CWE-364: Signal Handler Race Condition | Glexia
CWE-364 (Signal Handler Race Condition) weakness overview with consequences, detection methods, mitigations, related CVEs and MITRE ATT&CK context.
Glexia's Take · Automated analysis
CWE-364: Signal Handler Race Condition
Signal Handler Race Condition represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Executive Impact
- Integrity Confidentiality Availability: Modify Application Data Modify Memory DoS: Crash, Exit, or Restart Execute Unauthorized Code or Commands: It may be possible to cause data corruption and possibly execute arbitrary code by modifying global variables or data structures at unexpected times, violating the assumptions of code that uses this global data.
- Access Control: Gain Privileges or Assume Identity: If a signal handler interrupts code that is executing with privileges, it may be possible that the signal handler will also be executed with elevated privileges, possibly making subsequent exploits more severe.
Developer Pattern
CWE-364 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.
Automation confidence
high confidence from CWE-364, 4.20.
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Official CWE Definition
CWE-364: Signal Handler Race Condition
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)
Developer And Remediation Guidance
How teams prevent and detect this weakness
Causes
- This code registers the same signal handler function with two different signals (CWE-831). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits. The handler function uses global state (globalVar and logMessage), and it can be called by both the SIGHUP and SIGTERM signals. An attack scenario might follow these lines: The program begins execution, initializes logMessage, and registers the signal handlers for SIGHUP and SIGTERM. The program begins its "normal" functionality, which is simplified as sleep(), but could be any functionality that consumes some time. The attacker sends SIGHUP, which invokes handler (call this "SIGHUP-handler"). SIGHUP-handler begins to execute, calling syslog(). syslog() calls malloc(), which is non-reentrant. malloc() begins to modify metadata to manage the heap. The attacker then sends SIGTERM. SIGHUP-handler is interrupted, but syslog's malloc call is still executing and has not finished modifying its metadata. The SIGTERM handler is invoked. SIGTERM-handler records the log message using syslog(), then frees the logMessage variable. At this point, the state of the heap is uncertain, because malloc is still modifying the metadata for the heap; the metadata might be in an inconsistent state. The SIGTERM-handler call to free() is assuming that the metadata is inconsistent, possibly causing it to write data to the wrong location while managing the heap. The result is memory corruption, which could lead to a crash or even code execution, depending on the circumstances under which the code is running. Note that this is an adaptation of a classic example as originally presented by Michal Zalewski [REF-360]; the original example was shown to be exploitable for code execution. Also note that the strdup(argv[1]) call contains a potential buffer over-read (CWE-126) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array.
- The following code registers a signal handler with multiple signals in order to log when a specific event occurs and to free associated memory before exiting. However, the following sequence of events may result in a double-free (CWE-415): a SIGHUP is delivered to the process sh() is invoked to process the SIGHUP This first invocation of sh() reaches the point where global1 is freed At this point, a SIGTERM is sent to the process the second invocation of sh() might do another free of global1 this results in a double-free (CWE-415) This is just one possible exploitation of the above code. As another example, the syslog call may use malloc calls which are not async-signal safe. This could cause corruption of the heap management structures. For more details, consult the example within "Delivering Signals for Fun and Profit" [REF-360].
Remediation
- Requirements: Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
- Architecture and Design: 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.
- Implementation: 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.
Detection
- 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.)
Mappings
Related CVEs, CWEs, and ATT&CK context
Related CWEs
- CWE-123: Write-what-where Condition
- CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
- CWE-415: Double Free
- CWE-416: Use After Free
- CWE-368: Context Switching Race Condition
- CWE-432: Dangerous Signal Handler not Disabled During Sensitive Operations
- CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe
- CWE-831: Signal Handler Function Associated with Multiple Signals
ATT&CK Relevance
ATT&CK relevance is shown only when reviewed or responsibly inferred.