Class Virus
Platform DOS

Technical Details

The “Vacsina” (not “VACCINA”) and “Yankee” family comprises more than 40
non-memory resident, benign parasitic viruses. Each of them bears in
its body a label consisting of three bytes: F4h, 7Ah, NNh. The third byte, NNh,
is considered as a number of the virus in the family (number of the virus
version), and hereafter will be met in the names of viruses (“Vacsina.NN” and
“Yankee.NN”). After activation, the viruses of this family write the bytes
7Fh, 39h, and NNh into the memory at the addresses 0000:00C5, 0000:00C6,
0000:00C7. Viruses of the family are distinguished for recovering files
infected by previous versions of the viruses of that family and reinfecting
them. The viruses “Vacsina.NN” contain the word “VACSINA” in their bodies.

The viruses infect COM and EXE files that are executed (viruses of versions
up to 26h) or loaded into the memory (viruses of versions 26h and above).
The viruses infect COM files in a standard way: the viruses of versions up to
9th check the first byte in a COM file being infected. If this byte is not
equal to E9h (JMP), the file will not be infected. The viruses of higher
versions infect .COM files independently on their first byte. Apart from
this, the “VACSINA” viruses increase the length of the infected file up to a
paragraph. Versions 2Ah and above append 4 additional bytes to the file
after it has been infected.

The viruses of earlier versions (up to 23h) infect EXE files in a special
manner – the files are transformed to the COM file format. For this purpose,
the file is appended with a small fragment of the virus code (132 bytes) that
adjusts the addresses according to the address table upon loading the file
into the memory for execution; the first three bytes of the file (JMP to the
fragment) are also altered. The 132 bytes appended to the file do not
spread the virus. Their function is only to adjust the program addresses upon
its execution. The file processed in this way will be executed by the
operating system, and will be infected by many viruses as a COM file. As such,
an EXE file is run, control is transferred to the address relocation
fragment, which checks the address table in the file header, and corrects the
loaded program. Then control is transferred to the start address noted in
the file header.

The viruses hook the interrupt vector 21h. Some members of the “Yankee” family
hook INT 1, 3, 9, and 1Ch.

The viruses make sound effects: as a file is infected by a “VACSINA” virus, a
sound signal (BELL) is made; the “Yankee” viruses, depending on some
preconditions (when pressing ALT-Ctrl-Del simultaneously or at 5:00 p.m.), play
the “Yankee Doodle Dandy” melody. Under some conditions, the “Vacsina.06” virus
deciphers and displays the string: “Az sum vasta lelja.”

There are several “piquancies” in these viruses: versions 18h and above
determine the initial value of the 21h interrupt vector (address of the
interrupt handler in DOS), and upon incorporating into files, cause an interrupt
on that value. In this way, the viruses bypass memory-resident antiviral
monitors. To determinate the address of the interrupt handler, the following
algorithm is used:

  • set the 01h interrupt (entry at tracing; this interrupt is called in
    debugging mode after performing every instruction by the processor) to the
    subroutine determining the true value of the interrupt vector;
  • turn on the tracing mode;
  • call a “harmless” interrupt of the function 21h (from the standpoint of
    memory-resident anti-viruses).

Upon executing a “harmless” function of INT 21h, instructions of all programs,
which have altered the interrupt vector 21h, are executed in sequential
order, then the operating system commands handling this interrupt are
performed. After every executed command, control is transferred over to
the virus subroutine, which determines the initial value of the interrupt
vector. It checks the address of the last executed command, and if this
address points to the memory area belonging to the operating system, it turns
the debugging mode off, and treats the address of this command as the initial
value of the 21h interrupt vector.

00000 � … �
�Operating �
+——————>�system �
� +—————-�
� INT 21h +—————-�
+——————-�User �——- Programs, having modified
+——————>�program � � interrupt vector 21h
� +—————-� �
� � … � �
� INT 21h +—————-� �
+——————-�User �—+
+——————>�program �
� +—————-�
� � … �
� +—————-�
� �Virus �
� INT 21h � ————-�
+——————-�Command calling �
� interrupt 21h �
^ � � ————-�
� +————>�Program for �
� �determination of�
+—————-�vector �
FFFFF � … �

Versions 21h and above hook debugger actions, and, on the condition that the body
of the virus is debugged, stop the debugging process. During an attempt to dump
an infected file under the debugger, the file “cures itself”.

Version 21h and above encode themselves by linear block code, and keep checking
bytes in their body. Periodically, the viruses check themselves for the presence
of modifications, and correct them if possible.

Versions 2Ch and above deactivate the memory-resident part of the “PingPong” virus. Versions 2Eh and above take some counter
measures against the “Cascade” viruses.

Modifications of “VACSINA” and “Yankee”

The “VACSINA” and “Yankee” viruses are rather “popular”: apart from several
dozen “original” viruses, their mutations, in some cases rather
expertly copied from the original, have started to crop up. Thus, in
Irkutsk (Russia), the virus (“Yankee.1905”), challenging the right to be
referred to as “Yankee.53”, has been found. But not withstanding its number
(53h), it uses only one “piquancy” – traces INT 21h.

“Yankee.1049,1150,1202” – are harmless memory-resident file viruses. They
infect COM and EXE files when they are executed (INT 21h, “Yankee.1049” –
ax=4B00h, “Yankee.1202,1150” – ah= 4Bh). The viruses alter the first 32
bytes of a file, and attach themselves to its end, and in many respects, coincide with the
“Yankee” viruses. The 21h vector is used.

“Yankee.3045” contains the text strings: “LOGIN.EXE SUPERVISOR. HESLO.”.

“Yankee.Flip.2167”: depending on the system date, it hooks INT 8, 9, and 10h, and
“flips” the screen.


“Yankee” family. This virus is encrypted, sometimes it rolls the
symbols ‘/’, ”, ‘-‘, ‘|’.


“Yankee” family. It infects COM and EXE files, with the exception of COMMAND.COM,
that are executed. It appends 4 control bytes to COM files. On Monday at 2:00 p.m., it decrypts and displays, to the center of the screen, the following message:
“Independent Estonia presents”, then plays a tune and reboots the computer.

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