Home > CAPEC List > CAPEC-29: Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions (Version 3.2)  

CAPEC-29: Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions

Attack Pattern ID: 29
Abstraction: Standard
Status: Draft
Presentation Filter:
+ Description
This attack targets a race condition occurring between the time of check (state) for a resource and the time of use of a resource. A typical example is file access. The adversary can leverage a file access race condition by "running the race", meaning that they would modify the resource between the first time the target program accesses the file and the time the target program uses the file. During that period of time, the adversary could replace or modify the file, causing the application to behave unexpectedly.
+ Likelihood Of Attack


+ Typical Severity


+ Relationships

The table below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.

ChildOfMeta Attack PatternMeta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.26Leveraging Race Conditions
ParentOfDetailed Attack PatternDetailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.27Leveraging Race Conditions via Symbolic Links

The table below shows the views that this attack pattern belongs to and top level categories within that view.

+ Execution Flow
  1. The adversary explores to gauge what level of access he has.
  1. The adversary confirms access to a resource on the target host. The adversary confirms ability to modify the targeted resource.
  1. The adversary decides to leverage the race condition by "running the race", meaning that he would modify the resource between the first time the target program accesses the file and the time the target program uses the file. During that period of time, the adversary can replace the resource and cause an escalation of privilege.
+ Prerequisites
A resource is access/modified concurrently by multiple processes.
The adversary is able to modify resource.
A race condition exists while accessing a resource.
+ Skills Required
[Level: Medium]
This attack can get sophisticated since the attack has to occur within a short interval of time.
+ Consequences

The table below specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. 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 pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Modify Data
Access Control
Gain Privileges
Alter Execution Logic
Read Data
Resource Consumption
+ Mitigations
Use safe libraries to access resources such as files.
Be aware that improper use of access function calls such as chown(), tempfile(), chmod(), etc. can cause a race condition.
Use synchronization to control the flow of execution.
Use static analysis tools to find race conditions.
Pay attention to concurrency problems related to the access of resources.
+ Example Instances
The Net Direct client for Linux before 6.0.5 in Nortel Application Switch 2424, VPN 3050 and 3070, and SSL VPN Module 1000 extracts and executes files with insecure permissions, which allows local users to exploit a race condition to replace a world-writable file in /tmp/NetClient and cause another user to execute arbitrary code when attempting to execute this client, as demonstrated by replacing /tmp/NetClient/client. See also: CVE-2007-1057

The following code illustrates a file that is accessed multiple times by name in a publicly accessible directory. A race condition exists between the accesses where an adversary can replace the file referenced by the name.

include <sys/types.h> include <fcntl.h> include <unistd.h> define FILE "/tmp/myfile" define UID 100 void test(char *str) {
int fd; fd = creat(FILE, 0644); if(fd == -1)
chown(FILE, UID, -1); /* BAD */ close(fd);
} int main(int argc, char **argv) {
char *userstr; if(argc > 1) {
userstr = argv[1]; test(userstr);
} return 0;


+ References
[REF-131] J. Viega and G. McGraw. "Building Secure Software". Addison-Wesley. 2002.
[REF-107] Fortify Software. "SAMATE - Software Assurance Metrics And Tool Evaluation". Test Case ID 1598. National Institute of Standards and Technology (NIST). 2006-06-22. <http://samate.nist.gov/SRD/view_testcase.php?tID=1598>.
+ Content History
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modification DateModifierOrganization
2017-01-09CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2017-08-04CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases, Attack_Prerequisites, Description Summary, Examples-Instances
2018-07-31CAPEC Content TeamThe MITRE Corporation
Updated References
2019-09-30CAPEC Content TeamThe MITRE Corporation
Updated @Abstraction
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Page Last Updated or Reviewed: September 30, 2019