Talos Vulnerability Report


libxls xls_preparseWorkSheet MULBLANK Code Execution Vulnerability

November 15, 2017
CVE Number



An exploitable integer overflow vulnerability exists in the xls_preparseWorkSheet function of libxls 1.4 when handling a MULBLANK record. A specially crafted XLS file can cause a memory corruption resulting in remote code execution. An attacker can send malicious XLS file to trigger this vulnerability.

Tested Versions

libxls 1.4 readxl package 1.0.0 for R (tested using Microsoft R 4.3.1)

Product URLs


CVSSv3 Score

8.8 - CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H CVSSv3 Calculator: https://www.first.org/cvss/calculator/3.0


CWE-680: Integer Overflow to Buffer Overflow


libxls is a C library supported on windows, mac, cygwin which can read Microsoft Excel File Format ( XLS ) files. The library is used by the readxl package that can be installed in the R programming language.

The integer overflow appears in the xls_preparseWorkSheet function, so let's take a look at the vulnerable code:

Line 970 void xls_preparseWorkSheet(xlsWorkSheet* pWS)
Line 971 {
Line 985         read = ole2_read(buf, 1,tmp.size,pWS->workbook->olestr);
Line 986        assert(read == tmp.size);
Line 987        // xls_showBOF(&tmp);
Line 988         switch (tmp.id)
Line 989         {
Line 1012        case 0x00BE:        //MULBLANK
Line 1013            if (pWS->rows.lastcol<xlsShortVal(((MULBLANK*)buf)->col) + (tmp.size - 6)/2 - 1)
Line 1014                pWS->rows.lastcol=xlsShortVal(((MULBLANK*)buf)->col) + (tmp.size - 6)/2 - 1;
Line 1015            if (pWS->rows.lastrow<xlsShortVal(((MULBLANK*)buf)->row))
Line 1016                pWS->rows.lastrow=xlsShortVal(((MULBLANK*)buf)->row);
Line 1017            break;     

The general purpose of the xls_preparseWorkSheet function is to obtain the maximal size of col and row value from records present in the worksheet and update the lastcol and lastrow fields with that value. As we can see in lines 1013-1014 an integer overflow can occur. This can have two potential impacts. In one case, the maximum value stored in lastcol will not be updated even if the MULBLANK col field is greater than the value in lastcol. The other case will result in the lastcol value being updated to the overflowed value.

The malformed MULBLANK record is located at offset : 0xCB1F and looks as follows:

0xCB1F      BE 00 20 00 AA AA FF FF

Setting breakpoint at line 1007 we can obtain the following information:

(gdb) p/x tmp
$1 = {id = 0xbe, size = 0x20}
(gdb) p/x *((MULBLANK*)buf)
$3 = {row = 0xaaaa, col = 0xffff, rk = 0x60f3e4}
(gdb) p/x pWS->rows.lastcol
$4 = 0x0

stepping further after line 1014:

(gdb) p/x pWS->rows.lastcol
$5 = 0x2

We see that lastcol field has been updated with overflowed value 0x2. It has further consequences in function xls_makeTable where based on the lastcol field an array for cells is allocated:

Line 409    void xls_makeTable(xlsWorkSheet* pWS)
Line 410    {
Line 412        struct st_row_data* tmp;
Line 418        for (t=0;t<=pWS->rows.lastrow;t++)
Line 420            tmp=&pWS->rows.row[t];
Line 425            tmp->cells.count = pWS->rows.lastcol+1;
Line 426            tmp->cells.cell=(struct st_cell_data *)calloc(tmp->cells.count,sizeof(struct st_cell_data));                    

next during the final parsing of the malformed MULBLANK record in xls_addCell, an out of bound write occurs because the col value being used as index for the cell array is greater than amount of allocated elements for that array.

Line 446    struct st_cell_data *xls_addCell(xlsWorkSheet* pWS,BOF* bof,BYTE* buf)
Line 447    {
Line 448        struct st_cell_data*    cell;
Line 457        cell=&row->cells.cell[xlsShortVal(((COL*)buf)->col)];
Line 458        cell->id=bof->id;
Line 459        cell->xf=xlsShortVal(((COL*)buf)->xf);  

At line 457 the cell element points to a value outside of the array range which leads to an out-of-bounds write at lines 458,459...

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7bce936 in xls_addCell (pWS=0x60f3a0, bof=0x7fffffffdb80, buf=0x607c50 
<incomplete sequence \345>) at xls.c:458
458         cell->id=bof->id;
gdb-peda$ p cell
$8 = (struct st_cell_data *) 0xdea898
gdb-peda$ vmmap 0xdea898
Warning: not found or cannot access procfs

Crash Information

Microsoft R crash

(94c.c74): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Users\Icewall\Documents\R\win-
library\3.4\readxl\libs\x64\readxl.dll - 
00000000`6534b191 668903          mov     word ptr [rbx],ax ds:00000000`312f5f58=????
0:000> kb 10
 # RetAddr           : Args to Child                                                           : Call Site
00 00000000`6534c8e2 : 00000000`48332fd0 00000000`31135ff0 00000000`00000006 00000000`00000000 : 
01 00000000`65382714 : 00000000`044079c0 00000000`00000000 00000000`00000000 00000000`00000000 : 
02 00000000`6534572b : 00000000`04407750 00000000`044076b0 00000000`00000000 00007ffe`179295c9 : readxl!
03 00000000`65343ddb : 00000000`044079b0 00007ffe`02922b4f 00000000`00000000 00000000`00000000 : readxl!
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Program Files\Microsoft\R Open\R-
3.4.1\bin\x64\R.dll - 
04 00000000`6c7977cf : 00000000`25cecec8 00000000`10cfdf98 00000000`6cbfdbd0 00000000`04407cd0 : 
05 00000000`6c797f04 : 00000000`00000272 00000000`04407cc8 00000000`00000139 00000000`04407f00 : 
06 00000000`6c7ee8c8 : 00000000`23747fa0 00000000`2607c7c0 00000000`23747fa0 00000000`6cbfdcb8 : 
07 00000000`6c7f11dd : 00000000`6cbfdc08 00000000`6cbfdd20 00000000`6cbfdc90 00000000`6c7c070c : R!Rf_eval+0x6f8
08 00000000`6c7ee6ba : 00000000`23748010 00000000`14595aa8 00000000`23748010 00000000`25e0b818 :   
09 00000000`6c7f0550 : 00000000`31f17618 00000000`145dbd58 00000000`25f56dc0 00000000`264e3c08 : R!Rf_eval+0x4ea
0a 00000000`6c7f08d2 : 00000000`237b8cb8 00000000`261e92b0 00000000`237b8cb8 00000000`264e4270 : 
0b 00000000`6c7e28ba : 00000000`6cbfdbd0 00000000`2607f130 00000000`14590600 00000000`6c7f08d2 : 
0c 00000000`6c7ee341 : 00000000`31efd850 00000000`264e4270 00000000`2607cf08 00000000`6c7eebd5 : 
0d 00000000`6c7eeb9c : 00000000`31dfaad8 0000002e`2375f578 00000000`00000000 00000000`2607c4e8 : R!Rf_eval+0x171
0e 00000000`6c7ee3dc : 00000000`00000000 00000000`14590670 00000000`2607cf08 00000000`324006f8 : R!Rf_eval+0x9cc
0f 00000000`6c82251b : 00000000`2fa88528 00000000`14590788 00000000`23992b28 00000000`6c7f08d2 : R!Rf_eval+0x20c
0:000> lmv m readxl
Browse full module list
start             end                 module name
00000000`65340000 00000000`6543f000   readxl     (export symbols)       C:\Users\Icewall\Documents\R\win-
    Loaded symbol image file: C:\Users\Icewall\Documents\R\win-library\3.4\readxl\libs\x64\readxl.dll
    Image path: C:\Users\Icewall\Documents\R\win-library\3.4\readxl\libs\x64\readxl.dll
    Image name: readxl.dll
    Browse all global symbols  functions  data
    Timestamp:        Wed Aug 30 18:38:23 2017 (59A6E9FF)
    CheckSum:         001018F6
    ImageSize:        000FF000
    Translations:     0000.04b0 0000.04e4 0409.04b0 0409.04e4

Linux version   

RAX: 0xdea898 
RBX: 0xb6a8c0 --> 0xaaaa0201 
RCX: 0x7fffffffdb80 --> 0x2000bd 
RDX: 0xbe 
RSI: 0x7fffffffdb80 --> 0x2000bd 
RDI: 0xffffffff 
RBP: 0x7fffffffdb60 --> 0x7fffffffdc00 --> 0x7fffffffdc70 --> 0x401610 (<__libc_csu_init>:      push   r15)
RSP: 0x7fffffffdb00 --> 0x60f3a0 --> 0x8000000c90b 
RIP: 0x7ffff7bce936 (<xls_addCell+144>: mov    WORD PTR [rax],dx)
R8 : 0x60f360 --> 0x0 
R9 : 0x800000003000000 
R10: 0x6f ('o')
R11: 0x7ffff7bce8a6 (<xls_addCell>:     push   rbp)
R12: 0x400b60 (<_start>:        xor    ebp,ebp)
R13: 0x7fffffffdd50 --> 0x2 
R14: 0x0 
R15: 0x0
EFLAGS: 0x10202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
   0x7ffff7bce92b <xls_addCell+133>:    mov    rax,QWORD PTR [rbp-0x50]
   0x7ffff7bce92f <xls_addCell+137>:    movzx  edx,WORD PTR [rax]
   0x7ffff7bce932 <xls_addCell+140>:    mov    rax,QWORD PTR [rbp-0x28]
=> 0x7ffff7bce936 <xls_addCell+144>:    mov    WORD PTR [rax],dx
   0x7ffff7bce939 <xls_addCell+147>:    mov    rax,QWORD PTR [rbp-0x58]
   0x7ffff7bce93d <xls_addCell+151>:    movzx  eax,WORD PTR [rax+0x4]
   0x7ffff7bce941 <xls_addCell+155>:    cwde   
   0x7ffff7bce942 <xls_addCell+156>:    mov    edi,eax
0000| 0x7fffffffdb00 --> 0x60f3a0 --> 0x8000000c90b 
0008| 0x7fffffffdb08 --> 0x607c50 --> 0xbbbbbbbbffffaaaa 
0016| 0x7fffffffdb10 --> 0x7fffffffdb80 --> 0x2000bd 
0024| 0x7fffffffdb18 --> 0x60f3a0 --> 0x8000000c90b 
0032| 0x7fffffffdb20 --> 0x60f3a0 --> 0x8000000c90b 
0040| 0x7fffffffdb28 --> 0x60f360 --> 0x0 
0048| 0x7fffffffdb30 --> 0x800000003000000 
0056| 0x7fffffffdb38 --> 0xdea898 
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x00007ffff7bce936 in xls_addCell (pWS=0x60f3a0, bof=0x7fffffffdb80, buf=0x607c50 
<incomplete sequence \345>) at xls.c:458
458         cell->id=bof->id;
gdb-peda$ bt
#0  0x00007ffff7bce936 in xls_addCell (pWS=0x60f3a0, bof=0x7fffffffdb80, buf=0x607c50 
<incomplete sequence \345>) at xls.c:458
#1  0x00007ffff7bd0b1a in xls_parseWorkSheet (pWS=0x60f3a0) at xls.c:1150
#2  0x0000000000400ff8 in main (argc=0x2, argv=0x7fffffffdd58) at xls2csv.c:149
#3  0x00007ffff781c830 in __libc_start_main (main=0x400d78 <main>, argc=0x2, argv=0x7fffffffdd58, init=<optimized out>, fini=
 <optimized out>, 
rtld_fini=<optimized out>, stack_end=0x7fffffffdd48) at ../csu/libc-
#4  0x0000000000400b89 in _start ()
gdb-peda$ exploitable -m
__main__:102: UserWarning: GDB v7.11 may not support required Python API
Warning: machine string printing is deprecated and may be removed in a future release.
DESCRIPTION:Access violation on destination operand
OTHER_RULES:AccessViolation (28/29)
EXPLANATION:The target crashed on an access violation at an address matching the destination operand of the instruction. This 
     likely indicates a write access 
     violation, which means the attacker may control the write 
     address and/or value.
Description: Access violation on destination operand
Short description: DestAv (9/29)
Hash: 34754c1fb7e7f36e18e374f783e4c876.34754c1fb7e7f36e18e374f783e4c876
Exploitability Classification: EXPLOITABLE
Explanation: The target crashed on an access violation at an address matching the destination operand of the instruction. This    
     likely indicates a write access 
     violation, which means the attacker may control the 
     write address and/or value.
Other tags: AccessViolation (28/29)


2017-10-25 - Vendor Disclosure
2017-11-15 - Public Release


Discovered by Marcin 'Icewall' Noga of Cisco Talos.