Talk:EICAR test file

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[edit] Confirmed successes and failures

Please give the date (preferably in a signature) and nature of the scan (i.e. plaintext, encrypted, hdd, email, ...). Be sure to fully update your software so that your signed date is accurate.

Doesn't this violate Wikipedia's policy on No Original Research? --DragonHawk 20:39, 1 July 2006 (UTC)
Yes, but not badly, and at least the results are verifiable. Btw, "McAfee Total Protection for Small Businesses" obliterated the thing without giving me a chance to tell it not to. Stevage 13:48, 9 August 2006 (UTC)

[edit] Successes

[edit] Failures

  • Grisoft AVG Anti-Virus doesn't appear to recognize the string, even in plaintext. 74.132.209.231 03:02, 28 June 2006 (UTC)
  • ClamWin 0.88.2.3 doesn't appear to recognize the string when copy/pasted into a text editor (used UltraEdit). 74.132.209.231 17:55, 28 June 2006 (UTC)
  • AntiVir version 7, when saved as a plaintext file -- malo (tlk) (cntrbtns) 20:26, 28 June 2006 (UTC)
  • Avast! 4.7, plaintext completely unrecognized. -- malo (tlk) (cntrbtns) 20:26, 28 June 2006 (UTC)

[edit] Why do they detect it?

The one thing this article doesn't tell me is *why* AV programs pick it up. Are they specially taught to recognise this string, or is there something inherently "virus-like" in the file? When you think about it, since the file *is* benign, there's no particular reason they should do anything about it. It's like calling the fire brigade to tell them you're about to set off the fire alarm, then complaining when they don't show up. Stevage 13:48, 9 August 2006 (UTC)

It's more like holding fire drills, or testing your smoke alarm -- without starting a fire. Detecting the file means that your virus scanner is probably working properly. Furthermore, there's the possibility of testing your virus scanner's capability to detect viruses in compressed files, for example, without having to get an actual virus and then get your virus scanner to check it. You may want to use the Wikipedia:Reference desk for questions like this in future. Dysprosia 07:21, 10 August 2006 (UTC)
Reference desk? No, the article should explain more about how it works. heqs 14:33, 23 October 2006 (UTC)
The above guy has a point. If he still had to ask questions about basic information then this article needs expanding. The Kinslayer 12:26, 10 January 2007 (UTC)

[edit] It makes use of self-modifying code

It makes use of self-modifying code to work around technical issues that this constraint makes on the execution of the test string.

What does this sentance mean? Regards, Ben Aveling 05:17, 28 November 2006 (UTC)


I have restored this sentence, because it was very significative to my understanding of the EICAR file itself.

The executable code should end with two assembly language instructions: INT 0x21, INT 0x20, which respectively means "print that string" and "end". However, the instruction code for INT is 0xCD, which is not an ASCII character available from keyboard (though it is printable).

The actual file ends with the four-byte instruction pair DEC AX; SUB CX,[BX+SI+2A], which is responsible for the H+H* substring.

The EICAR file overwrites these last four bytes, in order to get the 0x21, 0x20 sequence. Most of the previous code (before the string offset) is only purposed for modifying this bytes (and passing the parameters to INT 21h).


Initial condition: AX=0 (I don't know why... it seems to be a precondition)

CS:0101  354F21        XOR     AX,214F    //AX=214F
CS:0104  50            PUSH    AX          
CS:0105  254041        AND     AX,4140    //AX=0140
CS:0108  50            PUSH    AX
CS:0109  5B            POP     BX         //BX<-AX

Now we have BX=0140

CS:010A  345C          XOR     AL,5C       
CS:010C  50            PUSH    AX
CS:010D  5A            POP     DX
CS:010E  58            POP     AX
CS:010F  353428        XOR     AX,2834
CS:0112  50            PUSH    AX
CS:0113  5E            POP     SI

At this point we have SI = 097B (the code is weird because the keyboard-ASCII requirement).

CS:0114  2937          SUB     [BX],SI

word ptr [BX] contains 2B48, hence 2B48-097B = 21CD (note the bytes are reverted within the word).

CS:0116  43            INC     BX
CS:0117  43            INC     BX
CS:0118  2937          SUB     [BX],SI

Increment BX by 2, and repeat the trick. Now word ptr [BX] contains 2A48, hence 2B48-097B = 20CD.

CS:011A  7D24          JGE     0140

This condition always evaluates true. The string EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$ is stored in CS:011B through CS:0139.

CS:0140  CD21          INT     21
CS:0142  CD20          INT     20

Voilà!

Rjgodoy (talk) 09:48, 25 March 2008 (UTC)

If it is not clear enough I could try to explain it better... Rjgodoy (talk) 09:54, 25 March 2008 (UTC)