iOS解压缩_路径穿越_解析
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几年前,盘古实验室公布的一个由目录穿越导致RCE的漏洞,称为ZipperDown。
由于现有的ios App基本上采用SSZipArchive或Ziparchive来实现解压,因此漏洞是来自使用第三方Zip库解压Zip文件的过程中没有对Zip内文件名做校验导致的;例如 SSZipArchive解压时会把文件名直接拼接到目标路径后面,如果文件名中含有“../”则可以实现目录的上一级跳转,从而实现应用内任意目录的跳转,进一步可以实现文件覆盖。
(如果你想测试下这个漏洞,可去github下载这个Demo:https://github.com/muzipiao/ZipperDown)
这件事之后Ziparchive修复了该漏洞。代码中增加了一个 - (NSString *)_sanitizedPath 方法用于专门处理压缩包内的文件名,清除其中有害部分。
SSZipArchive.m 的 _sanitizedPath 方法的源代码:
- (NSString *)_sanitizedPath
{
// Change Windows paths to Unix paths: https://en.wikipedia.org/wiki/Path_(computing)
// Possible improvement: only do this if the archive was created on a non-Unix system
NSString *strPath = [self stringByReplacingOccurrencesOfString:@"\" withString:@"/"];
// Percent-encode file path (where path is defined by https://tools.ietf.org/html/rfc8089)
// The key part is to allow characters "." and "/" and disallow "%".
// CharacterSet.urlPathAllowed seems to do the job
#if (__MAC_OS_X_VERSION_MIN_REQUIRED >= 1090 || __IPHONE_OS_VERSION_MIN_REQUIRED >= 70000 || __WATCH_OS_VERSION_MIN_REQUIRED >= 20000 || __TV_OS_VERSION_MIN_REQUIRED >= 90000)
strPath = [strPath stringByAddingPercentEncodingWithAllowedCharacters:NSCharacterSet.URLPathAllowedCharacterSet];
#else
// Testing availability of @available (https://stackoverflow.com/a/46927445/1033581)
#if __clang_major__ < 9
// Xcode 8-
if (floor(NSFoundationVersionNumber) > NSFoundationVersionNumber10_8_4) {
#else
// Xcode 9+
if (@available(macOS 10.9, iOS 7.0, watchOS 2.0, tvOS 9.0, *)) {
#endif
strPath = [strPath stringByAddingPercentEncodingWithAllowedCharacters:NSCharacterSet.URLPathAllowedCharacterSet];
} else {
strPath = [strPath stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
}
#endif
// `NSString.stringByAddingPercentEncodingWithAllowedCharacters:` may theorically fail: https://stackoverflow.com/questions/33558933/
// But because we auto-detect encoding using `NSString.stringEncodingForData:encodingOptions:convertedString:usedLossyConversion:`,
// we likely already prevent UTF-16, UTF-32 and invalid Unicode in the form of unpaired surrogate chars: https://stackoverflow.com/questions/53043876/
// To be on the safe side, we will still perform a guard check.
if (strPath == nil) {
return nil;
}
// Add scheme "file:///" to support sanitation on names with a colon like "file:a/../../../usr/bin"
strPath = [@"file:///" stringByAppendingString:strPath];
// Sanitize path traversal characters to prevent directory backtracking. Ignoring these characters mimicks the default behavior of the Unarchiving tool on macOS.
// "../../../../../../../../../../../tmp/test.txt" -> "tmp/test.txt"
// "a/b/../c.txt" -> "a/c.txt"
strPath = [NSURL URLWithString:strPath].standardizedURL.absoluteString;
// Remove the "file:///" scheme
strPath = [strPath substringFromIndex:8];
// Remove the percent-encoding
#if (__MAC_OS_X_VERSION_MIN_REQUIRED >= 1090 || __IPHONE_OS_VERSION_MIN_REQUIRED >= 70000 || __WATCH_OS_VERSION_MIN_REQUIRED >= 20000 || __TV_OS_VERSION_MIN_REQUIRED >= 90000)
strPath = strPath.stringByRemovingPercentEncoding;
#else
// Testing availability of @available (https://stackoverflow.com/a/46927445/1033581)
#if __clang_major__ < 9
// Xcode 8-
if (floor(NSFoundationVersionNumber) > NSFoundationVersionNumber10_8_4) {
#else
// Xcode 9+
if (@available(macOS 10.9, iOS 7.0, watchOS 2.0, tvOS 9.0, *)) {
#endif
strPath = strPath.stringByRemovingPercentEncoding;
} else {
strPath = [strPath stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
}
#endif
return strPath;
}
这个方法清理了路径中的有害部分:
(1)把windows下的路径转成Unix下的路径,把路径中的"\"替换成"/":
NSString *strPath = [self stringByReplacingOccurrencesOfString:@"\" withString:@"/"];
(2)路径中允许"." 和 "/" ,但不能有"%",对百分号做了处理:
strPath = [strPath stringByAddingPercentEncodingWithAllowedCharacters:NSCharacterSet.URLPathAllowedCharacterSet];
效果示例:
处理前:
http%3a%2f%2fwww.rfc-editor.org%2finfo%2frfc1738%3e
处理后:
http%253a%252f%252fwww.rfc-editor.org%252finfo%252frfc1738%253e
可以看到 "%" 被 "%25" 替换了
(3)添加 scheme 的 "file:///" 前缀, 以便转成NSURL时可以处理路径中的"../"从而获取绝对路径:
//为路径增加 "file:///" 前缀
strPath = [@"file:///" stringByAppendingString:strPath];
//路径转成NSURL,并使用 .standardizedURL.absoluteString 从而最终清理路径中的"../"获得最终的绝对路径
strPath = [NSURL URLWithString:strPath].standardizedURL.absoluteString;
// 删除 "file:///" 前缀
strPath = [strPath substringFromIndex:8];
效果示例:
原路径:test/../code.png
添加前缀后:file:///test/../code.png
NSURL处理后获得绝对路径:file:///code.png
删除前缀:code.png
(4)清除百分号编码
strPath = strPath.stringByRemovingPercentEncoding;
效果示例:
处理前:
http%253a%252f%252fwww.rfc-editor.org%252finfo%252frfc1738%253e
处理后:
http%3a%2f%2fwww.rfc-editor.org%2finfo%2frfc1738%3e
可以看到 "%25" 被 "%" 替换了。
经过以上几步的处理,最终得到可用的strPath
拓展:
从源码的调试中,也发现一个小问题:
当文件名包含".."时,文件名被读取时会被默认加入一个""转义符,并最终造成了路径穿越。
实践步骤如下:
原文件名是"..code.png", test 是文件夹
原文件路径是"test/..code.png",被读取时转义为"test/..\code.png"
在 _sanitizedPath 方法中把 "\" 替换成了 "/",因此 "test/..\code.png" 变成了 "test/../code.png":
为"test/../code.png"增加了"file:///"前缀:
接着又做绝对路径的处理,"file:///test/../code.png" 变成了 "file:///code.png",test文件夹没了!
删除"file:///"前缀,得到"code.png",最终 _sanitizedPath 方法返回值就是"code.png":
执行完解压后,app的Documents路径下, "..code.png"文件穿越到了test文件夹外:
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