bugThe FreeType Project - Bugs: bug #43589, FreeType 2.5.3 WOFF parsing...


You are not allowed to post comments on this tracker with your current authentication level.

bug #43589: FreeType 2.5.3 WOFF parsing heap-based buffer overflow due to integer overflow

Submitter:  Mateusz Jurczyk <j00ru>
Submitted:  Wed 12 Nov 2014 04:46:52 PM UTC
Severity:  3 - Normal Item Group:  Crash
Status:  Fixed Privacy:  Public
Assigned to:  wl Open/Closed:  Closed
Planned Release:  2.5.4

Thu 13 Nov 2014 08:12:18 AM UTC, comment #2: 

The fix LGTM, thanks!

Mateusz Jurczyk <j00ru>
Group Member
Wed 12 Nov 2014 08:06:56 PM UTC, comment #1: 

Thanks for the report.  Fixed in git, please check.

Werner LEMBERG <wl>
Group administrator
Wed 12 Nov 2014 04:46:52 PM UTC, original submission:  

In the freetype/src/sfnt/sfobjs.c file, there is a "woff_open_font" function responsible for processing WOFF (Web Open Font Format) file structures. Among other things, it unpacks potentially zlib-compressed SFNT tables and loads them into a new internal memory stream. In the pre-unpacking code, the following table handling snippet is found:

576:      if ( table->Offset != woff_offset                         ||
577:           table->Offset + table->CompLength > woff.length      ||
578:           sfnt_offset + table->OrigLength > woff.totalSfntSize ||
579:           table->CompLength > table->OrigLength                )
580:      {
581:        error = FT_THROW( Invalid_Table );
582:        goto Exit;
583:      }

Here, basic sanitization is performed to make sure that header entries denoting compressed/uncompressed sizes do not cross certain boundaries and are logically valid. However, on 32-bit builds of FreeType, both expressions in lines 577 and 578 ("table->Offset + table->CompLength" and "sfnt_offset + table->OrigLength") may overflow the 32-bit space if enormously large values of "CompLength" or "OrigLength" are provided in the file.

While the potential "CompLength" overflow does not to anything because of a further FT_FRAME_ENTER macro invocation in line 652, the impact is much higher for "OrigLength". Having the field store an inadequately large value may result in a heap-based buffer overflow in the table decompression, as "OrigLength" is passed as the "output_len" parameter to the FT_Gzip_Uncompress function:

668:        error = FT_Gzip_Uncompress( memory,
669:                                    sfnt + table->OrigOffset, &output_len,
670:                                    stream->cursor, table->CompLength );

Even though the value used to achieve an integer overflow must be very large, we can control the number of bytes written to the output buffer very accurately through the zlib-compressed stream, which greatly facilitates practical exploitation of this vulnerability.

Attached please find a file (poc.woff) which can be used to reproduce the issue: it causes FreeType (or in fact, zlib) to attempt to write 1000000 "A" bytes into a 57072 byte long heap-based buffer, resulting in the following AddressSanitizer report:

==24751== ERROR: AddressSanitizer: SEGV on unknown address 0xf4300000 (pc 0xf5db17fa sp 0xffce7270 bp 0xf4300000 T0)
AddressSanitizer can not provide additional info.
    #0 0xf5db17f9 (/lib/i386-linux-gnu/libz.so.1+0x77f9)
    #1 0xf5db3bec (/lib/i386-linux-gnu/libz.so.1+0x9bec)
    #2 0xf6157d6b in FT_Gzip_Uncompress freetype2/src/gzip/ftgzip.c:719
    #3 0xf60ee94a in woff_open_font freetype2/src/sfnt/sfobjs.c:668
    #4 0xf60eefe3 in sfnt_open_font freetype2/src/sfnt/sfobjs.c:764
    #5 0xf60ef84c in sfnt_init_face freetype2/src/sfnt/sfobjs.c:871
    #6 0xf601513e in tt_face_init freetype2/src/truetype/ttobjs.c:535
    #7 0xf5fca1f7 in open_face freetype2/src/base/ftobjs.c:1191
    #8 0xf5fcd80a in FT_Open_Face freetype2/src/base/ftobjs.c:2123
    #9 0xf5fca51f in FT_New_Face freetype2/src/base/ftobjs.c:1254
    #10 0x804b5a8 in get_face ft2demos-2.5.3/src/ftbench.c:705
    #11 0x804bc64 in main ft2demos-2.5.3/src/ftbench.c:924
SUMMARY: AddressSanitizer: SEGV ??:0 ??
==24751== ABORTING

Mateusz Jurczyk <j00ru>
Group Member


(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attached Files
file #32423:  poc.woff added by j00ru (38KiB - application/font-woff)


Depends on the following items: None found

Items that depend on this one: None found


Carbon-Copy List
  • -email is unavailable- added by wl (Posted a comment)
  • -email is unavailable- added by j00ru (Submitted the item)

    There are 0 votes so far. Votes easily highlight which items people would like to see resolved in priority, independently of the priority of the item set by tracker managers.


    Follow 6 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2014-11-12 wl StatusNone Fixed
        PrivacyPrivate Public
        Assigned toNone wl
        Open/ClosedOpen Closed
        Planned ReleaseNone 2.5.4
    2014-11-12 j00ru Attached File- Added poc.woff, #32423

    Back to the top

    Powered by Savane 3.13-758e.
    Corresponding source code