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yTNEF/Evolution TNEF Attachment decoder plugin directory traversal &
buffer overflow vulnerabilities
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Yorick Koster, June 2009

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Abstract
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yTNEF & the Evolution TNEF Attachment decoder plugin are affected by
several directory traversal and buffer overflow vulnerabilities. The 
directory traversal vulnerabilities allow attackers to overwrite or 
create local files with the privileges of the target user. Exploiting 
the buffer overflow vulnerabilities allows for arbitrary code execution 
with the privileges of the target user.

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See also
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- #2009-013 yTNEF/Evolution TNEF attachment decoder input sanitization 
errors [2]

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Tested version
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These vulnerabilities were discovered using the latest (stable) versions
of Evolution (currently 2.62.2) and yTNEF (currently 2.6). The 
vulnerabilities were verified on the following Linux distributions:

 - GNOME version of Mandriva Linux 2009 Spring running Evolution 
2.26.1.1 (Evolution plugin installed by default)
 - Ubuntu 9.04 running Evolution 2.26.1 (with 
evolution-plugins-experimental package installed)

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Affected functions
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The following functions are affected by these issues:

Evolution plugin:
  * processTnef()
  * saveVCard()
  * saveVCalendar()
  * saveVTask()

yTNEF:
  * ProcessTNEF()
  * SaveVCard()
  * SaveVCalendar()
  * SaveVTask()

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Fix
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There is currently no fix available.

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Introduction
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Transport Neutral Encapsulation Format (TNEF) is a proprietary e-mail 
attachment format used by Microsoft Outlook and Microsoft Exchange 
Server. A plugin [3] for Evolution exists that provides basic support 
for TNEF encoded e-mails. This plugin uses the ytnef library [4] 
(libytnef) for processing TNEF messages. It borrows code from the ytnef 
program, which is a program to work with procmail to decode TNEF streams
(winmail.dat attachments). Both applications share (almost) code and 
are, because of this, both affected by the issues described in this 
document.

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Evolution TNEF Attachment decoder plugin
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The plugin is started on e-mail attachments that have a MIME type of 
either application/vnd.ms-tnef or application/ms-tnef. It creates a 
temporary directory under ~/.evolution/cache/tmp using the format 
tnef-attachment-XXXXXX. The TNEF attachment is saved as 
.evo-attachment.tnef.

void
org_gnome_format_tnef(void *ep, EMFormatHookTarget *t)
{
  [...]
  
  tmpdir = e_mkdtemp("tnef-attachment-XXXXXX");
  if (tmpdir == NULL)
    return;
  
  filepath = tmpdir;
  
  name = g_build_filename(tmpdir, ".evo-attachment.tnef", 
NULL);
  
  out = camel_stream_fs_new_with_name(name, O_RDWR|O_CREAT, 0666);


The saved file is parsed by TNEFParseFile(), the result is stored in a 
struct of the type TNEFStruct. This struct is passed to the function 
processTnef(), which tries to extract all relevant data and attachments 
from the TNEF stream. Each relevant part of the TNEF stream is stored 
within the previously created temporary directory that are made 
available to the end user as separate e-mail attachments.

  /* Extracting the winmail.dat */
  TNEFInitialize(tnef);
  tnef->Debug = verbose;
  if (TNEFParseFile(name, tnef) == -1) {
    printf("ERROR processing file\n");
  }
  processTnef(tnef);
  
  TNEFFree(tnef);
  /* Extraction done */

------------------------------------------------------------------------
yTNEF
------------------------------------------------------------------------

yTNEF processes TNEF files in a similar manner. It receives a file name 
from the command line, calls TNEFParseFile() that creates a struct 
TNEFStruct after which ProcessTNEF() is called. If ProcessTNEF() finds 
attachments it can process, these attachments will be saved locally. The
ProcessTNEF() function is almost the same as the processTnef() function
of the Evolution plugin.

int main(int argc, char ** argv) {
  [...]
  
  for(i=1; i<argc; i++) {
    [...]
  
    TNEFInitialize(&TNEF);
    TNEF.Debug = verbose;
    if (TNEFParseFile(argv[ i], &TNEF) == -1) {
      printf("ERROR processing file\n");
      continue;
    }
    ProcessTNEF(TNEF);
    TNEFFree(&TNEF);
  }
}

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Directory traversal
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If a TNEF file is processed, both yTNEF and the Evolution plugin will 
save certain types of TNEF structures. Special processing functions are 
available for Contacts, Tasks & Appointments. These functions are 
called if the Message Class is set to a certain value.

void processTnef(TNEFStruct *tnef) {
  [...]
  
  /* First see if this requires special processing. */
  /* ie: it's a Contact Card, Task, or Meeting request (vCal/vCard) 
*/
  if (tnef->messageClass[0] != 0)  {
    if (strcmp(tnef->messageClass, "IPM.Contact") == 0) {
      saveVCard(tnef);
    }
    if (strcmp(tnef->messageClass, "IPM.Task") == 0) {
      saveVTask(tnef);
    }
    if (strcmp(tnef->messageClass, "IPM.Appointment") == 0) {
      saveVCalendar(tnef);
      foundCal = 1;
    }
  }
  
  if ((filename = MAPIFindUserProp(&(tnef->MapiProperties),
        PROP_TAG(PT_STRING8,0x24))) != MAPI_UNDEFINED) {
    if (strcmp(filename->data, "IPM.Appointment") == 0) {
      /* If it's "indicated" twice, we don't want to 
save 2 calendar entries. */
      if (foundCal == 0) {
        saveVCalendar(tnef);
      }
    }
  }

There is also code that treats TNEF structures with the Message Class 
set to IPM.Microsoft Mail.Note. In the Evolution plugin, this code is 
never called as the global variable saveRTF is set to zero. In case of 
yTNEF this global variable is controlled by the command line.

if (strcmp(TNEF.messageClass, "IPM.Microsoft Mail.Note") == 0)
 {
  if ((saveRTF == 1) && (TNEF.subject.size > 0)) {
    // Description
    if ((filename=MAPIFindProperty(&(TNEF.MapiProperties),
        PROP_TAG(PT_BINARY, PR_RTF_COMPRESSED)))
      != MAPI_UNDEFINED) {
[...]

After the structures mentioned before have been processed, all other 
attachments are also saved locally. The file names used to save the 
attachments are obtained from the TNEF data. In case of normal 
attachments, the code first looks if the TNEF data contains MAPI 
properties and if so, it will look for specific properties. If these 
exists, a file name is extracted from these properties. If the 
properties do not exist, the attachment's title is used. This title
is also set through a TNEF structure. If this title is also not 
available, a default file name will be used instead.

if ((RealAttachment == 1) || (saveintermediate == 1)) {
/* Ok, it's not an embedded stream, so now we */
/* process it. */
if ((filename = MAPIFindProperty(&(p->MAPI), 
    PROP_TAG(30,0x3707))) 
  == MAPI_UNDEFINED) {
  if ((filename = MAPIFindProperty(&(p->MAPI), 
        PROP_TAG(30,0x3001))) 
      == MAPI_UNDEFINED) {
    filename = &(p->Title);
  }
}
if (filename->size == 1) {
  filename = (variableLength*)malloc(sizeof(variableLength));
  filename->size = 20;
  filename->data = (char*)malloc(20);
  sprintf(filename->data, "file_%03i.dat", count);
}
if (filepath == NULL) {
  sprintf(ifilename, "%s", filename->data);
} else {
  sprintf(ifilename, "%s/%s", filepath, filename->data);
}
for(i=0; i<strlen(ifilename); i++) 
  if (ifilename[ i] == ' ') 
    ifilename[ i] = '_';
  
  if ((fptr = fopen(ifilename, "wb"))==NULL) {
    printf("ERROR: Error writing file to disk!");
  } else {
    if (object == 1) {
    fwrite(filedata->data + 16, 
      sizeof(BYTE), 
      filedata->size - 16, 
      fptr);
    } else {
    fwrite(filedata->data, 
      sizeof(BYTE), 
      filedata->size, 
      fptr);
    }
    fclose(fptr);
  }
}

Before a new file is created, all spaces within the file name are 
replaced with the underscore character. No additional sanitation is 
performed on the file name. Because of this, it is possible to traverse 
outside of the temporary directory and create or overwrite any file with
the privileges of the target user. This allows an attacker to execute 
arbitrary code for example by overwriting ~/.bashrc.

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Buffer overflow
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Beside the directory traversal, it is also possible to trigger a buffer 
overflow by supplying an overly long file name. This is possible, 
because the file name is copied in a fixed size buffer (256 bytes). In 
the Evolution plugin, this triggers a buffer overflow on the heap. In 
case of yTNEF the file name is copied in a buffer on the stack, thus 
allowing for a stack-based buffer overflow to occur.

Evolution plugin:

void processTnef(TNEFStruct *tnef) {
  [...]
  ifilename = (char *) g_malloc(sizeof(char) * 256);
  [...]
    if (filepath == NULL) {
      sprintf(ifilename, "%s", filename->data);
    } else {
      sprintf(ifilename, "%s/%s", filepath, filename->data);
    }

yTNEF:

void ProcessTNEF(TNEFStruct TNEF) {
  [...]      
  char ifilename[256];
  [...]
    if (filepath == NULL) {
      sprintf(ifilename, "%s", filename->data);
    } else {
      sprintf(ifilename, "%s/%s", filepath, filename->data);
    }

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References
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[1] http://www.akitasecurity.nl/advisory.php?id=AK20090601
[2] http://www.ocert.org/advisories/ocert-2009-013.html
[3] http://www.go-evolution.org/Tnef
[4] http://sourceforge.net/projects/ytnef/

------------------------------------------------------------------------
-- 
------------------------------------------------------------------------
Akita Software Security (Kvk 37144957)
http://www.akitasecurity.nl/
------------------------------------------------------------------------
Key fingerprint = 5FC0 F50C 8B3A 4A61 7A1F  2BFF 5482 D26E D890 5A65
http://keyserver.pgp.com/vkd/DownloadKey.event?keyid=0x5482D26ED8905A65