Interesting tidbit in iDefense Security Advisory 06.26.07

[ken at krvw.com (Kenneth Van Wyk)]

SC-L

I'm not quite so sure why this one (below) caught my eye -- we _all_  
get tons of product advisories -- but it did.  In particular, two  
things jump out at me:

1) the original author of the defect thought that s/he was doing  
things correctly in using strncpy (vs. strcpy).
2) the original author had apparently been doing static source  
analysis using David Wheeler's Flawfinder tool, as we can tell from  
the comments.

Yet, a simple coding mistake was made in calculating the length of a  
buffer and passing that incorrect length to strncpy.  The result was  
a buffer overrun on the stack, just like the millions that we've all  
seen.

Mind you, the overrun can only be exploited when specific characters  
are used as input to the loop in the code.  Thus, I'm inclined to  
think that this is an interesting example of a bug that would have  
been extraordinarily difficult to find using black box testing, even  
fuzzing.  The iDefense team doesn't say how the (anonymous) person  
who reported it found it, but I for one would be really curious to  
hear that story.

Just some random thoughts this afternoon...  Perhaps I'm still  
getting over the jet lag after returning from the FIRST conference in  
Seville.

Cheers,

Ken van Wyk
SC-L Moderator


Begin forwarded message:

> From: iDefense Labs <labs-no-reply at idefense.com>
> Date: June 26, 2007 3:53:46 PM EDT
> To: vulnwatch at vulnwatch.org, full-disclosure at lists.grok.org.uk,  
> bugtraq at securityfocus.com
> Subject: iDefense Security Advisory 06.26.07: RealNetworks  
> RealPlayer/HelixPlayer SMIL wallclock Stack Overflow Vulnerability
>
> RealNetworks RealPlayer/HelixPlayer SMIL wallclock Stack Overflow
> Vulnerability
>
> iDefense Security Advisory 06.26.07
> http://labs.idefense.com/intelligence/vulnerabilities/
> Jun 26, 2007
>
> I. BACKGROUND
>
> RealPlayer is an application for playing various media formats,
> developed by RealNetworks Inc. HelixPlayer is the open source version
> of RealPlayer. More information can be found at the URLs shown below.
>
> http://www.real.com/realplayer.html
> http://helixcommunity.org/
>
> Synchronized Multimedia Integration Language (SMIL) is a markup  
> language
> used to specify the use of several multi-media concepts when rendering
> media. Some such concepts are timing, transitions, and embedding. More
> information is available from WikiPedia at the following URL.
>
> http://en.wikipedia.org/wiki/ 
> Synchronized_Multimedia_Integration_Language
>
> II. DESCRIPTION
>
> Remote exploitation of a buffer overflow within RealNetworks'  
> RealPlayer
> and HelixPlayer allows attackers to execute arbitrary code in the  
> context
> of the user.
>
> The issue specifically exists in the handling of HH:mm:ss.f time  
> formats
> by the 'wallclock' functionality within the code supporting SMIL2. An
> excerpt from the code follows.
>
>    924    HX_RESULT
>    925    SmilTimeValue::parseWallClockValue(REF(const char*) pCh)
>    926    {
>    ...
>    957        char buf[10]; /* Flawfinder: ignore */
>    ...
>    962        while (*pCh)
>    963        {
>    ...
>    972             else if (isspace(*pCh) || *pCh == '+' || *pCh ==  
> '-'
> || *pCh == 'Z')
>    973             {
>    974                 // this will find the last +, - or Z...  
> which is
> what we want.
>    975                 pTimeZone = pCh;
>    976             }
>    ...
>    982             ++pCh;
>    983        }
>    ...
>   1101        if (pTimePos)
>   1102        {
>   1103        //HH:MM...
>   ....
>   1133          if (*(pos-1) == ':')
>   1134          {
>   ....
>   1148            if (*(pos-1) == '.')
>   1149            {
>   1150            // find end.
>   1151            UINT32 len = 0;
>   1152            if (pTimeZone)
>   1153            {
>   1154                len = pTimeZone - pos;
>   1155            }
>   1156            else
>   1157            {
>   1158                len = end - pos;
>   1159            }
>   1160            strncpy(buf, pos, len); /* Flawfinder: ignore */
>
> The stack buffer is declared to be 10 bytes on line 957. You can see
> that it has a comment which will cause the FlawFinder program to  
> ignore
> this buffer.
>
> The loop, which begins on line 962, runs through the parameter to the
> function looking for characters that denote different sections of the
> time format. When it encounters white space, or the +, -, or Z
> characters it will record the location for later use. If a time was
> located and it contains both a colon and a period the vulnerable code
> will be reached.
>
> The length of data to copy into the stack buffer is calculated  
> either on
> line 1154 or line 1158 depending on whether or not a timezone is  
> present.
> Neither calculations take into consideration the constant length of  
> the
> 'buf' buffer and therefore a stack-based buffer overflow can occur on
> line 1160. Again, notice that this unsafe use of strncpy() is also
> marked with a FlawFinder ignore comment.
>
> III. ANALYSIS
>
> Exploitation requires that an attacker persuade a user to supply
> RealPlayer or HelixPlayer with a maliciously crafted SMIL file. For
> example, this can be accomplished by convincing them to visit a
> malicious web page.
>
> The data that is used to overflow the buffer is quite limited in the
> range of characters that are allowed. However, given the ease of
> address space manipulation within web browsers, exploitation is not
> substantially impacted by this limitation.
>
> The RealPlayer plug-in can be referenced within a web browser by using
> CFCDAA03-8BE4-11cf-B84B-0020AFBBCCFA CLSID.
>
> IV. DETECTION
>
> iDefense has confirmed the existence of this vulnerability in version
> 10.5-GOLD of RealNetworks' RealPlayer and HelixPlayer. Confirmation of
> the existence this vulnerability within HelixPlayer was done via  
> source
> code review. Older versions are assumed to be vulnerable.
>
> V. WORKAROUND
>
> For Windows systems, setting the kill-bit for the associated CLSID,
> despite greatly reducing the media player's functionality, will
> mitigate exploitation. It should be noted that the CLSID listed may  
> not
> be the only CLSID allowing access to the vulnerable code.
>
> VI. VENDOR RESPONSE
>
> RealNetworks has addressed this vulnerability by releasing fixed
> versions of their software.
>
> RealNetworks has not provided iDefense with any links referring to
> updated packages or advisories. Installing the latest version from
> their web site will address the vulnerability.
>
> VII. CVE INFORMATION
>
> The Common Vulnerabilities and Exposures (CVE) project has assigned  
> the
> name CVE-2007-3410 to this issue. This is a candidate for inclusion in
> the CVE list (http://cve.mitre.org/), which standardizes names for
> security problems.
>
> VIII. DISCLOSURE TIMELINE
>
> 10/02/2006  Initial vendor notification
> 10/03/2006  Initial vendor response
> 06/26/2007  Public disclosure
>
> IX. CREDIT
>
> The discoverer of this vulnerability wishes to remain anonymous.
>
> Get paid for vulnerability research
> http://labs.idefense.com/methodology/vulnerability/vcp.php
>
> Free tools, research and upcoming events
> http://labs.idefense.com/
>
> X. LEGAL NOTICES
>
> Copyright ? 2007 iDefense, Inc.
>
> Permission is granted for the redistribution of this alert
> electronically. It may not be edited in any way without the express
> written consent of iDefense. If you wish to reprint the whole or any
> part of this alert in any other medium other than electronically,
> please e-mail customerservice at idefense.com for permission.
>
> Disclaimer: The information in the advisory is believed to be accurate
> at the time of publishing based on currently available information.  
> Use
> of the information constitutes acceptance for use in an AS IS  
> condition.
>  There are no warranties with regard to this information. Neither the
> author nor the publisher accepts any liability for any direct,
> indirect, or consequential loss or damage arising from use of, or
> reliance on, this information.


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