Covert Channel

A covert channel is a path that can be used to transfer information in a way not intended by the system's designers.


Typically the system has not given authorization for the transmission and has no knowledge of its occurrence.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In this example, the attacker observes how long an authentication takes when the user types in the correct password.

When the attacker tries their own values, they can first try strings of various length. When they find a string of the right length, the computation will take a bit longer, because the for loop will run at least once. Additionally, with this code, the attacker can possibly learn one character of the password at a time, because when they guess the first character right, the computation will take longer than a wrong guesses. Such an attack can break even the most sophisticated password with a few hundred guesses.

def validate_password(actual_pw, typed_pw):

  if len(actual_pw) <> len(typed_pw):
    return 0

  for i in len(actual_pw):
    if actual_pw[i] <> typed_pw[i]:
      return 0

  return 1

Note that in this example, the actual password must be handled in constant time as far as the attacker is concerned, even if the actual password is of an unusual length. This is one reason why it is good to use an algorithm that, among other things, stores a seeded cryptographic one-way hash of the password, then compare the hashes, which will always be of the same length.

See Also

Comprehensive Categorization: Resource Control

Weaknesses in this category are related to resource control.

SFP Secondary Cluster: Covert Channel

This category identifies Software Fault Patterns (SFPs) within the Covert Channel cluster.

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Entries with Maintenance Notes

CWE entries in this view have maintenance notes. Maintenance notes are an indicator that an entry might change significantly in future versions. This view was created...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.

Common Weakness Enumeration content on this website is copyright of The MITRE Corporation unless otherwise specified. Use of the Common Weakness Enumeration and the associated references on this website are subject to the Terms of Use as specified by The MITRE Corporation.