Covert Timing Channel
Weakness ID: 385 (Weakness Base)Status: Incomplete
+ Description

Description Summary

Covert timing channels convey information by modulating some aspect of system behavior over time, so that the program receiving the information can observe system behavior and infer protected information.

Extended Description

In some instances, knowing when data is transmitted between parties can provide a malicious user with privileged information. Also, externally monitoring the timing of operations can potentially 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.

Covert channels are frequently classified as either storage or timing channels. Some examples of covert timing channels are the system's paging rate, the time a certain transaction requires to execute, the time it takes to gain access to a shared bus)

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms



+ Common Consequences

Information leakage.

+ Likelihood of Exploit


+ Demonstrative Examples

Example 1

(Bad Code)
Example Language: 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.

+ Potential Mitigations

Phase: Architecture and Design

Whenever possible, specify implementation strategies that do not introduce time variances in operations.

Phase: Implementation

Often one can artificially manipulate the time which operations take or -- when operations occur -- can remove information from the attacker.

Phase: Implementation

It is reasonable to add artificial or random delays so that the amount of CPU time consumed is independent of the action being taken by the application.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory361Time and State
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class514Covert Channel
Development Concepts699
Research Concepts (primary)1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPCovert Timing Channel
+ Content History
Submission DateSubmitterOrganizationSource
LandwehrExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Common Consequences, Relationships, Other Notes, Taxonomy Mappings
2008-10-14CWE Content TeamMITREInternal
updated Description
2009-07-27CWE Content TeamMITREInternal
updated Description, Other Notes, Potential Mitigations