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Rule 09. Input Output (FIO)

Created by Jeffrey Gennari, last modified by David Svoboda on May 06, 2025

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Information for Editors
To have a new guideline automatically listed above be sure to label it fio and rule.

Risk Assessment Summary

Rule	Severity	Likelihood	Detectable	Repairable	Priority	Level
FIO30-C	High	Likely	Yes	No	P18	L1
FIO32-C	Medium	Unlikely	No	No	P2	L3
FIO34-C	High	Probable	Yes	Yes	P18	L1
FIO37-C	High	Probable	Yes	Yes	P18	L1
FIO38-C	Low	Probable	Yes	No	P4	L3
FIO39-C	Low	Likely	Yes	No	P6	L2
FIO40-C	Low	Probable	Yes	Yes	P6	L2
FIO41-C	Low	Unlikely	Yes	Yes	P3	L3
FIO42-C	Medium	Unlikely	No	No	P2	L3
FIO44-C	Medium	Unlikely	No	No	P2	L3
FIO45-C	High	Probable	No	No	P6	L2
FIO46-C	Medium	Unlikely	No	No	P2	L3
FIO47-C	High	Unlikely	Yes	No	P6	L2

Related Rules and Recommendations

Avoid having unreachable code

Copy of Rule Template

DRD00. Do not store sensitive information on external storage (SD card) unless encrypted first

DRD04-J. Do not log sensitive information

DRD11. Ensure that sensitive data is kept secure

DRD12. Do not trust data that is world writable

DRD22. Do not cache sensitive information

DRD23. Do not use world readable or writeable to share files between apps

DRD25. Use constant-time encryption

DRD27-J. For OAuth, use an explicit intent method to deliver access tokens

FIO00-J. Do not operate on files in shared directories

FIO00-PL. Do not use bareword file handles

FIO01-C. Be careful using functions that use file names for identification

FIO01-J. Create files with appropriate access permissions

FIO01-PL. Do not operate on files that can be modified by untrusted users

FIO02-C. Canonicalize path names originating from tainted sources

FIO02-J. Detect and handle file-related errors

FIO03-C. Do not make assumptions about fopen() and file creation

FIO03-J. Remove temporary files before termination

FIO04-J. Release resources when they are no longer needed

FIO05-C. Identify files using multiple file attributes

FIO05-J. Do not expose buffers or their backing arrays methods to untrusted code

FIO06-C. Create files with appropriate access permissions

FIO06-J. Do not create multiple buffered wrappers on a single byte or character stream

FIO07-J. Do not let external processes block on IO buffers

FIO08-C. Take care when calling remove() on an open file

FIO08-J. Distinguish between characters or bytes read from a stream and -1

FIO09-C. Be careful with binary data when transferring data across systems

FIO09-J. Do not rely on the write() method to output integers outside the range 0 to 255

FIO10-C. Take care when using the rename() function

FIO10-J. Ensure the array is filled when using read() to fill an array

FIO11-C. Take care when specifying the mode parameter of fopen()

FIO11-J. Do not convert between strings and bytes without specifying a valid character encoding

FIO12-J. Provide methods to read and write little-endian data

FIO13-C. Never push back anything other than one read character

FIO13-J. Do not log sensitive information outside a trust boundary

FIO14-C. Understand the difference between text mode and binary mode with file streams

FIO14-J. Perform proper cleanup at program termination

FIO15-C. Ensure that file operations are performed in a secure directory

FIO15-J. Do not reset a servlet's output stream after committing it

FIO16-J. Canonicalize path names before validating them

FIO17-C. Do not rely on an ending null character when using fread()

FIO18-C. Never expect fwrite() to terminate the writing process at a null character

FIO19-C. Do not use fseek() and ftell() to compute the size of a regular file

FIO20-C. Avoid unintentional truncation when using fgets() or fgetws()

FIO21-C. Do not create temporary files in shared directories

FIO22-C. Close files before spawning processes

FIO23-C. Do not exit with unflushed data in stdout or stderr

FIO24-C. Do not open a file that is already open

FIO30-C. Exclude user input from format strings

FIO30-PL. Use compatible character encodings when performing network or file I/O

FIO32-C. Do not perform operations on devices that are only appropriate for files

FIO34-C. Distinguish between characters read from a file and EOF or WEOF

FIO37-C. Do not assume that fgets() or fgetws() returns a nonempty string when successful

FIO38-C. Do not copy a FILE object

FIO39-C. Do not alternately input and output from a stream without an intervening flush or positioning call

FIO40-C. Reset strings on fgets() or fgetws() failure

FIO41-C. Do not call getc(), putc(), getwc(), or putwc() with a stream argument that has side effects

FIO42-C. Close files when they are no longer needed

FIO44-C. Only use values for fsetpos() that are returned from fgetpos()

FIO45-C. Avoid TOCTOU race conditions while accessing files

FIO46-C. Do not access a closed file

FIO47-C. Use valid format strings

FIO50-CPP. Do not alternately input and output from a file stream without an intervening positioning call

FIO50-J. Do not make assumptions about file creation

FIO51-CPP. Close files when they are no longer needed

FIO51-J. Identify files using multiple file attributes

FIO52-J. Do not store unencrypted sensitive information on the client side

FIO53-J. Use the serialization methods writeUnshared() and readUnshared() with care

Rec. 01. File I/O and Logging (FIO)

Rec. 07. File Input and Output (FIO)

Rec. 09. Input Output (FIO)

Rec. 13. Input Output (FIO)

Rule 01. File I/O and Logging (FIO)

Rule 07. File Input and Output (FIO)

Rule 07. Input Output (FIO)

Rule 09. Input Output (FIO)

Rule 13. Input Output (FIO)

Rule Template

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