Covert timing channel

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This is a Vulnerability. To view all vulnerabilities, please see the Vulnerability Category page.

Last revision (mm/dd/yy): 02/20/2009

Vulnerabilities Table of Contents


Unintended information about data gets leaked through observing the timing of events.


  • Confidentiality: Information leakage.

Exposure period

  • Design: Protocols usually have timing difficulties implicit in their design.
  • Implementation: Sometimes a timing covert channel can be dependent on implementation strategy. Example: Using conditionals may leak information, but using table lookup will not.



Required resources




Likelihood of exploit


Sometimes simply knowing when data is sent between parties can provide a malicious user with information that should be unauthorized.

Other times, externally monitoring the timing of operations can reveal sensitive data. For example, some cryptographic operations can leak their internal state if the time it takes to perform the operation changes, based on the state. In such cases, it is good to switch algorithms or implementation techniques. It is also reasonable to add artificial stalls to make the operation take the same amount of raw CPU time in all cases.

Risk Factors

  • Talk about the factors that make this vulnerability likely or unlikely to actually happen
  • Discuss the technical impact of a successful exploit of this vulnerability
  • Consider the likely [business impacts] of a successful attack


In Python:

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

In this example, the attacker can observe how long an authentication takes when the user types in the correct password. When the attacker tries his own values, he can first try strings of various length. When he finds 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 he guesses the first character right, the computation will take longer than when he guesses wrong. Such an attack can break even the most sophisticated password with a few hundred guesses.

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.

Related Attacks

Related Vulnerabilities

Related Controls

  • Control 1
  • Control 2
  • Design: Whenever possible, specify implementation strategies that do not introduce time variances in operations.
  • Implementation: Often one can artificially manipulate the time which operations take or - when operations occur - can remove information from the attacker.

Related Technical Impacts


Note: A reference to related CWE or CAPEC article should be added when exists. Eg: