Full Report
Intro Hello again and welcome to the third of our series. On today’s blog post we are going to see what is and how can we abuse a double free(). We are also going to take advantage of leaks that happen when doing double free()‘s and see some examples of code execution using said leaks – we are making our execution ride on frees! As a last note, we are going to step things up a notch in this blog post and we are going to be using gdb as it will be crucial from now on. Sadly, ascii art doesn’t cut it anymore.
Analysis Summary
Malware analysis and TTP summarization based on the provided article excerpt:
# Tool/Technique: Double Free Vulnerability Exploitation
## Overview
This document summarizes the technique of abusing a "double free()" vulnerability in userland heap management, specifically utilizing the resulting corruption and memory leaks to achieve arbitrary code execution on Linux systems. The analysis heavily relies on using the GNU Debugger (`gdb`).
## Technical Details
- Type: Technique (Memory Corruption/Exploitation Primitive)
- Platform: Linux (Userland heap exploitation, referencing `ptmalloc2` internals)
- Capabilities: Leads to memory leaks (specifically `main_arena->top`) and potentially write-what-where primitives, enabling control over heap structures and function pointers like `__malloc_hook`.
- First Seen: The concept of double free is long-standing; this specific exploitation method is discussed in the context of posts around 2017 concerning glibc heap management.
## MITRE ATT&CK Mapping
The core technique involves exploiting a weakness to manipulate execution flow.
- **TA0002 - Execution**
- T1059 - Command and Scripting Interpreter
- T1059.004 - Unix Shell
- **TA0004 - Privilege Escalation** (Often the result of exploiting such flaws)
- T1068 - Exploitation for Privilege Escalation
- **TA0008 - Lateral Movement** (If the exploited binary runs with network access or higher privileges)
- **TA0011 - Command and Control** (If the final payload retrieves data or executes remote commands)
## Functionality
### Core Capabilities
- **Causing Memory Corruption:** Occurs when `free()` is called twice on the same memory chunk, leading to undefined behavior if basic heap checks fail or are bypassed.
- **Memory Leaks:** The initial double-free operation can leak internal heap metadata, such as `main_arena->top`, which provides an address reference point.
- **Attacking `__malloc_hook`:** By leaking `main_arena->top`, an attacker can calculate the distant address of the `__malloc_hook` function pointer (offsetting by `0x68` bytes in the example).
### Advanced Features
- **Bypassing Double-Free Checks:** Modern allocators like ptmalloc2 often detect simple, adjacent double frees (`SIGABRT`). Exploitation requires specific conditions or using vulnerabilities in structures holding pointers.
- **Fastbin Manipulation:** The technique relies on manipulating free fastchunks. A double-free results in a rogue `FD` (Forward Data) pointer being placed in the free list structure.
- **Arbitrary Write Primitive (Write-What-Where):** By manipulating the `FD` pointer of a freed fastchunk to point near `__malloc_hook`, an attacker can trigger the next allocation (`malloc`) to read the rogue pointer. A subsequent allocation is then directed to the attacker-controlled location (`malloc_hook-0x20-3`), where the attacker writes/overwrites the target function pointer (`__malloc_hook`) with the address of shellcode (`jackpot` function).
## Indicators of Compromise
(Note: The article describes a proof-of-concept exploitation technique, not a specific piece of malware with persistent artifacts.)
- File Hashes: N/A (Technique)
- File Names: N/A (Technique)
- Registry Keys: N/A (Linux userland technique)
- Network Indicators: N/A (The final execution step—calling `malloc` after overwriting the hook—leads to execution, potentially initiating network activity, but no C2 is specified in the setup.)
- Behavioral Indicators:
- Multiple, sequential `free()` calls on the same memory address.
- Unusually rapid allocation/deallocation cycles immediately following the double free.
- Program termination due to segmentation faults or abort signals (`SIGABRT`) if heap checks fail before exploitation succeeds.
- Execution flow branching unexpectedly into system function calls (e.g., shell execution) after routine `malloc()` calls.
## Associated Threat Actors
Double free is a generic exploitation technique used in high-impact vulnerability disclosures (e.g., CVE-2017-9078 in Dropbear SSH, vulnerability in OpenSSL). No specific threat actor is exclusively associated with this low-level heap primitive itself; rather, sophisticated actors utilize it when applicable vulnerabilities are discovered.
## Detection Methods
- Signature-based detection: Difficult for general heap conditions unless specific, known vulnerable code paths are fingerprintable.
- Behavioral detection: Monitoring unusual heap metadata manipulation, unexpected jumps in execution flow following memory allocation primitives, and hooks being set on internal functions like `__malloc_hook`.
- YARA rules: Not applicable for detecting the technique itself, but could be designed to detect specific proof-of-concept or exploit code exhibiting the calculated offsets (e.g., offsets related to `0x68` or `malloc_hook-0x20-3`).
## Mitigation Strategies
- **Code Review:** Thoroughly inspect code paths, particularly those involving complex cleanup routines or structs, to ensure memory is freed exactly once.
- **Memory Safety Languages:** Utilize languages that prevent memory corruption errors automatically.
- **Compiler/OS Protections:** Ensure modern mitigations are enabled, though these techniques aim to bypass them (e.g., ASLR, DEP).
- **Dynamic Analysis/Fuzzing:** Employ tools like fuzzers (as noted in the article) to uncover rarely hit code paths that might contain double-free mistakes.
- **Use Debuggers:** Employ debugging tools like `gdb` during development/testing to trace memory allocation/deallocation events.
## Related Tools/Techniques
- Heap Overflow/Buffer Overflows (often used in conjunction to set up the initial state or overwrite pointers).
- Use-After-Free (UAF) vulnerabilities.
- Exploitation relying on `__mmap_hook` or other glibc hooks.