#32 ZDI-CAN-27517: New Vulnerability Report

[Zero Day Initiative]

ZDI-CAN-27517: FontForge GUtils BMP File Parsing Heap-based Buffer Overflow Remote Code Execution Vulnerability

-- CVSS -----------------------------------------

7.8: AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

-- ABSTRACT -------------------------------------

Trend Micro's Zero Day Initiative has identified a vulnerability affecting the following products:
FontForge - FontForge

-- VULNERABILITY DETAILS ------------------------
* Version tested:387146a241b36bcdf6ce229c5a3fe367ed3854a1
* Installer file:-
* Platform tested:ubuntu 24.04 desktop edition

---

Analysis

heap overflow exists within the FontForge BMP parser.
It didn't validate the length of the pixels when reading from BMP within the readpixels() function.
It overflowed around 0x100 bytes with arbitrary content.
The overflow will be triggered when victim open the malicious svg file with the fontforge application.

static int readpixels(FILE *file,struct bmpheader *head) {
    int i,ii,j,ll,excess;

    fseek(file,head->offset,0);

    ll = head->width;
    if ( head->bitsperpixel==8 && head->compression==0 ) {
        excess = ((ll+3)/4)*4 -ll;
        for ( i=0; i<head->height; ++i ) {
            fread(head->byte_pixels+i*ll,1,ll,file);
            for ( j=0; j<excess; ++j )
                (void) getc(file);
        }
    } else if ( head->bitsperpixel==8 ) {
        /* 8 bit RLE */
        int ii = 0;
        while ( ii<head->height*head->width ) {
            int cnt = getc(file);                               // (1) `cnt` is extracted from the file without validation
                                                                //     it should be in bounds of (height*head->width)-ii
            if ( cnt!=0 ) {
                int ch = getc(file);
                while ( --cnt>=0 )
                    head->byte_pixels[ii++] = ch;               // (2) overflow here
            } else {
                cnt = getc(file);                               // (3) same pattern repeats
                if ( cnt>= 3 ) {
                    int odd = cnt&1;
                    while ( --cnt>=0 )
                        head->byte_pixels[ii++] = getc(file);   // (3) overflow here
                    if ( odd )
                        getc(file);
                } else if ( cnt==0 ) {  /* end of scan line */
                    ii = ((ii+head->width-1)/head->width) * head->width;
                } else if ( cnt==1 ) {
        break;
                } else if ( cnt==2 ) {
                    int x=getc(file);
                    int y = getc(file);
                    y += ii/head->width;
                    x += ii%head->width;
                    ii = y*head->width + x;
                }
            }
        }

reproduce steps

mkdir build ; cd build
cmake -GNinja ..   -DCMAKE_C_FLAGS="-fsanitize=address -g"   -DCMAKE_CXX_FLAGS="-fsanitize=address -g"   -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address"
ninja
./bin/fontforge [poc.svg]

ASAN report

=================================================================
==8294==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x50b0000a3ed0 at pc 0x73e28866c2b7 bp 0x7fffe4cd1e10 sp 0x7fffe4cd1e00
WRITE of size 1 at 0x50b0000a3ed0 thread T0
    #0 0x73e28866c2b6 in readpixels /root/fontforge/gutils/gimagereadbmp.c:193
    #1 0x73e28866c2b6 in GImageRead_Bmp /root/fontforge/gutils/gimagereadbmp.c:362
    #2 0x73e288c7a6e6 in GImageFromDataURI /root/fontforge/fontforge/svg.c:2399
    #3 0x73e288c7a6e6 in SVGParseImage /root/fontforge/fontforge/svg.c:2443
    #4 0x73e288c7a6e6 in _SVGParseSVG /root/fontforge/fontforge/svg.c:2762
    #5 0x73e288c77849 in _SVGParseSVG /root/fontforge/fontforge/svg.c:2709
    #6 0x73e288c7b203 in SVGParseSVG /root/fontforge/fontforge/svg.c:2868
    #7 0x73e288c7b60d in SVGParseGlyphBody /root/fontforge/fontforge/svg.c:2880
    #8 0x73e288c7f626 in SVGParseGlyph /root/fontforge/fontforge/svg.c:2984
    #9 0x73e288c7f626 in SVGParseFont /root/fontforge/fontforge/svg.c:3430
    #10 0x73e288c7f626 in _SFReadSVG /root/fontforge/fontforge/svg.c:3617
    #11 0x73e288b282cc in _ReadSplineFont /root/fontforge/fontforge/splinefont.c:1218
    #12 0x73e288b28fb0 in LoadSplineFont /root/fontforge/fontforge/splinefont.c:1420
    #13 0x73e28878466f in ViewPostScriptFont /root/fontforge/fontforge/fontviewbase.c:1387
    #14 0x5e33865e7396 in fontforge_main /root/fontforge/fontforgeexe/startui.c:1017
    #15 0x73e285a2a1c9 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #16 0x73e285a2a28a in __libc_start_main_impl ../csu/libc-start.c:360
    #17 0x5e3386145f74 in _start (/usr/local/bin/fontforge+0x172f74) (BuildId: 68c80c361f3adf7aea14524aeb7eb3e62357bdcb)

0x50b0000a3ed0 is located 0 bytes after 112-byte region [0x50b0000a3e60,0x50b0000a3ed0)
allocated by thread T0 here:
    #0 0x73e2894fd9c7 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x73e288669e94 in GImageRead_Bmp /root/fontforge/gutils/gimagereadbmp.c:357

SUMMARY: AddressSanitizer: heap-buffer-overflow /root/fontforge/gutils/gimagereadbmp.c:193 in readpixels
Shadow bytes around the buggy address:
  0x50b0000a3c00: 00 fa fa fa fa fa fa fa fa fa fd fd fd fd fd fd
  0x50b0000a3c80: fd fd fd fd fd fd fd fa fa fa fa fa fa fa fa fa
  0x50b0000a3d00: fd fd fd fd fd fd fd fd fd fd fd fd fd fa fa fa
  0x50b0000a3d80: fa fa fa fa fa fa fd fd fd fd fd fd fd fd fd fd
  0x50b0000a3e00: fd fd fd fd fa fa fa fa fa fa fa fa 00 00 00 00
=>0x50b0000a3e80: 00 00 00 00 00 00 00 00 00 00[fa]fa fa fa fa fa
  0x50b0000a3f00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x50b0000a3f80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x50b0000a4000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x50b0000a4080: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x50b0000a4100: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==8294==ABORTING

-- CREDIT ---------------------------------------
This vulnerability was discovered by:
volticks (@movx64 on twitter) working with Trend Micro Zero Day Initiative

-- FURTHER DETAILS ------------------------------

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