Time-of-check to time-of-use: Difference between revisions

Revision as of 20:26, 29 September 2019

Revision as of 05:27, 9 November 2019

Line 1:

In [[software development]], '''time-of-check to time-of-use''' ('''TOCTOU''', '''TOCTTOU''' or '''TOC/TOU''') is a class of [[software bug]]s caused by a [[race condition]] involving the ''checking'' of the state of a part of a system (such as a security credential) and the ''use'' of the results of that check.

A simple example is as follows: Consider a Web application that allows a user to edit pages, and also allows administrators to lock pages to prevent editing. A user requests to edit a page, getting a form which can be used to alter its content. Before the user submits the form, an administrator locks the page, which should prevent editing. However, since editing has already begun, when the user submits the form, those edits (which have already been made) are accepted. When the user began editing, the appropriate authorization was ''checked'', and the user was indeed allowed to edit. However, the authorization was ''used'' later, at a time when edits should no longer have been allowed.

TOCTOU race conditions are common in [[Unix]] between operations on the [[File system#Metadata|file system]],<ref>{{Cite web|url=https://www.usenix.org/conference/fast-05/tocttou-vulnerabilities-unix-style-file-systems-anatomical-study|title=TOCTTOU Vulnerabilities in UNIX-Style File Systems: An Anatomical Study|last=Wei|first=Jinpeng|last2=Pu|first2=Calton|website=www.usenix.org|access-date=2019-01-14}}</ref> but can occur in other contexts, including local [[Unix ___domain socket|sockets]] and improper use of [[database transaction]]s. In the early 1990s, the mail utility of BSD 4.3 UNIX had an [[Exploit (computer security)|exploitable]] race condition for temporary files because it used the [http://man7.org/linux/man-pages/man3/mktemp.3.html <code>mktemp()</code>] function.<ref>{{cite web |author=Shangde Zhou(周尚德) |date=1991-10-01 |title=A Security Loophole in Unix |url=http://cdblp.cn/paper/UNIX%E7%9A%84%E4%B8%80%E4%B8%AA%E6%BC%8F%E6%B4%9E/94334.html |url-status=dead |archiveurl=https://archive.is/20130116041403/http://cdblp.cn/paper/UNIX%E7%9A%84%E4%B8%80%E4%B8%AA%E6%BC%8F%E6%B4%9E/94334.html |archivedate=2013-01-16 }}</ref>