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The Open Group Base Specification Issue 6 \[[Open Group 04|AA. C References#Open Group 04]\] does not specify the permissions the file is created with, so these are [implementation-defined|BB. Definitions#implementation-defined behavior]. However, Issue 7 (that is, POSIX.1-2008) specifies them as {{S_IRUSR\|S_IWUSR}} (0600) \[[Austin Group 08|AA. C References#Austin Group 08]\]. |
Implementation Details
For glibc versions 2.0.6 and earlier, the file is created with permissions 0666; for glibc versions 2.0.7 and later, the file is created with permissions 0600. On NetBSD, the file is created with permissions 0600. This creates a security risk in that an attacker will have write access to the file immediately after creation. Consequently, programs need a private version of the mkstemp() function in which this issue is known to be fixed.
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The tool Compass Rose is able to detect violations of this recommendation. Specifically, Rose reports use of tmpnam(), tmpnam_s(), tmpfile(), and mktemp().
References
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\[[Austin Group 08|AA. C References#Austin Group 08]\] \[[HP 03|AA. C References#HP 03]\] \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Sections 7.19.4.4, "The {{tmpnam}} function," 7.19.4.3, "The {{tmpfile}} function," and 7.19.5.3, "The {{fopen}} function" \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "EWR Path Traversal" \[[ISO/IEC TR 24731-1-2007|AA. C References#ISO/IEC TR 24731-1-2007]\] Sections 6.5.1.2, "The {{tmpnam_s}} function," 6.5.1.1, "The {{tmpfile_s}} function," and 6.5.2.1, "The {{fopen_s}} function" \[[Kennaway 00|AA. C References#Kennaway 00]\] \[[Open Group 04|AA. C References#Open Group 04]\] [{{mktemp()}}|http://www.opengroup.org/onlinepubs/000095399/functions/mktemp.html], [{{mkstemp()}}|http://www.opengroup.org/onlinepubs/009695399/functions/mkstemp.html], [{{open()}}|http://www.opengroup.org/onlinepubs/009695399/functions/open.html] \[[Seacord 05a|AA. C References#Seacord 05a]\] Chapter 3, "File I/O", Chapter 7 \[[Wheeler 03|AA. C References#Wheeler 03]\] [Chapter 7, "Structure Program Internals and Approach"|http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/avoid-race.html#TEMPORARY-FILES] \[[Viega 03|AA. C References#Viega 03]\] Section 2.1, "Creating Files for Temporary Use" \[[Kennaway 00|AA. C References#Kennaway 00]\] \[[HP 03|AA. C References#HP 03]\] |
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FIO42-C. Ensure files are properly closed when they are no longer needed 09. Input Output (FIO)