2

Yesterday a file that should not exist appeared in my Downloads folder.

Virustotal knows the file since 2012 and reports it as clean - but it seems other people have had it appear.

Does anyone know more? Anyone else seen this? I assume it's harmless and caused by a bug, but maybe not?

File: a.out 1759 bytes

MD5 f61595bec6cc87903ce1677723c878cb

SHA1 fbe66f197516eed6bb357db1bb19ed0d2c3d8fa8

SHA256 e20c67711743553e16501b65a9921f59643edafd32d46e37f916d43e0023ffc3

ssdeep
12:Bvw/bg0scb46s4czclqJ/dXrI2+1E2X3VE6XVZ6tXcq/+/39WuZ8KVXMnikezGFE:GfE6s+u+NHdX+tMqQMOMW


    Disassembly:
a.out:     file format elf64-x86-64


Disassembly of section .interp:

0000000000400158 <.interp>:
  400158:   2f                      (bad)  
  400159:   6c                      insb   (%dx),%es:(%rdi)
  40015a:   69 62 2f 6c 64 36 34    imul   $0x3436646c,0x2f(%rdx),%esp
  400161:   2e 73 6f                jae,pn 4001d3 <_GLOBAL_OFFSET_TABLE_+0x4001d3>
  400164:   2e 31 00                xor    %eax,%cs:(%rax)

Disassembly of section .hash:

0000000000400168 <.hash>:
  400168:   01 00                   add    %eax,(%rax)
    ...

Disassembly of section .dynstr:

0000000000400178 <.dynstr>:
  400178:   00 6c 69 62             add    %ch,0x62(%rcx,%rbp,2)
  40017c:   72 74                   jb     4001f2 <_GLOBAL_OFFSET_TABLE_+0x4001f2>
  40017e:   2e 73 6f                jae,pn 4001f0 <_GLOBAL_OFFSET_TABLE_+0x4001f0>
  400181:   2e 31 00                xor    %eax,%cs:(%rax)

Disassembly of section .eh_frame:

0000000000400188 <.eh_frame>:
  400188:   14 00                   adc    $0x0,%al
  40018a:   00 00                   add    %al,(%rax)
  40018c:   00 00                   add    %al,(%rax)
  40018e:   00 00                   add    %al,(%rax)
  400190:   01 7a 52                add    %edi,0x52(%rdx)
  400193:   00 01                   add    %al,(%rcx)
  400195:   78 10                   js     4001a7 <_GLOBAL_OFFSET_TABLE_+0x4001a7>
  400197:   01 1b                   add    %ebx,(%rbx)
  400199:   0c 07                   or     $0x7,%al
  40019b:   08 90 01 00 00 24       or     %dl,0x24000001(%rax)
  4001a1:   00 00                   add    %al,(%rax)
  4001a3:   00 1c 00                add    %bl,(%rax,%rax,1)
    ...
  4001ae:   00 00                   add    %al,(%rax)
  4001b0:   00 0e                   add    %cl,(%rsi)
  4001b2:   10 46 0e                adc    %al,0xe(%rsi)
  4001b5:   18 4a 0f                sbb    %cl,0xf(%rdx)
  4001b8:   0b 77 08                or     0x8(%rdi),%esi
  4001bb:   80 00 3f                addb   $0x3f,(%rax)
  4001be:   1a 3b                   sbb    (%rbx),%bh
  4001c0:   2a 33                   sub    (%rbx),%dh
  4001c2:   24 22                   and    $0x22,%al
  4001c4:   00 00                   add    %al,(%rax)
    ...

Disassembly of section .dynamic:

00000000006001c8 <_DYNAMIC>:
  6001c8:   01 00                   add    %eax,(%rax)
  6001ca:   00 00                   add    %al,(%rax)
  6001cc:   00 00                   add    %al,(%rax)
  6001ce:   00 00                   add    %al,(%rax)
  6001d0:   01 00                   add    %eax,(%rax)
  6001d2:   00 00                   add    %al,(%rax)
  6001d4:   00 00                   add    %al,(%rax)
  6001d6:   00 00                   add    %al,(%rax)
  6001d8:   04 00                   add    $0x0,%al
  6001da:   00 00                   add    %al,(%rax)
  6001dc:   00 00                   add    %al,(%rax)
  6001de:   00 00                   add    %al,(%rax)
  6001e0:   68 01 40 00 00          pushq  $0x4001
  6001e5:   00 00                   add    %al,(%rax)
  6001e7:   00 05 00 00 00 00       add    %al,0x0(%rip)        # 6001ed <_DYNAMIC+0x25>
  6001ed:   00 00                   add    %al,(%rax)
  6001ef:   00 78 01                add    %bh,0x1(%rax)
  6001f2:   40 00 00                add    %al,(%rax)
  6001f5:   00 00                   add    %al,(%rax)
  6001f7:   00 06                   add    %al,(%rsi)
  6001f9:   00 00                   add    %al,(%rax)
  6001fb:   00 00                   add    %al,(%rax)
  6001fd:   00 00                   add    %al,(%rax)
  6001ff:   00 78 01                add    %bh,0x1(%rax)
  600202:   40 00 00                add    %al,(%rax)
  600205:   00 00                   add    %al,(%rax)
  600207:   00 0a                   add    %cl,(%rdx)
  600209:   00 00                   add    %al,(%rax)
  60020b:   00 00                   add    %al,(%rax)
  60020d:   00 00                   add    %al,(%rax)
  60020f:   00 0c 00                add    %cl,(%rax,%rax,1)
  600212:   00 00                   add    %al,(%rax)
  600214:   00 00                   add    %al,(%rax)
  600216:   00 00                   add    %al,(%rax)
  600218:   0b 00                   or     (%rax),%eax
  60021a:   00 00                   add    %al,(%rax)
  60021c:   00 00                   add    %al,(%rax)
  60021e:   00 00                   add    %al,(%rax)
  600220:   18 00                   sbb    %al,(%rax)
  600222:   00 00                   add    %al,(%rax)
  600224:   00 00                   add    %al,(%rax)
  600226:   00 00                   add    %al,(%rax)
  600228:   15 00 00 00 00          adc    $0x0,%eax
    ...

Did not run compile jobs at that time.

Ubuntu 12.04.04 LTS 64bit.

File appeared while downloading packages from sourceforge with Firefox.

Bears same timestamp as one of the downloads.

Computer is connected to internet via router w/ firewall.


  • I am certain the file is not of my own creation.
  • Browser does not show any downloads apart from the ones I initiated.
  • No site likely to infect visited at that point in time - unless Nixnote's sourceforge page got hacked.
  • stat reveals nothing of note, no Birth date set.

readelf output:

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          64 (bytes into file)
  Start of section headers:          744 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         5
  Size of section headers:           64 (bytes)
  Number of section headers:         10
  Section header string table index: 7

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000400158  00000158
       000000000000000f  0000000000000000   A       0     0     1
  [ 2] .hash             HASH             0000000000400168  00000168
       000000000000000c  0000000000000004   A       3     0     8
  [ 3] .dynsym           DYNSYM           0000000000400178  00000178
       0000000000000000  0000000000000018   A       4     1     8
  [ 4] .dynstr           STRTAB           0000000000400178  00000178
       000000000000000c  0000000000000000   A       0     0     1
  [ 5] .eh_frame         PROGBITS         0000000000400188  00000188
       0000000000000040  0000000000000000   A       0     0     8
  [ 6] .dynamic          DYNAMIC          00000000006001c8  000001c8
       00000000000000d0  0000000000000010  WA       4     0     8
  [ 7] .shstrtab         STRTAB           0000000000000000  00000298
       000000000000004c  0000000000000000           0     0     1
  [ 8] .symtab           SYMTAB           0000000000000000  00000568
       0000000000000138  0000000000000018           9     9     8
  [ 9] .strtab           STRTAB           0000000000000000  000006a0
       000000000000003f  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

There are no section groups in this file.

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x0000000000000118 0x0000000000000118  R E    8
  INTERP         0x0000000000000158 0x0000000000400158 0x0000000000400158
                 0x000000000000000f 0x000000000000000f  R      1
      [Requesting program interpreter: /lib/ld64.so.1]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000001c8 0x00000000000001c8  R      200000
  LOAD           0x00000000000001c8 0x00000000006001c8 0x00000000006001c8
                 0x00000000000000d0 0x00000000000000d0  RW     200000
  DYNAMIC        0x00000000000001c8 0x00000000006001c8 0x00000000006001c8
                 0x00000000000000d0 0x00000000000000d0  RW     8

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .hash .dynsym .dynstr .eh_frame 
   03     .dynamic 
   04     .dynamic 

Dynamic section at offset 0x1c8 contains 8 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000004 (HASH)               0x400168
 0x0000000000000005 (STRTAB)             0x400178
 0x0000000000000006 (SYMTAB)             0x400178
 0x000000000000000a (STRSZ)              12 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000000 (NULL)               0x0

There are no relocations in this file.

There are no unwind sections in this file.

Symbol table '.dynsym' contains 0 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name

Symbol table '.symtab' contains 13 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000400158     0 SECTION LOCAL  DEFAULT    1 
     2: 0000000000400168     0 SECTION LOCAL  DEFAULT    2 
     3: 0000000000400178     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000400178     0 SECTION LOCAL  DEFAULT    4 
     5: 0000000000400188     0 SECTION LOCAL  DEFAULT    5 
     6: 00000000006001c8     0 SECTION LOCAL  DEFAULT    6 
     7: 00000000006001c8     0 OBJECT  LOCAL  DEFAULT    6 _DYNAMIC
     8: 0000000000000000     0 OBJECT  LOCAL  DEFAULT    6 _GLOBAL_OFFSET_TABLE_
     9: 0000000000600298     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    10: 0000000000600298     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _start
    12: 0000000000600298     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start

Histogram for bucket list length (total of 1 buckets):
 Length  Number     % of total  Coverage
      0  1          (100.0%)

No version information found in this file.
0

If you run the command ld -lrt, ld creates a file "a.out" with the same hash.

You probably created this file with a wrongly typed ls -lrt command.

|improve this answer|||||
1

a.out is the default output filename for compiling something with the GCC suite of compilers.

If you've been downloading things that need compiling and have run make or gcc recently, that's probably your answer. You could probably work out what it is with strings a.out | less and seeing what strings it contains; that tends to be a bit easier than hexing.


If you're sure it's not something of your own creation, check the creation date (stat a.out) and check your browsers' download histories. That might suggest where you've downloaded it from.

Failing that, check your normal browsing history. Might explain what you were up to (and if you came into contact with anything likely to infect you).

|improve this answer|||||
  • I know that. I did not do any compiling at that point and certainly not in ~/Downloads directly. Strings only returns this: /lib/ld64.so.1 librt.so.1 ;*3$" – Rock Jun 10 '14 at 11:28
  • a.out is also the name of the object file format itself. – Nathan Osman Jun 10 '15 at 21:09
0

Maybe you downloaded that file from sourceforge as an installer? If it is a virus, it is good to check... Press Ctrl-Alt-T and a terminal will open. Type

file ~/Downloads/a.out

If it does not say anything bad, use the readelf command and the elfedit command to investigate it. Post the output here.

JP

|improve this answer|||||
  • posted the output of readelf – Rock Jun 10 '14 at 12:54
0

It happened to me as well (I'm using Ubuntu 16.04 LTS and the file appeared in my home folder). When I backtracked and looked at what commands I've used in terminal, I found that I was playing around with the 'as' command while trying to wrap my head around assembly language. There's a small chance users type 'as' instead of 'ls' and this happens (maybe?).

Just a thought.

|improve this answer|||||
  • When I type locate a.out I see many files. – WinEunuuchs2Unix Mar 20 '18 at 3:28

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