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In case of set-user-ID and set-group-ID programs, when the effective user-ID and group-ID are different from those of the real user, it is important to drop not only the user-level privileges but also the group privileges. While doing so, the order of revocation must be correct.

Wiki MarkupPOSIX defines {{setgid()}} to have the following behavior \ [[Open Group 2004|AA. Bibliography#Open Group 04]\]:

If the process has appropriate privileges, setgid() shall set the real group ID, effective group ID, and the saved set-group-ID of the calling process to gid.

If the process does not have appropriate privileges, but gid is equal to the real group ID or the saved set-group-ID, setgid() shall set the effective group ID to gid; the real group ID and saved set-group-ID shall remain unchanged.

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MITRE CWE: CWE-696, "Incorrect Behavior Order"

Bibliography

Wiki Markup\[[Chen 2002|AA. Bibliography#Chen 02] \] "Setuid Demystified" \
[[Dowd 2006|AA. Bibliography#Dowd 06]\] Chapter 9, "UNIX I: Privileges and Files" \[
[Open Group 2004|AA. Bibliography#Open Group 04]\] \[ [{{setuid()}}|http://www.opengroup.org/onlinepubs/009695399/functions/setuid.html]\], and \ [[{{setgid()}}|http://www.opengroup.org/onlinepubs/009695399/functions/setgid.html]\] \[[Tsafrir 2008|AA. Bibliography#Tsafrir 08]\] "The Murky Issue of Changing Process Identity: Revising 'Setuid ]
[Tsafrir 2008] "The Murky Issue of Changing Process Identity: Revising 'Setuid Demystified'"

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      50. POSIX (POS)