CWE-208: Observable Timing DiscrepancyWeakness ID: 208 Vulnerability Mapping:
ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities
Abstraction: BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. |
 Description Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not. Extended Description In security-relevant contexts, even small variations in timing can be exploited by attackers to indirectly infer certain details about the product's internal operations. For example, in some cryptographic algorithms, attackers can use timing differences to infer certain properties about a private key, making the key easier to guess. Timing discrepancies effectively form a timing side channel. Common Consequences  This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.| Scope | Impact | Likelihood |
|---|
Confidentiality
Access Control
| Technical Impact: Read Application Data; Bypass Protection Mechanism | |
Relationships This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Research Concepts" (CWE-1000) | Nature | Type | ID | Name |
|---|
| ChildOf |  Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 203 | Observable Discrepancy | | ParentOf | Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 1254 | Incorrect Comparison Logic Granularity | | CanPrecede |  Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 327 | Use of a Broken or Risky Cryptographic Algorithm | | CanPrecede | Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 385 | Covert Timing Channel |
Relevant to the view "Software Development" (CWE-699) | Nature | Type | ID | Name |
|---|
| MemberOf |  Category - a CWE entry that contains a set of other entries that share a common characteristic. | 199 | Information Management Errors |
 Relevant to the view "Architectural Concepts" (CWE-1008) | Nature | Type | ID | Name |
|---|
| MemberOf | Category - a CWE entry that contains a set of other entries that share a common characteristic. | 1012 | Cross Cutting |
Modes Of Introduction The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.| Phase | Note |
|---|
| Architecture and Design | COMMISSION: This weakness refers to an incorrect design related to an architectural security tactic. | | Implementation | | | Operation | |
Demonstrative Examples Example 1 Consider an example hardware module that checks a user-provided password to grant access to a user. The user-provided password is compared against a golden value in a byte-by-byte manner. (bad code) Example Language: Verilog
always_comb @ (posedge clk)
begin
assign check_pass[3:0] = 4'b0;
for (i = 0; i < 4; i++) begin
if (entered_pass[(i*8 - 1) : i] eq golden_pass([i*8 - 1) : i])
assign check_pass[i] = 1;
continue;
else
assign check_pass[i] = 0;
break;
end
assign grant_access = (check_pass == 4'b1111) ? 1'b1: 1'b0;
end
Since the code breaks on an incorrect entry of password, an attacker can guess the correct password for that byte-check iteration with few repeat attempts. To fix this weakness, either the comparison of the entire string should be done all at once, or the attacker is not given an indication whether pass or fail happened by allowing the comparison to run through all bits before the grant_access signal is set.
always_comb @ (posedge clk)
begin
assign check_pass[3:0] = 4'b0;
for (i = 0; i < 4; i++) begin
if (entered_pass[(i*8 - 1) : i] eq golden_pass([i*8 -1) : i])
assign check_pass[i] = 1;
continue;
else
assign check_pass[i] = 0;
continue;
end
assign grant_access = (check_pass == 4'b1111) ? 1'b1: 1'b0;
end
Example 2 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 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
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. Observed Examples | Reference | Description |
|---|
| Java-oriented framework compares HMAC signatures using String.equals() instead of a constant-time algorithm, causing timing discrepancies |
| Smartphone OS uses comparison functions that are not in constant time, allowing side channels |
| Password-checking function in router terminates validation of a password entry when it encounters the first incorrect character, which allows remote attackers to obtain passwords via a brute-force attack that relies on timing differences in responses to incorrect password guesses, aka a timing side-channel attack. |
| SSL implementation does not perform a MAC computation if an incorrect block cipher padding is used, which causes an information leak (timing discrepancy) that may make it easier to launch cryptographic attacks that rely on distinguishing between padding and MAC verification errors, possibly leading to extraction of the original plaintext, aka the "Vaudenay timing attack." |
| Virtual machine allows malicious web site operators to determine the existence of files on the client by measuring delays in the execution of the getSystemResource method. |
| Product uses a shorter timeout for a non-existent user than a valid user, which makes it easier for remote attackers to guess usernames and conduct brute force password guessing. |
| Product immediately sends an error message when a user does not exist, which allows remote attackers to determine valid usernames via a timing attack. |
| FTP server responds in a different amount of time when a given username exists, which allows remote attackers to identify valid usernames by timing the server response. |
| Browser allows remote attackers to determine the existence of arbitrary files by setting the src property to the target filename and using Javascript to determine if the web page immediately stops loading, which indicates whether the file exists or not. |
Functional Areas
- Cryptography
- Authentication
Memberships This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources. Vulnerability Mapping Notes Usage: ALLOWED (this CWE ID could be used to map to real-world vulnerabilities) | Reason: Acceptable-Use | Rationale: This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities. | Comments: Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction. |
Notes Relationship Often primary in cryptographic applications and algorithms. Taxonomy Mappings | Mapped Taxonomy Name | Node ID | Fit | Mapped Node Name |
|---|
| PLOVER | | | Timing discrepancy infoleak |
Content History | Submissions |
|---|
| Submission Date | Submitter | Organization |
|---|
2006-07-19
(CWE Draft 3, 2006-07-19) | PLOVER | | | | Modifications |
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| Modification Date | Modifier | Organization |
|---|
| 2008-07-01 | Eric Dalci | Cigital | | updated Time_of_Introduction | | 2008-09-08 | CWE Content Team | MITRE | | updated Relationships, Other_Notes, Relationship_Notes, Taxonomy_Mappings | | 2008-10-14 | CWE Content Team | MITRE | | updated Description | | 2010-09-27 | CWE Content Team | MITRE | | updated Relationships | | 2011-03-29 | CWE Content Team | MITRE | | updated Name | | 2011-06-01 | CWE Content Team | MITRE | | updated Common_Consequences | | 2012-05-11 | CWE Content Team | MITRE | | updated Observed_Examples, Relationships | | 2014-06-23 | CWE Content Team | MITRE | | updated Other_Notes, Related_Attack_Patterns | | 2014-07-30 | CWE Content Team | MITRE | | updated Relationships | | 2017-05-03 | CWE Content Team | MITRE | | updated Related_Attack_Patterns | | 2017-11-08 | CWE Content Team | MITRE | | updated Applicable_Platforms, Modes_of_Introduction, Relationships | | 2020-02-24 | CWE Content Team | MITRE | | updated Description, Name, Relationships | | 2020-06-25 | CWE Content Team | MITRE | | updated Relationships | | 2023-01-31 | CWE Content Team | MITRE | | updated Related_Attack_Patterns | | 2023-04-27 | CWE Content Team | MITRE | | updated Observed_Examples, Relationships | | 2023-06-29 | CWE Content Team | MITRE | | updated Mapping_Notes | | 2023-10-26 | CWE Content Team | MITRE | | updated Demonstrative_Examples, Observed_Examples | 2024-02-29
(CWE 4.14, 2024-02-29) | CWE Content Team | MITRE | | updated Demonstrative_Examples | | Previous Entry Names |
|---|
| Change Date | Previous Entry Name |
|---|
| 2011-03-29 | Timing Discrepancy Information Leak | | | 2020-02-24 | Information Exposure Through Timing Discrepancy | |
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