscalculated command sent
10. Sequencing error, failure to issue commands in the correct sequence
11. Command sent in wrong software state or software in an incorrect or unanticipated state
12. An incorrect timestamp on the command
13. Missing software error handling on incorrect commands
14. Status messages on command execution not provided
15. Memory corruption, critical data variables overwritten in memory
16. Inconsistent syntax
17. Inconsistent command options
18. Similarly named commands
19. Inconsistent error handling rules
20. Incorrect automated command sequence built into script containing single commands that can remove multiple inhibits to a hazard
Flight computer errors1. Board support package software error
2. Boot load software error
3. Boot Programmable Read-Only Memory (PROM) corruption preventing reset
4. Buffer overrun
5. CPU overload
6. Cycle jitter
7. Cycle over-run
8. Deadlock
9. Livelock
10. Reset during program upload (PROM corruption)
11. Reset with no restart
12. Single or double event upset/bit flip or hardware induced error
13. Time to reset greater than time to failure
14. Unintended persistent data/configuration on reset
15. Watchdog active during reboot causing infinite boot loop
16. Watchdog failure
17. Failure to detect and transition to redundant or backup computer
18. Incorrect or stale data in redundant or backup computer
Operating systems errors1. Application software incompatibility with upgrades/patches to an operating system
2. Defects in Real-Time Operating System (RTOS) Board Support software
3. Missing or incorrect software error handling
4. Partitioning errors
5. Shared resource errors
6. Single or double event upset/bit flip
7. Unexpected operating system software response to user input
8. Excessive functionality
9. Missing function
10. Wrong function
11. Inadequate protection against operating system bugs
12. Unexpected and aberrant software behavior
Programmable logic device errors1. High cyclomatic complexity levels (above 15)
2. Errors in programming and simulation tools used for Programmable Logic Controller (PLC) development
3. Errors in the programmable logic device interfaces
4. Errors in the logic design
5. Missing software error handling in the logic design
6. PLC logic/sequence error
7. Single or double event upset/bit flip or hardware induced error
8. Timing errors
9. Unexpected operating system software response to user input
10. Excessive functionality
11. Missing function
12. Wrong function
13. Unexpected and aberrant software behavior
Flight system time management errors1. Incorrect data latency/sampling rates
2. Failure to terminate/complete process in a given time
3. Incorrect time sync
4. Latent data (Data delayed or not provided in required time) 5. Mission elapsed time timing issues and distribution
6. Incorrect function execution, performing a function at the wrong time, out of sequence, or when the program is in the wrong state
7. Race conditions
8. The software cannot respond to an off-nominal condition within the time needed to prevent a hazardous event
9. Time function runs fast/slow 10. Time skips (e.g., Global Positioning System time correction)
11. Loss or incorrect time sync across flight system components
12. Loss or incorrect time Synchronization between ground and spacecraft Interfaces
13. Unclear software timing requirements
14. Asynchronous systems or components
15. Deadlock conditions
16. Livelocks conditions
Coding, logic, and algorithm failures, algorithm specification errors1. Auto-coding errors as a cause
2. Bad configuration data/no checks on external input files and data
3. Division by zero
4. Wrong sign
5. Syntax errors
6. Error coding software algorithm
7. Error in positioning algorithm
8. Case/type/conversion error/unit mismatch
9. Buffer overflows
10. High cyclomatic complexity levels (above 15)
11. Dead code or unused code
12. Endless do loops
13. Erroneous outputs
14. Failure of flight computer software to transition to or operate in a correct mode or state
15. Failure to check safety-critical outputs for reasonableness and hazardous values and correct timing
16. Failure to generate a process error upon detection of arithmetic error (such as divide-by-zero)
17. Failure to create a software error log report when an unexpected event occurs
18. Inadvertent memory modification
19. Incorrect "if-then" and incorrect "else"
20. Missing default case in a switch statement
21. Incorrect implementation of a software change, software defect, or software non-conformance
22. Incorrect number of functions or mathematical iteration
23. Incorrect software operation if no commands are received or if a loss of commanding capability exists (inability to issue commands)
24. Insufficient or poor coding reviews, inadequate software peer reviews
25. Insufficient use of coding standards
26. Interface errors
27. Missing or inadequate static analysis checks on code
28. Missing or incorrect parameter range and boundary checking
29. Non-functional loops
30. Overflow or underflow in the calculation
31. Precision mismatch
32. Resource contention (e.g., thrashing: two or more processes accessing a shared resource)
33. Rounding or truncation fault
34. Sequencing error (e.g., failure to issue commands in the correct sequence)
35. Software is initialized to an unknown state; failure to properly initialize all system and local variables are upon startup, including clocks
36. Too many or too few parameters for the called function
37. Undefined or non-initialized data
38. Untested COTS, MOTS, or reused code
39. Incomplete end-to-end testing
40. Incomplete or missing software stress test
41. Errors in the data dictionary or data dictionary processes
42. Confusing feature names
43. More than one name for the same feature
44. Repeated code modules
45. Failure to initialize a loop-control
46. Failure to initialize (or reinitialize) pointers
47. Failure to initialize (or reinitialize) registers
48. Failure to clear a flag
49. Scalability errors
50. Unexpected new behavior or defects introduced in newer or updated COTS modules
51. Not addressing pointer closure
Fault tolerance and fault management errors1. Missing software error handling
2. Missing or incorrect fault detection logic
3. Missing or incorrect fault recovery logic
4. Problems with the execution of emergency safing operations
5. Failure to halt all hazard functions after an interlock failure
6. The software cannot respond to an off-nominal condition within the time needed to prevent a hazardous event
7. Common mode software faults
8. A hazard causal factor occurrence isn't detected
9. False positives in fault detection algorithms
10. Failure to perform prerequisite checks before the execution of safety-critical software commands
11. Failure to terminate/complete process in a given time
12. Memory corruption, critical data variables overwritten in memory
13. Single or double event upset/bit flip or hardware induced error
14. Incorrect interfaces, errors in interfaces
15. Missing self-test capabilities
16. Failing to consider stress on the hardware
17. Incomplete end-to-end testing
18. Incomplete or missing software stress test
19. Errors in the data dictionary or data dictionary processes
20. Failure to provide or ensure secure access for input data, commanding, and software modifications

Software process errors1. Failure to implement software development processes or implementing inadequate processes
2. Inadequate software assurance support and reviews
3. Missing or inadequate software assurance audits
4. Failure to follow the documented software development processes
5. Missing, tailored, or incomplete implementation of the safety-critical software requirements in NPR 7150.2
6. Missing, tailored, or incomplete implementation of the safety-critical software requirements in Space Station Program 50038, Computer-Based Control System Safety Requirements
7. Incorrect or incomplete testing
8. Inadequate testing of reused or heritage software
9. Failure to open a software problem report when an unexpected event occurs
10. Failure to include hardware personnel in reviews of software changes, software implementation, peer reviews, and software testing
11. Failure to perform a safety review on all software changes and software defects
12. Defects in COTS, MOTS, or OSS Software,
13. Failure to perform assessments of available bug fixes and updates available in COTS software
14. Insufficient use of coding standards
15. Missing or inadequate static analysis checks on code
16. Incorrect version loaded
17. Incorrect configuration values or data
18. No checks on external input files and data
19. Errors in configuration data changes being uploaded to spacecraft
20. Software/avionics simulator/emulator errors and defects
21. Unverified software
22. High cyclomatic complexity levels (over 15)
23. Incomplete or inadequate software requirements analysis
24. Compound software requirements
25. Incomplete or inadequate software hazard analysis
26. Incomplete or inadequate software safety analysis
27. Incomplete or inadequate software test data analysis
28. Unrecorded software defects found during informal and formal software testing
29. Auto-coding tool faults and defects
30. Errors in design models
31. Software errors in hardware simulators due to a lack of understanding of hardware requirements
32. Incomplete or inadequate software test data analysis
33. Inadequate built-in-test coverage
34. Inadequate regression testing and unit test coverage of flight software application-level source code
35. Failure to test all nominal and planned contingency scenarios (breakout and re-rendezvous, launch abort) and complete mission duration (launch to docking to splashdown) in the hardware in the loop environment
36. Incomplete testing of unexpected conditions, boundary conditions, and software/interface inputs
37. Use of persistence of test data, files, or config files in an operational scenario
38. Failure to provide multiple paths or triggers from safe states to hazardous states
39. Interface control documents and interface requirements documents errors
40. System requirements errors
41. Misunderstanding of hardware configuration and operation
42. Hardware requirements and interface errors, Incorrect description of the software/hardware functions and how they are to perform
43. Missing or incorrect software requirements or specifications
44. Missing software error handling
45. Requirements/design errors not fully defined, detected, and corrected)
46. Failure to identify the safety-critical software items
47. Failure to perform a function, performing the wrong function, performing the function incompletely
48. An inadvertent/unauthorized event, an unexpected, unwanted event, an out-of-sequence event, the failure of a planned event to occur
49. The magnitude or direction of an event is wrong
50. Out-of-sequence event protection
51. Multiple events/actions trigger simultaneously (when not expected)
52. Error or exception handling missing or incomplete
53. Inadvertent or incorrect mode transition for required vehicle functional operation; undefined or incorrect mode transition criteria; unauthorized mode transition
54. Failure of flight software to correctly initiate proper transition mode
55. Software state transition error
56. Software termination is an unknown state
57. Errors in the software data dictionary values
Human-machine interface errors1. Incorrect data (unit conversion, incorrect variable type)
2. Stale data
3. Poor design of human machine interface
4. Too much, too little, incorrect data displayed
5. Ambiguous or incorrect messages
6. User display locks up/fails
7. Missing software error handling
8. Unsolicited command (command issued inadvertently, cybersecurity issue, or without cause)
9. Wrong command or a miscalculated command sent
10. Failure to display information or messages to a user
11. Display refresh rate leads to an incorrect operator response
12. Lack of ordering scheme for hazardous event queues (such as alerts) in the human-computer interface (i.e., priority versus time of arrival, for example, when an abort must go to the top of the queue)
13. Incorrect labeling of operator controls in the human interface software
14. Failure to check for constraints in algorithms/specifications and valid boundaries
15. Failure of human interface software to check operator inputs
16. Failure to pass along information or messages
17. No onscreen instructions
18. Undocumented features
19. States that appear impossible to exit
20. No cursor
21. Failure to acknowledge an input
22. Failure to advise when a change takes effect
23. Wrong, misleading, or confusing information
24. Poor aesthetics in the screen layout
25. Menu layout errors
26. Dialog box layout errors
27. Obscured instructions
28. Misuse of color
29. Failure to allow tabbing navigation to edit fields (mouse only input)
Security and virus errors1. Denial or interruption of service
2. Spoofed or jammed inputs 3. Missing capabilities to detect insider threat activities
4. Inadvertent or intentional memory modification
5. Inadvertent or unplanned mode transition
6. Missing software error handling or detect handling
7. Unsolicited command
70 of 70
Software Cause Areas to Consider Potential Software Causes
8. Stack-based buffer overflows
9. Heap-based attacks
10. Cybersecurity vulnerability or computer virus
11. Inadvertent access to ground system software
12. Destruct commands incorrectly allowed in a hands-off zone
13. Communication to/from an unexpected system on the network
Unknown Unknowns errors1. Undetected software defects
2. Unknown limitations for COTS (operational, environmental, stress)
3. COTS extra capabilities
4. Incomplete or inadequate software safety analysis for COTS components
5. Compiler behavior errors or undefined compiler behavior
6. Software defects and investigations that are unresolved before the flight

See also Topic 8.04 - Additional Requirements Considerations for Use with Safety-Critical Software

Note:  Software is classified as safety-critical if the software is determined by and traceable to hazard analysis. See appendix A for guidelines associated with addressing software in hazard definitions. See SWE-205 - Determination of Safety-Critical Software. Consideration for other independent means of protection (software, hardware, barriers, or administrative) should be a part of the system hazard definition process. 

Note:  Fault tolerant systems are built to handle most probable, and some less probable but hazardous, faults. Taking care of the faults will usually help prevent the software, or the system, from going into failure.  The down-side to fault tolerance is that it requires multiple checks and monitoring at very low levels.  If a system is failure tolerant, it will ignore most faults and only respond to higher-level failures. A presumption is that it requires less work and is simpler to detect, isolate, stop, or recover from the failures. A project must weigh the costs and benefits of each approach and determine what will provide the most safety for the least cost and effort.

1.3 Additional Guidance

Links to Additional Guidance materials for this subject have been compiled in the Relevant Links table. Click here to see the Additional Guidance in the Resources tab.

2. Resources

2.1 References

2.2 Tools

Tools to aid in compliance with this SWE, if any, may be found in the Tools Library in the NASA Engineering Network (NEN). 

NASA users find this in the Tools Library in the Software Processes Across NASA (SPAN) site of the Software Engineering Community in NEN. 

The list is informational only and does not represent an “approved tool list”, nor does it represent an endorsement of any particular tool.  The purpose is to provide examples of tools being used across the Agency and to help projects and centers decide what tools to consider.

2.3 Additional Guidance

Additional guidance related to this requirement may be found in the following materials in this Handbook:

2.4 Center Process Asset Libraries

SPAN - Software Processes Across NASA
SPAN contains links to Center managed Process Asset Libraries. Consult these Process Asset Libraries (PALs) for Center-specific guidance including processes, forms, checklists, training, and templates related to Software Development. See SPAN in the Software Engineering Community of NEN. Available to NASA only.  197

See the following link(s) in SPAN for process assets from contributing Centers (NASA Only). 

SPAN Links

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