COMMON WEAKNESS ENUMERATIONDraft

CWE-479

Signal Handler Use of a Non-reentrant Function

The product defines a signal handler that calls a non-reentrant function.

Abstraction

Variant

Structure

Simple

Likelihood of Exploit

Low

Understand the problem

What is the problem?

The product defines a signal handler that calls a non-reentrant function.

Extended description

Non-reentrant functions are functions that cannot safely be called, interrupted, and then recalled before the first call has finished without resulting in memory corruption. This can lead to an unexpected system state and unpredictable results with a variety of potential consequences depending on context, including denial of service and code execution. Many functions are not reentrant, but some of them can result in the corruption of memory if they are used in a signal handler. The function call syslog() is an example of this. In order to perform its functionality, it allocates a small amount of memory as "scratch space." If syslog() is suspended by a signal call and the signal handler calls syslog(), the memory used by both of these functions enters an undefined, and possibly, exploitable state. Implementations of malloc() and free() manage metadata in global structures in order to track which memory is allocated versus which memory is available, but they are non-reentrant. Simultaneous calls to these functions can cause corruption of the metadata.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

Risk

What does successful exploitation lead to?

Execute Unauthorized Code or Commands

Affected security scopes: Integrity, Confidentiality, Availability

It may be possible to execute arbitrary code through the use of a write-what-where condition.
Modify MemoryModify Application Data

Affected security scopes: Integrity

Signal race conditions often result in data corruption.

Defence

How is it prevented and fixed?

Requirements

Security requirement

Detail
Require languages or libraries that provide reentrant functionality, or otherwise make it easier to avoid this weakness.

Architecture and Design

Architecture and design approach

Detail
Design signal handlers to only set flags rather than perform complex functionality.

Implementation

Implementation-phase defence

Detail
Ensure that non-reentrant functions are not found in signal handlers.

Implementation

Implementation-phase defence

Detail
Use sanity checks to reduce the timing window for exploitation of race conditions. This is only a partial solution, since many attacks might fail, but other attacks still might work within the narrower window, even accidentally.

Effectiveness: Defense in Depth

Verification

How is it detected?

Automated Static Analysis

High
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.)