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The POSIX {{setuid()}} function has complex semantics and platform-specific behavior  \[[Open Group 04|AA. References#OpenBibliography#Open Group 04]\].

This compliant solution was implemented in sendmail, a popular mail transfer agent, to determine if superuser privileges were successfully dropped \[[Wheeler 03|AA. References#WheelerBibliography#Wheeler 03]\]. If the {{setuid()}} call succeeds after (supposedly) dropping privileges permanently, privileges were not dropped as intended.

Processes on Linux have two additional values called {{fsuid}} and {{fsgid}}. These indicate the privileges used when accessing files on the file system. These values normally shadow the effective user ID and effective group ID, but the {{setfsuid()}} and {{setfsgid()}} functions allow them to be changed.  Since changes to the {{euid}} and {{egid}} normally also apply to {{fsuid}} and {{fsgid}}, a program relinquishing root privileges need not be concerned with setting {{fsuid}} or {{fsgid}} to safe values. However, there has been at least one kernel bug that violated this invariant (\[[Chen 02|AA. References#ChenBibliography#Chen 02]\] and \[[Tsafrir 08|AA. References#TsafrirBibliography#Tsafrir 08]\]). Consequently, a prudent program will check that {{fsuid}} and {{fsgid}} have harmless values after relinquishing privileges.

\[[Chen 02|AA. References#ChenBibliography#Chen 02]\] "Setuid Demystified"
\[[Dowd 06|AA. References#DowdBibliography#Dowd 06]\] Chapter 9, "Unix I: Privileges and Files"
\[[ISO/IEC PDTR 24772|AA. References#ISOBibliography#ISO/IEC PDTR 24772]\] "XYO Privilege Sandbox Issues"
\[[MITRE 07|AA. References#MITREBibliography#MITRE 07]\] [CWE ID 250|http://cwe.mitre.org/data/definitions/250.html], "Execution with Unnecessary Privileges," [CWE ID 273|http://cwe.mitre.org/data/definitions/273.html], "Failure to Check Whether Privileges Were Dropped Successfully"
\[[Open Group 04|AA. References#OpenBibliography#Open Group 04]\] [{{setuid()}}|http://www.opengroup.org/onlinepubs/009695399/functions/setuid.html], [{{getuid()}}|http://www.opengroup.org/onlinepubs/009695399/functions/getuid.html], [{{seteuid()}}|http://www.opengroup.org/onlinepubs/009695399/functions/seteuid.html]
\[[Tsafrir 08|AA. References#TsafrirBibliography#Tsafrir 08]\] "The Murky Issue of Changing Process Identity: Revising 'Setuid Demystified'"
\[[Wheeler 03|AA. References#WheelerBibliography#Wheeler 03]\] [Section 7.4, "Minimize Privileges"|http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/minimize-privileges.html]