Signal Handler Function Associated with Multiple Signals

Incomplete Variant

Structure: Simple

Description

This vulnerability occurs when a single function is registered to handle multiple different operating system signals, creating potential race conditions if that function isn't carefully designed.

Extended Description

When the same function handles multiple signals, an attacker can exploit timing issues by sending a second signal while the handler is still processing the first. If the handler uses shared resources like global variables or non-reentrant functions, this interruption can corrupt program state, potentially leading to crashes, denial of service, or even code execution. Even without classic race conditions, this pattern creates risk. Attackers can trigger the handler multiple times by sending different signals sequentially, causing unintended side effects. This is especially dangerous for handlers designed to run only once, as forced re-execution can bypass security controls or destabilize the application.

Common Consequences 1

Scope: AvailabilityIntegrityConfidentialityAccess ControlOther

Impact: DoS: Crash, Exit, or RestartExecute Unauthorized Code or CommandsRead Application DataGain Privileges or Assume IdentityBypass Protection MechanismVaries by Context

The most common consequence will be a corruption of the state of the product, possibly leading to a crash or exit. However, if the signal handler is operating on state variables for security relevant libraries or protection mechanisms, the consequences can be far more severe, including protection mechanism bypass, privilege escalation, or information exposure.

Demonstrative Examples 2

This code registers the same signal handler function with two different signals.

Code Example:Bad
C
c

ID : DX-26

This code registers the same signal handler function with two different signals (Signal Handler Function Associated with Multiple Signals). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits.

Code Example:

Bad

/* artificially increase the size of the timing window to make demonstration of this weakness easier. /

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 (Buffer Over-read) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array.

References 2

Delivering Signals for Fun and Profit

Michal Zalewski

https://lcamtuf.coredump.cx/signals.txt(2023-04-07)

ID: REF-360

Race Condition: Signal Handling

https://vulncat.fortify.com/en/detail?id=desc.structural.cpp.race_condition_signal_handling#:~:text=Signal%20handling%20race%20conditions%20can,installed%20to%20handle%20multiple%20signals.s(2023-04-07)

ID: REF-361

Related Weaknesses

ChildOf:

Signal Handler Race Condition (CWE-364)