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CWE-14: Compiler Removal of Code to Clear Buffers

 
Compiler Removal of Code to Clear Buffers
Weakness ID: 14 (Weakness Base)Status: Draft
+ Description

Description Summary

Sensitive memory is cleared according to the source code, but compiler optimizations leave the memory untouched when it is not read from again, aka "dead store removal."

Extended Description

This compiler optimization error occurs when:

1. Secret data are stored in memory.

2. The secret data are scrubbed from memory by overwriting its contents.

3. The source code is compiled using an optimizing compiler, which identifies and removes the function that overwrites the contents as a dead store because the memory is not used subsequently.

+ Time of Introduction
  • Implementation
  • Build and Compilation
+ Applicable Platforms

Languages

C

C++

+ Common Consequences
ScopeEffect

Technical Impact: Read memory; Bypass protection mechanism

This weakness will allow data that has not been cleared from memory to be read. If this data contains sensitive password information, then an attacker can read the password and use the information to bypass protection mechanisms.

+ Detection Methods

Black Box

This specific weakness is impossible to detect using black box methods. While an analyst could examine memory to see that it has not been scrubbed, an analysis of the executable would not be successful. This is because the compiler has already removed the relevant code. Only the source code shows whether the programmer intended to clear the memory or not, so this weakness is indistinguishable from others.

White Box

This weakness is only detectable using white box methods (see black box detection factor). Careful analysis is required to determine if the code is likely to be removed by the compiler.

+ Demonstrative Examples

Example 1

The following code reads a password from the user, uses the password to connect to a back-end mainframe and then attempts to scrub the password from memory using memset().

(Bad Code)
Example Language:
void GetData(char *MFAddr) {
char pwd[64];
if (GetPasswordFromUser(pwd, sizeof(pwd))) {

if (ConnectToMainframe(MFAddr, pwd)) {

// Interaction with mainframe
}
}
memset(pwd, 0, sizeof(pwd));
}

The code in the example will behave correctly if it is executed verbatim, but if the code is compiled using an optimizing compiler, such as Microsoft Visual C++ .NET or GCC 3.x, then the call to memset() will be removed as a dead store because the buffer pwd is not used after its value is overwritten [18]. Because the buffer pwd contains a sensitive value, the application may be vulnerable to attack if the data are left memory resident. If attackers are able to access the correct region of memory, they may use the recovered password to gain control of the system.

It is common practice to overwrite sensitive data manipulated in memory, such as passwords or cryptographic keys, in order to prevent attackers from learning system secrets. However, with the advent of optimizing compilers, programs do not always behave as their source code alone would suggest. In the example, the compiler interprets the call to memset() as dead code because the memory being written to is not subsequently used, despite the fact that there is clearly a security motivation for the operation to occur. The problem here is that many compilers, and in fact many programming languages, do not take this and other security concerns into consideration in their efforts to improve efficiency.

Attackers typically exploit this type of vulnerability by using a core dump or runtime mechanism to access the memory used by a particular application and recover the secret information. Once an attacker has access to the secret information, it is relatively straightforward to further exploit the system and possibly compromise other resources with which the application interacts.

+ Potential Mitigations

Phase: Implementation

Store the sensitive data in a "volatile" memory location if available.

Phase: Build and Compilation

If possible, configure your compiler so that it does not remove dead stores.

Phase: Architecture and Design

Where possible, encrypt sensitive data that are used by a software system.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory2Environment
Development Concepts699
Seven Pernicious Kingdoms (primary)700
ChildOfCategoryCategory503Byte/Object Code
Development Concepts (primary)699
ChildOfCategoryCategory633Weaknesses that Affect Memory
Resource-specific Weaknesses (primary)631
ChildOfCategoryCategory729OWASP Top Ten 2004 Category A8 - Insecure Storage
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfWeakness BaseWeakness Base733Compiler Optimization Removal or Modification of Security-critical Code
Research Concepts (primary)1000
ChildOfCategoryCategory747CERT C Secure Coding Section 49 - Miscellaneous (MSC)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory883CERT C++ Secure Coding Section 49 - Miscellaneous (MSC)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory895SFP Cluster: Information Leak
Software Fault Pattern (SFP) Clusters (primary)888
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Affected Resources
  • Memory
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
7 Pernicious KingdomsInsecure Compiler Optimization
PLOVERSensitive memory uncleared by compiler optimization
OWASP Top Ten 2004A8Insecure Storage
CERT C Secure CodingMSC06-CBe aware of compiler optimization when dealing with sensitive data
CERT C++ Secure CodingMSC06-CPPBe aware of compiler optimization when dealing with sensitive data
+ References
[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 9, "A Compiler Optimization Caveat" Page 322. 2nd Edition. Microsoft. 2002.
Michael Howard. "When scrubbing secrets in memory doesn't work". BugTraq. 2002-11-05. <http://cert.uni-stuttgart.de/archive/bugtraq/2002/11/msg00046.html>.
Michael Howard. "Some Bad News and Some Good News". Microsoft. 2002-10-21. <http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncode/html/secure10102002.asp>.
Joseph Wagner. "GNU GCC: Optimizer Removes Code Necessary for Security". Bugtraq. 2002-11-16. <http://www.derkeiler.com/Mailing-Lists/securityfocus/bugtraq/2002-11/0257.html>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01CigitalExternal
updated Time_of_Introduction
2008-09-08MITREInternal
updated Relationships, Other_Notes, Taxonomy_Mappings
2008-10-14MITREInternal
updated Relationships
2008-11-24MITREInternal
updated Applicable_Platforms, Description, Detection_Factors, Other_Notes, Potential_Mitigations, Relationships, Taxonomy_Mappings, Time_of_Introduction
2009-05-27MITREInternal
updated Demonstrative_Examples
2010-02-16MITREInternal
updated References
2011-06-01MITREInternal
updated Common_Consequences
2011-09-13MITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11MITREInternal
updated Common_Consequences, References, Relationships
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Insecure Compiler Optimization
Page Last Updated: June 23, 2014