CWE-416: Use After Free

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

Detailed description

Characteristics

• Dangling pointer: a pointer that no longer points to valid memory, often after it has been freed
• UAF: commonly used acronym for Use After Free
• Use-After-Free

Common consequences

• Integrity

  Modify Memory

  The use of previously freed memory may corrupt valid data, if the memory area in question has been allocated and used properly elsewhere.

• Availability

  DoS: Crash, Exit, or Restart

  If chunk consolidation occurs after the use of previously freed data, the process may crash when invalid data is used as chunk information.

• Confidentiality

  Read Memory

  Read operations on freed memory can sometimes leak sensitive information instead of causing a crash

• Integrity, Confidentiality, Availability

  Execute Unauthorized Code or Commands

  If malicious data is entered before chunk consolidation can take place, it may be possible to take advantage of a write-what-where primitive to execute arbitrary code. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Mitigations

• Architecture and Design · Language SelectionChoose a language that provides automatic memory management.
• Implementation · Attack Surface ReductionWhen freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.Effectiveness: Defense in DepthIf a bug causes an attempted access of this pointer, then a NULL dereference could still lead to a crash or other unexpected behavior, but it will reduce or eliminate the risk of code execution.

Detection methods

• FuzzingFuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.Effectiveness: High
• Automated Static AnalysisAutomated 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.)Effectiveness: High
• Automated Dynamic AnalysisUse tools that are integrated during compilation to insert runtime error-checking mechanisms related to memory safety errors, such as AddressSanitizer (ASan) for C/C++ [REF-1518].Effectiveness: ModerateCrafted inputs are necessary to reach the code containing the error, such as generated by fuzzers. Also, these tools may reduce performance, and they only report the error condition - not the original mistake that led to the error.

Representative vulnerabilities

Sources and references

• MITRE CWE
• CWE Terms of Use

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