mdw@git.distorted.org.uk Git - qmail/blob - forgeries.7

   1 .TH forgeries 7
   2 .SH "NAME"
   3 forgeries \- how easy it is to forge mail
   4 .SH "SUMMARY"
   5 An electronic mail message can easily be forged.
   6 Almost everything in it,
   7 including the return address,
   8 is completely under the control of the sender.
   9
  10 An electronic mail message can be manually traced to its origin
  11 if (1) all system administrators of intermediate machines
  12 are both cooperative and competent,
  13 (2) the sender did not break low-level TCP/IP security,
  14 and
  15 (3) all intermediate machines are secure.
  16
  17 Users of
  18 .I cryptography
  19 can automatically ensure the integrity and secrecy
  20 of their mail messages, as long as
  21 the sending and receiving machines are secure.
  22 .SH "FORGERIES"
  23 Like postal mail,
  24 electronic mail can be created entirely at the whim of the sender.
  25 .BR From ,
  26 .BR Sender ,
  27 .BR Return-Path ,
  28 and
  29 .BR Message-ID
  30 can all contain whatever information the sender wants.
  31
  32 For example, if you inject a message through
  33 .B sendmail
  34 or
  35 .B qmail-inject
  36 or
  37 .BR SMTP ,
  38 you can simply type in a
  39 .B From
  40 field.
  41 In fact,
  42 .B qmail-inject
  43 lets you set up
  44 .BR MAILUSER ,
  45 .BR MAILHOST ,
  46 and
  47 .B MAILNAME
  48 environment variables
  49 to produce your desired
  50 .B From
  51 field on every message.
  52 .SH "TRACING FORGERIES"
  53 Like postal mail,
  54 electronic mail is postmarked when it is sent.
  55 Each machine that receives an electronic mail message
  56 adds a
  57 .B Received
  58 line to the top.
  59
  60 A modern
  61 .B Received
  62 line contains quite a bit of information.
  63 In conjunction with the machine's logs,
  64 it lets a competent system administrator
  65 determine where the machine received the message from,
  66 as long as the sender did not break low-level TCP/IP security
  67 or security on that machine.
  68
  69 Large multi-user machines often come with inadequate logging software.
  70 Fortunately, a system administrator can easily obtain a copy of a
  71 931/1413/Ident/TAP server, such as
  72 .BR pidentd .
  73 Unfortunately,
  74 some system administrators fail to do this,
  75 and are thus unable to figure out which local user
  76 was responsible for generating a message.
  77
  78 If all intermediate system administrators are competent,
  79 and the sender did not break machine security or low-level TCP/IP security,
  80 it is possible to trace a message backwards.
  81 Unfortunately, some traces are stymied by intermediate system
  82 administrators who are uncooperative or untrustworthy.
  83 .SH "CRYPTOGRAPHY"
  84 The sender of a mail message may place his message into a
  85 .I cryptographic
  86 envelope stamped with his seal.
  87 Strong cryptography guarantees that any two messages with the same seal
  88 were sent by the same cryptographic entity:
  89 perhaps a single person, perhaps a group of cooperating people,
  90 but in any case somebody who knows a secret originally held
  91 only by the creator of the seal.
  92 The seal is called a
  93 .I public key\fR.
  94
  95 Unfortunately, the creator of the seal is often an insecure machine,
  96 or an untrustworthy central agency,
  97 but most of the time seals are kept secure.
  98
  99 One popular cryptographic program is
 100 .BR pgp .
 101 .SH "SEE ALSO"
 102 pgp(1),
 103 identd(8),
 104 qmail-header(8)