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Android5.1.1 - APK签名校验分析和修改源码绕过签名校验

@(Android研究)[APK签名校验|绕过签名校验]

APK签名校验分析

找到PackageParser类，该类在文件"frameworks/base/core/java/android/content/pm/PackageParser.java"中。PackageParser类的collectCertificates方法会对APK进行签名校验，在该方法会遍历APK中的所有文件，并对每个文件进行校验。下面是该方法的部分源码：

private static void collectCertificates(Package pkg, File apkFile, int flags)
throws PackageParserException {
final String apkPath = apkFile.getAbsolutePath();
StrictJarFile jarFile = null;
try {
jarFile = new StrictJarFile(apkPath);
......
// Verify that entries are signed consistently with the first entry
// we encountered. Note that for splits, certificates may have
// already been populated during an earlier parse of a base APK.
for (ZipEntry entry : toVerify) {
final Certificate[][] entryCerts = loadCertificates(jarFile, entry);
if (ArrayUtils.isEmpty(entryCerts)) {
throw new PackageParserException(INSTALL_PARSE_FAILED_NO_CERTIFICATES,
"Package " + apkPath + " has no certificates at entry "
+ entry.getName());
}
......
}
}
......
}

APK是一个ZIP格式的文件所以使用ZIP相关的类进行读写。上面代码中调用了loadCertificates方法，这个方法返回一个二维数组，如果APK中的文件签名校验失败那么loadCertificates方法会返回一个空数组(可能是null，可能是数组长度为0)，按照上面代码的逻辑如果数组为空则会抛出异常。

loadCertificates方法的代码见下：

private static Certificate[][] loadCertificates(StrictJarFile jarFile, ZipEntry entry)
throws PackageParserException {
InputStream is = null;
try {
// We must read the stream for the JarEntry to retrieve
// its certificates.
is = jarFile.getInputStream(entry);
readFullyIgnoringContents(is);
return jarFile.getCertificateChains(entry);
} catch (IOException | RuntimeException e) {
throw new PackageParserException(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,
"Failed reading " + entry.getName() + " in " + jarFile, e);
} finally {
IoUtils.closeQuietly(is);
}
}

上面代码中is是JarFile.JarFileInputStream类的对象。loadCertificates方法中调用了readFullyIgnoringContents方法，在readFullyIgnoringContents方法中会调用JarFile.JarFileInputStream.read方法读取APK中一项的数据，在read方法中会校验读取到的数据项的签名，如果签名校验失败，则会抛出SecurityException类型的异常，即签名校验失败。

JarFile类在"libcore/luni/src/main/java/java/util/jar/JarFile.java"文件中。

上面代码中调用了StrictJarFile.getCertificateChains方法，下面是它的代码：

public Certificate[][] getCertificateChains(ZipEntry ze) {
if (isSigned) {
return verifier.getCertificateChains(ze.getName());
}
return null;
}

StrictJarFile类在文件"libcore/luni/src/main/java/java/util/jar/StrictJarFile.java"中。

上面代码中isSigned的值是这么来的：

public StrictJarFile(String fileName) throws IOException {
this.nativeHandle = nativeOpenJarFile(fileName);
this.raf = new RandomAccessFile(fileName, "r");
try {
......
this.verifier = new JarVerifier(fileName, manifest, metaEntries);
isSigned = verifier.readCertificates() && verifier.isSignedJar();
} catch (IOException ioe) {
nativeClose(this.nativeHandle);
throw ioe;
}
guard.open("close");
}

当证书读取成功，并且当前APK经过了签名，则isSigned为true。

回到StrictJarFile.getCertificateChains方法中，当isSigned为true时会调用JarVerifier.getCertificateChains方法，下面是它的代码：

Certificate[][] getCertificateChains(String name) {
return verifiedEntries.get(name);
}

下面是类成员变量verifiedEntries的声明：

private final Hashtable verifiedEntries =
new Hashtable();

verifiedEntries是一个键值对，键是APK中经过了签名的文件名，如：classes.dex文件，值是证书数组。如果向已经签过名的APK中新添加一个文件然后安装这个APK，当程序逻辑执行到JarVerifier.getCertificateChains方法中时，在verifiedEntries变量中无法找到新添加的文件名(因为这个新文件是在APK签名之后添加)，那么JarVerifier.getCertificateChains方法将返回null。

绕过签名校验

源码修改点一

找到PackageParser.loadCertificates方法，下面是部分源码：

private static Certificate[][] loadCertificates(StrictJarFile jarFile, ZipEntry entry)
throws PackageParserException {
......
try {
......
} catch (IOException | RuntimeException e) {
throw new PackageParserException(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,
"Failed reading " + entry.getName() + " in " + jarFile, e);
}
} finally {
IoUtils.closeQuietly(is);
}
}

将上面代码catch块中的throw语句替换为：return null;

下面是修改后的代码：

private static Certificate[][] loadCertificates(StrictJarFile jarFile, ZipEntry entry)
throws PackageParserException {
InputStream is = null;
try {
......
} catch (IOException | RuntimeException e) {
return null;
} finally {
IoUtils.closeQuietly(is);
}
}

代码修改完后，当APK中文件签名校验失败时不会抛出异常，APK还会继续安装。

源码修改点二

找到PackageParser.collectCertificates方法，找到代码中调用loadCertificates方法的地方：

private static void collectCertificates(Package pkg, File apkFile, int flags)
throws PackageParserException {
......
try {
......
for (ZipEntry entry : toVerify) {
final Certificate[][] entryCerts = loadCertificates(jarFile, entry);
if (ArrayUtils.isEmpty(entryCerts)) {
throw new PackageParserException(INSTALL_PARSE_FAILED_NO_CERTIFICATES,
"Package " + apkPath + " has no certificates at entry "
+ entry.getName());
}
......
}
}......
}

将上面的throw语句替换为：continue;

修改后的代码：

private static void collectCertificates(Package pkg, File apkFile, int flags)
throws PackageParserException {
......
try {
......
for (ZipEntry entry : toVerify) {
final Certificate[][] entryCerts = loadCertificates(jarFile, entry);
if (ArrayUtils.isEmpty(entryCerts)) {
continue;
}
......
}
}......
}

代码修改完后，当遇到APK中没有经过签名的文件时不会抛出异常，APK还会继续安装。