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Date: Tue, 8 Mar 2011 23:20:28 +0000
From: Helgi Þormar Þorbjörnsson <>
Cc: Dan Rosenberg <>,
 Pierre Joye <>
Subject: Re: CVE Request: PEAR Installer 1.9.1 <= - Symlink Attack


On 1 Mar 2011, at 12:39, Helgi Þormar Þorbjörnsson wrote:

> On 1 Mar 2011, at 12:19, Dan Rosenberg wrote:
>>> Not sure it is fixable, or maybe using a lock on the symbolic link
>>> while fetching its target (to be tested to be sure that such locks
>>> cannot be overridden from shell).
>> The easiest way is to just open the target with the O_NOFOLLOW flag to
>> avoid following symlinks and abort on failure.  If you need to support
>> systems that don't have this flag, then perhaps you could consider
>> using an application-specific temporary directory instead of operating
>> in the world-writable /tmp.
> The PEAR installer does use /tmp (and whatever the Windows equivalent is) by default unless the user opts into a local installation or does indeed change the configuration to use other temp/download/cache directories so users can guard themselves with a good setup.
> A flag like that would be handy but doesn't exist (yet) in PHP. 
> I moved over to using the O_CREAT|O_EXCL equivalent in PHP when creating new files and lstat + fopen + fstat and comparing mode/ino/dev before writing to an existing file for the cache. I could add an nlink check to that as well.
> The current version I've been playing around with is located at - It is missing the nlink part but it should be able to deal with TOCTOU problems. That code snippet hasn't been committed as I consider it work-in-progress still.
> Any comments / suggestions are welcome, I did write that one quite late last night :-)

Here is the latest fix for the TOCTOU (e.g. time-of-check-time-of-use) problem: - A proper mix of lstat, fopen, fstat (to ensure no one has messed around with the file pointer between the check and getting the handler) as well as adding in a nlink check to make sure it is 1.

Hopefully this is enough to fix the problem you had with my earlier fixes and get me the CVE number.

- Helgi

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