Virus Name: Cascade
Aliases: BlackJack, Fall, Falling Letters, 1701, 1704, 1701 Mutation,
1704 Format, 1704-B
V Status: Common
Discovery: October, 1987
Symptoms: TSR; falling letters; .COM file growth; random reboots
Eff Length: 1,701 or 1,704 bytes
Type Code: PRsC - Parasitic Resident Encrypting .COM Infector
Detection Method: ViruScan, F-Prot, AVTK, NAV, ChAV,
Sweep, IBMAV, NAVDX, VAlert, PCScan,
NShld, LProt, Sweep/N, Innoc, NProt, AVTK/N,
Removal Instructions: F-Prot, or delete infected files
Originally, this virus was a Trojan horse which was disguised as a
program which was supposed to turn off the Num-Lock light when
the system was booted. The Trojan horse instead caused all the
characters on the screen to fall into a pile at the bottom of the
screen. In late 1987, the Trojan horse was changed by someone into
a memory resident .COM virus.
While the original virus had a length of 1,701 bytes and would
infect both true IBM PCs and clones, a variation exists of this
virus which is 3 bytes longer than the original virus and does not
infect true IBM PCs. Both viruses are functionally identical in all
Both of the viruses have some fairly unique qualities: Both use an
encryption algorithm to avoid detection and complicate any attempted
analysis of them. The activation mechanisms are based on a
sophisticated randomization algorithm incorporating machine checks,
monitor types, presence or absence of a clock card, and the time or
season of the year.
The viruses will activate on any machine with a CGA or VGA monitor
in the months of September, October, November, or December in the
years 1980 and 1988.
Known variant(s) of Cascade are:
1701 Mutation: Received in October, 1991, this variant of has
had two bytes modified in order to avoid detection.
1701-B: Same as 1701, except that it can activate in the Fall of
1701-Yap: Based on the original Cascade virus, this variant
adds 1,701 bytes to the .COM programs it infects. Its
memory resident TSR is 2,048 bytes, and hooks interrupt 21.
The encryption mechanism has been slightly altered to avoid
detection by some anti-viral utilities.
Origin: Unknown October, 1992.
1704 Format: Like the Cascade virus, but the disk is formatted
when the virus activates. Activation occurs during the
months of October, November, and December of any year except
Origin: Unknown January, 1989.
1704-C: Same as Cascade-B except that the virus can activate in
December of any year.
1704-D: Same as the 1704, except that the IBM selection has been
disabled so that it can infect true IBM PCs.
17Y4: Similar to the Cascade 1704 virus, the only difference is
one byte in the virus which has been altered.
Cascade-1621: Based on the original Cascade virus, this variant
adds 1,621 bytes to the .COM programs it infects. Its
memory resident TSR is 1,936 bytes, and hooks interrupt 21.
Attempts to execute .BAT files on infected systems may
result in the scrolling of the message "Insufficient
disk space", and the .BAT file not executing.
Origin: Unknown June, 1992.
Cascade 1701.E: Based on the original Cascade virus, this is a
minor variant. It drops a single character down the system
display when it is memory resident from September 1 to
December 31 of any year.
Origin: Unknown September, 1993.
Cascade.1702: Based on the original Cascade virus, this variant
adds 1,702 bytes to the .COM programs it infects. It is
a memory resident virus which employs a 1,936 byte TSR
hooking interrupts 21. The virus will be located at the end
of infected files.
Origin: Unknown July, 1994.
Cascade-1704G: Based on the original Cascade virus, this variant
has been altered to avoid detection by some anti-viral
programs by the addition of a NOP instruction in the virus'
decryption routine. It adds 1,704 bytes to the .COM programs
it infects, with the virus being located at the end of the
file. It is a memory resident virus which employs a 1,968
byte TSR hooking interrupt 21.
Origin: France May, 1993.
Cascade-1706: Based on the original Cascade virus, this variant
adds 1,706 bytes to the .COM programs it infects. It is
a memory resident virus which employs a 2,064 byte TSR
hooking interrupts 1C and 21. The virus will be located
at the end of infected files.
Origin: Unknown April, 1992.
Cascade-B: Similar to the Cascade virus, except that the
cascading display has been replaced with a system reboot
which will occur at random time intervals after the
Cunning: Based on the Cascade virus, a major change to the virus is
that it now plays music.
See: 1661 JoJo