Failure to Use Economy of Mechanism
Weakness ID: 637 (Weakness Class)Status: Draft
+ Description

Description Summary

The software uses a more complex mechanism than necessary, which could lead to resultant weaknesses when the mechanism is not correctly understood, modeled, configured, implemented, or used.

Extended Description

Security mechanisms should be as simple as possible. Complex security mechanisms may engender partial implementations and compatibility problems, with resulting mismatches in assumptions and implemented security. A corollary of this principle is that data specifications should be as simple as possible, because complex data specifications result in complex validation code. Complex tasks and systems may also need to be guarded by complex security checks, so simple systems should be preferred.

+ Alternate Terms
Unnecessary Complexity
+ Time of Introduction
  • Architecture and Design
  • Implementation
  • Operation
+ Applicable Platforms



+ Demonstrative Examples

Example 1

The IPSEC specification is complex, which resulted in bugs, partial implementations, and incompatibilities between vendors.

Example 2

HTTP Request Smuggling (CWE-444) attacks are feasible because there are not stringent requirements for how illegal or inconsistent HTTP headers should be handled. This can lead to inconsistent implementations in which a proxy or firewall interprets the same data stream as a different set of requests than the end points in that stream.

+ Observed Examples
CVE-2007-6067Support for complex regular expressions leads to a resultant algorithmic complexity weakness (CWE-407).
CVE-2007-1552Either a filename extension and a Content-Type header could be used to infer the file type, but the developer only checks the Content-Type, enabling unrestricted file upload (CWE-434).
CVE-2007-6479In Apache environments, a "filename.php.gif" can be redirected to the PHP interpreter instead of being sent as an image/gif directly to the user. Not knowing this, the developer only checks the last extension of a submitted filename, enabling arbitrary code execution.
CVE-2005-2148The developer cleanses the $ REQUEST superglobal array, but PHP also populates $ GET, allowing attackers to bypass the protection mechanism and conduct SQL injection attacks against code that uses $ GET.
+ Potential Mitigations

Avoid complex security mechanisms when simpler ones would meet requirements. Avoid complex data models, and unnecessarily complex operations. Adopt architectures that provide guarantees, simplify understanding through elegance and abstraction, and that can be implemented similarly. Modularize, isolate and do not trust complex code, and apply other secure programming principles on these modules (e.g., least privilege) to mitigate vulnerabilities.

+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class657Violation of Secure Design Principles
Development Concepts (primary)699
Research Concepts (primary)1000
+ Research Gaps

Since design issues are hard to fix, they are rarely publicly reported, so there are few CVE examples of this problem as of January 2008. Most publicly reported issues occur as the result of an implementation error instead of design, such as CVE-2005-3177 (failure to handle large numbers of resources) or CVE-2005-2969 (inadvertently disabling a verification step, leading to selection of a weaker protocol).

+ Causal Nature


+ References
Jerome H. Saltzer and Michael D. Schroeder. "The Protection of Information in Computer Systems". Proceedings of the IEEE 63. September, 1975. <>.
Sean Barnum and Michael Gegick. "Economy of Mechanism". 2005-09-13. <>.
+ Content History
Submission DateSubmitterOrganizationSource
2008-01-18Pascal MeunierPurdue UniversityExternal Submission
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Description, Relationships, Weakness Ordinalities
2009-01-12CWE Content TeamMITREInternal
updated Description, Name
Previous Entry Names
Change DatePrevious Entry Name
2009-01-12Design Principle Violation: Not Using Economy of Mechanism