CWE-385: Covert Timing Channel

Description

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.

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, a cryptographic operation can expose its internal state if the time it takes to perform the operation varies, 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, and the time it takes to gain access to a shared bus.

Potential Impact

Confidentiality, Other

Read Application Data, Other

Demonstrative Examples

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.

Bad

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.

Mitigations & Prevention

Architecture and 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.

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.

Taxonomy Mappings

Landwehr: — Timing
CLASP: — Covert Timing Channel

Frequently Asked Questions

What is CWE-385?

CWE-385 (Covert Timing Channel) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. 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 informat...

How can CWE-385 be exploited?

Attackers can exploit CWE-385 (Covert Timing Channel) to read application data, other. This weakness is typically introduced during the Architecture and Design, Implementation phase of software development.

How do I prevent CWE-385?

Key mitigations include: Whenever possible, specify implementation strategies that do not introduce time variances in operations.

What is the severity of CWE-385?

CWE-385 is classified as a Base-level weakness (Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.