CWE-364: Signal Handler Race Condition

CWE-364: Signal Handler Race Condition is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product uses a signal handler that introduces a race condition. Race conditions frequently occur in signal handlers, since signal handlers support asynchronous actions. These race conditions have a variety of root causes and symptoms. Attackers may be able to exploit a signal handler race condition to cause the product state to be corrupted, possibly leading to a denial of service or even code execution. These issues occur when non-reentrant functions, or state-sensitive actions occur in the signal handler, where they may be called at any time. These behaviors can violate assumptions being made by the "regular" code that is interrupted, or by other signal handlers that may also be invoked. If these functions are called at an inopportune moment - such as while a non-reentrant function is already running - memory corruption could occur that may be exploitable for code execution. Another signal race condition commonly found occurs when free is called within a signal handler, resulting in a double free and therefore a write-what-where condition. Even if a given pointer is set to NULL after it has been freed, a race condition still exists between the time the memory was freed and the pointer was set to NULL. This is especially problematic if the same signal handler has been set for more than one signal -- since it means that the signal handler itself may be reentered. There are several known behaviors related to signal handlers that have received the label of "signal handler race condition": Shared state (e.g. global data or static variables) that are accessible to both a signal handler and "regular" code Shared state between a signal handler and other signal handlers Use of non-reentrant functionality within a signal handler - which generally implies that shared state is being used. For example, malloc() and free() are non-reentrant because they may use global or static data structures for managing memory, and they are indirectly used by innocent-seeming functions such as syslog(); these functions could be exploited for memory corruption and, possibly, code execution. Association of the same signal handler function with multiple signals - which might imply shared state, since the same code and resources are accessed. For example, this can be a source of double-free and use-after-free weaknesses. Use of setjmp and longjmp, or other mechanisms that prevent a signal handler from returning control back to the original functionality While not technically a race condition, some signal handlers are designed to be called at most once, and being called more than once can introduce security problems, even when there are not any concurrent calls to the signal handler. This can be a source of double-free and use-after-free weaknesses. Signal handler vulnerabilities are often classified based on the absence of a specific protection mechanism, although this style of classification is discouraged in CWE because programmers often have a choice of several different mechanisms for addressing the weakness. Such protection mechanisms may preserve exclusivity of access to the shared resource, and behavioral atomicity for the relevant code: Avoiding shared state Using synchronization in the signal handler Using synchronization in the regular code Disabling or masking other signals, which provides atomicity (which effectively ensures exclusivity)

If exploited, CWE-364 (Signal Handler Race Condition) it can compromise Integrity, Confidentiality, Availability and Access Control, leading to outcomes such as Modify Application Data, Modify Memory, DoS: Crash, Exit, or Restart, Execute Unauthorized Code or Commands and Gain Privileges or Assume Identity.

Recommended mitigations for CWE-364 include: Design signal handlers to only set flags, rather than perform complex functionality. These flags can then be checked and acted upon within the main program loop. Only use reentrant functions within signal handlers. Also, use validation to ensure that state is consistent while performing asynchronous actions that affect the state of execution.

CWE-364 commonly affects C and C++. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

MITRE documents real CVEs mapped to CWE-364, including CVE-1999-0035, CVE-2001-0905, CVE-2001-1349, CVE-2004-0794 and CVE-2004-2259. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

A CWE (Common Weakness Enumeration) like CWE-364 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.