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Common Weakness Enumeration

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CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')

 
Loop with Unreachable Exit Condition ('Infinite Loop')
Weakness ID: 835 (Weakness Base)Status: Incomplete
+ Description

Description Summary

The program contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.

Extended Description

If the loop can be influenced by an attacker, this weakness could allow attackers to consume excessive resources such as CPU or memory.

+ Applicable Platforms

Languages

Language-independent

+ Common Consequences
ScopeEffect

Technical Impact: DoS: resource consumption (CPU); DoS: resource consumption (memory); DoS: amplification

An infinite loop will cause unexpected consumption of resources, such as CPU cycles or memory. The software's operation may slow down, or cause a long time to respond.

+ Demonstrative Examples

Example 1

In the following code the method processMessagesFromServer attempts to establish a connection to a server and read and process messages from the server. The method uses a do/while loop to continue trying to establish the connection to the server when an attempt fails.

(Bad Code)
Example Languages: C and C++ 
int processMessagesFromServer(char *hostaddr, int port) {
...
int servsock;
int connected;
struct sockaddr_in servaddr;

// create socket to connect to server
servsock = socket( AF_INET, SOCK_STREAM, 0);
memset( &servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(port);
servaddr.sin_addr.s_addr = inet_addr(hostaddr);

do {
// establish connection to server
connected = connect(servsock, (struct sockaddr *)&servaddr, sizeof(servaddr));

// if connected then read and process messages from server
if (connected > -1) {

// read and process messages
...
}

// keep trying to establish connection to the server
} while (connected < 0);

// close socket and return success or failure
...
}

However, this will create an infinite loop if the server does not respond. This infinite loop will consume system resources and can be used to create a denial of service attack. To resolve this a counter should be used to limit the number of attempts to establish a connection to the server, as in the following code.

(Good Code)
Example Languages: C and C++ 
int processMessagesFromServer(char *hostaddr, int port) {
...
// initialize number of attempts counter
int count = 0;
do {
// establish connection to server
connected = connect(servsock, (struct sockaddr *)&servaddr, sizeof(servaddr));

// increment counter
count++;

// if connected then read and process messages from server
if (connected > -1) {

// read and process messages
...
}

// keep trying to establish connection to the server
// up to a maximum number of attempts
} while (connected < 0 && count < MAX_ATTEMPTS);

// close socket and return success or failure
...
}

Example 2

For this example the method isReorderNeeded as part of a bookstore application that determines if a particular book needs to be reordered based on the current inventory count and the rate at which the book is being sold.

(Bad Code)
Example Language: Java 
public boolean isReorderNeeded(String bookISBN, int rateSold) {

boolean isReorder = false;

int minimumCount = 10;
int days = 0;

// get inventory count for book
int inventoryCount = inventory.getIventoryCount(bookISBN);

// find number of days until inventory count reaches minimum
while (inventoryCount > minimumCount) {

inventoryCount = inventoryCount - rateSold;
days++;

}

// if number of days within reorder timeframe
// set reorder return boolean to true
if (days > 0 && days < 5) {
isReorder = true;
}

return isReorder;
}

However, the while loop will become an infinite loop if the rateSold input parameter has a value of zero since the inventoryCount will never fall below the minimumCount. In this case the input parameter should be validated to ensure that a value of zero does not cause an infinite loop,as in the following code.

(Good Code)
Example Language: Java 
public boolean isReorderNeeded(String bookISBN, int rateSold) {
...

// validate rateSold variable
if (rateSold < 1) {
return isReorder;
}

...
}
+ Observed Examples
ReferenceDescription
Chain: off-by-one error leads to infinite loop using invalid hex-encoded characters.
Chain: self-referential values in recursive definitions lead to infinite loop.
NULL UDP packet is never cleared from a queue, leading to infinite loop.
Floating point conversion routine cycles back and forth between two different values.
Floating point conversion routine cycles back and forth between two different values.
Chain: improperly clearing a pointer in a linked list leads to infinite loop.
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base834Excessive Iteration
Development Concepts (primary)699
Research Concepts (primary)1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ References
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 7, "Looping Constructs", Page 327.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2011-03-22MITREInternal CWE Team
Modifications
Modification DateModifierOrganizationSource
2011-06-01MITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11MITREInternal
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings
Page Last Updated: June 23, 2014