The product calls a non-reentrant function in a concurrent context in which a competing code sequence (e.g. thread or signal handler) may have an opportunity to call the same function or otherwise influence its state.

Primary

Implementation

Integrity
Confidentiality
Other
Modify Memory
Read Memory
Modify Application Data
Read Application Data
Alter Execution Logic

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
High

Implementation
Use reentrant functions if available.

Implementation
Add synchronization to your non-reentrant function.

Implementation
In Java, use the ReentrantLock Class.

In this example, a signal handler uses syslog() to log a message:

char *message;void sh(int dummy) {syslog(LOG_NOTICE,"%s\n",message);sleep(10);exit(0);}int main(int argc,char* argv[]) {...signal(SIGHUP,sh);signal(SIGTERM,sh);sleep(10);exit(0);}If the execution of the first call to the signal handler is suspended after invoking syslog(), and the signal handler is called a second time, the memory allocated by syslog() enters an undefined, and possibly, exploitable state.

The following code relies on getlogin() to determine whether or not a user is trusted. It is easily subverted.

pwd = getpwnam(getlogin());if (isTrustedGroup(pwd->pw_gid)) {allow();} else {deny();}

CVE-2001-1349
unsafe calls to library functions from signal handler
https://www.cve.org/CVERecord?id=CVE-2001-1349

CVE-2004-2259
SIGCHLD signal to FTP server can cause crash under heavy load while executing non-reentrant functions like malloc/free.
https://www.cve.org/CVERecord?id=CVE-2004-2259

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

CWE Community
2008-04-11
Draft 9
2008-04-11
Submitted by members of the CWE community to extend early CWE versions

Eric Dalci
Cigital
2008-07-01
updated References, Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Relationships, References

CWE Content Team
MITRE
2009-03-10
updated Related_Attack_Patterns

CWE Content Team
MITRE
2010-09-27
updated Name, Observed_Examples, Potential_Mitigations, References, Relationships

CWE Content Team
MITRE
2010-12-13
updated Description, Name, Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Relationships

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Observed_Examples

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-06-25
updated Relationships

CWE Content Team
MITRE
2020-12-10
updated Common_Consequences

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated References, Relationships, Time_of_Introduction

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2023-10-26
updated Demonstrative_Examples

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Demonstrative_Examples

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Detection_Factors, Weakness_Ordinalities

Use of a Non-reentrant Function in an Unsynchronized Context
Use of a Non-reentrant Function in a Multithreaded Context