CWE

Common Weakness Enumeration

A Community-Developed List of Software & Hardware Weakness Types

CWE Top 25 Most Dangerous Weaknesses
Home > CWE List > CWE- Individual Dictionary Definition (4.2)  
ID

CWE-1254: Incorrect Comparison Logic Granularity

Weakness ID: 1254
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The product's comparison logic is performed over a series of steps rather than across the entire string in one operation. If there is a comparison logic failure on one of these steps, the operation may be vulnerable to a timing attack that can result in the interception of the process for nefarious purposes.
+ Extended Description

Comparison logic is used to compare a variety of objects including passwords, Message Authentication Codes (MACs), and responses to verification challenges. When comparison logic is implemented at a finer granularity (e.g., byte-by-byte comparison) and breaks in the case of a comparison failure, an attacker can exploit this implementation to identify when exactly the failure occurred. With multiple attempts, the attacker may be able to guesses the correct password/response to challenge and elevate their privileges.

+ Relationships

The table(s) below 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)
NatureTypeIDName
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.697Incorrect Comparison
ChildOfBaseBase - 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.208Observable Timing Discrepancy
PeerOfBaseBase - 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.1261Improper Handling of Single Event Upsets
+ Relevant to the view "Hardware Design" (CWE-1194)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1199General Circuit and Logic Design Concerns
+ 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.

PhaseNote
Architecture and Design
Implementation
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)

Architectures

Class: Architecture-Independent (Undetermined Prevalence)

Technologies

Class: Technology-Independent (Undetermined Prevalence)

+ Common Consequences

The table below 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.

ScopeImpactLikelihood
Confidentiality
Authorization

Technical Impact: Bypass Protection Mechanism

+ 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: Other 
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.

(informative)
 

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
+ Observed Examples
ReferenceDescription
The passwordCheck function in SAP Router 721 patch 117, 720 patch 411, 710 patch 029, and earlier terminates validation of a Route Permission Table entry password upon encountering 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.
+ Potential Mitigations

Phase: Implementation

The hardware designer should ensure that comparison logic is implemented so as to compare in one operation instead in smaller chunks.

+ References
[REF-1079] Joe Fitzpatrick. "SCA4n00bz - Timing-based Sidechannel Attacks for Hardware N00bz workshop". <https://github.com/securelyfitz/SCA4n00bz>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2020-02-12Arun Kanuparthi, Hareesh Khattri, Parbati Kumar Manna, Narasimha Kumar V MangipudiIntel Corporation
+ Modifications
Modification DateModifierOrganization
2020-08-20CWE Content TeamMITRE
updated Relationships
More information is available — Please select a different filter.
Page Last Updated: August 20, 2020