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      3     <title>Security Enhancements in Android 1.5 through 4.1</title>
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     26 <p>
     27 Android provides a multi-layered security model described in the <a href="/security/index.html">Android
     28 Security Overview</a>. Each update to Android includes dozens of
     29 security enhancements to protect users.  The following are some of the security
     30 enhancements introduced in Android versions 1.5 through 4.1:</p>
     31 
     32 <dl>
     33 <dt><strong>Android 1.5</strong></dt>
     34 <dd><ul>
     35 <li>ProPolice to prevent stack buffer overruns (-fstack-protector)</li>
     36 <li>safe_iop to reduce integer overflows</li>
     37 <li>Extensions to OpenBSD dlmalloc to prevent double free() vulnerabilities and
     38 to prevent chunk consolidation attacks.  Chunk consolidation attacks are a
     39 common way to exploit heap corruption.</li>
     40 <li>OpenBSD calloc to prevent integer overflows during memory allocation</li>
     41 </ul>
     42 </dd>
     43 
     44 <dt><strong>Android 2.3</strong></dt>
     45 <dd><ul>
     46 <li>Format string vulnerability protections (-Wformat-security -Werror=format-security)</li>
     47 <li>Hardware-based No eXecute (NX) to prevent code execution on the stack and heap</li>
     48 <li>Linux mmap_min_addr to mitigate null pointer dereference privilege
     49 escalation (further enhanced in Android 4.1)</li>
     50 </ul>
     51 </dd>
     52 
     53 <dt><strong>Android 4.0</strong></dt>
     54 <dd>Address Space Layout Randomization (ASLR) to randomize key locations in memory
     55 </dd>
     56 
     57 <dt><strong>Android 4.1</strong></dt>
     58 <dd><ul>
     59 <li>PIE (Position Independent Executable) support</li>
     60 <li>Read-only relocations / immediate binding (-Wl,-z,relro -Wl,-z,now)</li>
     61 <li>dmesg_restrict enabled (avoid leaking kernel addresses)</li>
     62 <li>kptr_restrict enabled (avoid leaking kernel addresses)</li>
     63 </ul>
     64 </dd>
     65 
     66 </dl>
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