Android provides a multi-layered security model described in the Android Security Overview. Each update to Android includes dozens of security enhancements to protect users. The following are some of the security enhancements introduced in Android versions 1.5 through 4.1:

Android 1.5

• ProPolice to prevent stack buffer overruns (-fstack-protector)
• safe_iop to reduce integer overflows
• Extensions to OpenBSD dlmalloc to prevent double free() vulnerabilities and to prevent chunk consolidation attacks. Chunk consolidation attacks are a common way to exploit heap corruption.
• OpenBSD calloc to prevent integer overflows during memory allocation

Android 2.3

• Format string vulnerability protections (-Wformat-security -Werror=format-security)
• Hardware-based No eXecute (NX) to prevent code execution on the stack and heap
• Linux mmap_min_addr to mitigate null pointer dereference privilege escalation (further enhanced in Android 4.1)

Android 4.0

Address Space Layout Randomization (ASLR) to randomize key locations in memory

Android 4.1

• PIE (Position Independent Executable) support
• Read-only relocations / immediate binding (-Wl,-z,relro -Wl,-z,now)
• dmesg_restrict enabled (avoid leaking kernel addresses)
• kptr_restrict enabled (avoid leaking kernel addresses)