Description

The software uses a signal handler that introduces a race condition.

Extended Description

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 software 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)

Likelihood of Exploit: Medium

Applicable Platforms
Language: C
Language: Sometimes
Language: C++
Language: Sometimes

Time Of Introduction

• Architecture and Design
• Implementation

Common Consequences

Detection Methods
None

Potential Mitigations

Relationships

Demonstrative Examples   (Details)

1. 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. (Demonstrative Example Id DX-48)
2. 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. (Demonstrative Example Id DX-26)

Observed Examples

1. CVE-1999-0035 : Signal handler does not disable other signal handlers, allowing it to be interrupted, causing other functionality to access files/etc. with raised privileges
2. CVE-2001-0905 : Attacker can send a signal while another signal handler is already running, leading to crash or execution with root privileges
3. CVE-2001-1349 : unsafe calls to library functions from signal handler
4. CVE-2004-0794 : SIGURG can be used to remotely interrupt signal handler; other variants exist
5. CVE-2004-2259 : handler for SIGCHLD uses non-reentrant functions

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

References:

1. .Delivering Signals for Fun and Profit.
2. .Race Condition: Signal Handling.
3. Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 13: Race Conditions." Page 205'. Published on 2010.
4. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 13, "Signal Vulnerabilities", Page 791.'. Published on 2006.