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Topic:                  unix mailbox parsing trouble in qpopper

Software affected:      qpopper 3.0 fc2 and probably others

Description:            malicious   user  can  remotely  post  message
                        with  spoofed  or incorrect headers (including
                        "Received:" one)  and  in  some  cases  bypass
                        virus  checking.  This can be used for sending
                        trojans or to attack vulnerabilities in MUA.


In  unix  systems  e-mail  delivered  to user is usually stored in his
mailbox,  which  has  predefined  format  (so-called "unix mailbox" or
"berkley  mailbox").  This  mailbox  holds  messages  in  plain format
separated  by  empty  line ("\n") and specially formed "From " header.
The pattern of the next message in mailbox is "\n\nForm ".
Then  local mail programs (f.e. mail.local) delivers message to user's
mailbox   it  searches  for  this  pattern and if message contains one
"From   "   will  be  commented out by '>' and additional '\n' will be
added  to  message  if  necessary.  This assumes mailbox integrity and
protects from e-mail spoofing.

Problem description:

qpopper  has vulnerability which allows for malicious user to generate
his  own  "From " with followed email headers and text. The problem is
in the way qpopper reads data from mailbox. Qpopper users fgets()-like
routine,  mfgets(),  which reads data from mailbox into the fixed 1024
byte  buffer and returns string in case either '\n' character received
or 1023 bytes read. Malicious user can put text like

AAAA...AAA(string of 1023 symbols)\n
From user Wed Dec  2 05:53 -0700 1992

In this case  mfgets() will return 3 strings:
"AAAA...AAA(string of 1023)symbols",
"From user Wed Dec 2 05:53 -0700 1992"
and  this  will be recognized as a beginning of the new message in the

Text  after "From " string will be recognized as a headers and text of
the   next  message,  allowing  to  generate  any  headers  and  text.
Additionally, this "internal" messages will be treated by any software
as  a  plain  text  inside message, without any MIME attachments. This
allows  to  bypass  virus  checking in case antiviral tools scans only
attached files.

Additional Info:
mail.local  also  uses  fgets() for reading input message, but default
buffer size is 2048, so "From " will not be commented.