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本文主要研究一下directory traversal attack及其防范

directory traversal attack

又称Path Traversal attack，即目录遍历攻击，旨在访问web服务器根目录外的文件/目录。通过是通过url或变量里头传递"../"来进行目录遍历。

通过url

比如

http://some_site.com.br/../../../../some dir/some file
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或者

http://some_site.com.br/../../../../etc/shadow
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通过变量名

通常是在文件下载接口中，比如

http://some_site.com.br/get-files?file=/etc/passwd
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或者

http://some_site.com.br/get-files?file=../../../../some dir/some file
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防范

针对url

spring security提供了DefaultHttpFirewall来进行处理，是为了防止一些web框架没有遵循servlet规范而进行的防范。 spring-security-web-4.2.3.RELEASE-sources.jar!/org/springframework/security/web/firewall/DefaultHttpFirewall.java

/*** Default implementation which wraps requests in order to provide consistent* values of the {@code servletPath} and {@code pathInfo}, which do not contain* path parameters (as defined in* <a href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396</a>). Different* servlet containers interpret the servlet spec differently as to how path* parameters are treated and it is possible they might be added in order to* bypass particular security constraints. When using this implementation, they* will be removed for all requests as the request passes through the security* filter chain. Note that this means that any segments in the decoded path* which contain a semi-colon, will have the part following the semi-colon* removed for request matching. Your application should not contain any valid* paths which contain semi-colons.* <p>* If any un-normalized paths are found (containing directory-traversal* character sequences), the request will be rejected immediately. Most* containers normalize the paths before performing the servlet-mapping, but* again this is not guaranteed by the servlet spec.** @author Luke Taylor*/
public class DefaultHttpFirewall implements HttpFirewall {private boolean allowUrlEncodedSlash;@Overridepublic FirewalledRequest getFirewalledRequest(HttpServletRequest request) throws RequestRejectedException {FirewalledRequest fwr = new RequestWrapper(request);if (!isNormalized(fwr.getServletPath()) || !isNormalized(fwr.getPathInfo())) {throw new RequestRejectedException("Un-normalized paths are not supported: " + fwr.getServletPath()+ (fwr.getPathInfo() != null ? fwr.getPathInfo() : ""));}String requestURI = fwr.getRequestURI();if (containsInvalidUrlEncodedSlash(requestURI)) {throw new RequestRejectedException("The requestURI cannot contain encoded slash. Got " + requestURI);}return fwr;}@Overridepublic HttpServletResponse getFirewalledResponse(HttpServletResponse response) {return new FirewalledResponse(response);}/*** <p>* Sets if the application should allow a URL encoded slash character.* </p>* <p>* If true (default is false), a URL encoded slash will be allowed in the* URL. Allowing encoded slashes can cause security vulnerabilities in some* situations depending on how the container constructs the* HttpServletRequest.* </p>** @param allowUrlEncodedSlash*            the new value (default false)*/public void setAllowUrlEncodedSlash(boolean allowUrlEncodedSlash) {this.allowUrlEncodedSlash = allowUrlEncodedSlash;}private boolean containsInvalidUrlEncodedSlash(String uri) {if (this.allowUrlEncodedSlash || uri == null) {return false;}if (uri.contains("%2f") || uri.contains("%2F")) {return true;}return false;}/*** Checks whether a path is normalized (doesn't contain path traversal* sequences like "./", "/../" or "/.")** @param path*            the path to test* @return true if the path doesn't contain any path-traversal character*         sequences.*/private boolean isNormalized(String path) {if (path == null) {return true;}for (int j = path.length(); j > 0;) {int i = path.lastIndexOf('/', j - 1);int gap = j - i;if (gap == 2 && path.charAt(i + 1) == '.') {// ".", "/./" or "/."return false;} else if (gap == 3 && path.charAt(i + 1) == '.' && path.charAt(i + 2) == '.') {return false;}j = i;}return true;}}
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这里会对url进行判断

通过变量

这种框架没有内置进行判断，需要自己在开发应用服务的时候额外关注。这里谈谈几种防范方法。

• 对变量名进行过滤

final Pattern INVALID_PATH_PATTERN = Pattern.compile("(\\.\\.\\/|\\.\\.\\\\)");
if(INVALID_PATH_PATTERN.matcher(path).find()){throw new BadRequestException("invalid path");
}
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• 利用absolutePath与canonicalPath

absolutePath不会处理../之类的，而canonicalPath会翻译../，判断两者是否相等即可判断是否有../

        if (!file.getAbsolutePath().equals(file.getCanonicalPath())) {throw new BadRequestException("invalid path");}
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小结

在编写文件下载服务的时候，需要特别关注directory traversal attack。通常url层面的web框架会帮你防范，但是变量层面的，则需要自己开发额外注意。

doc

• Path Traversal
• Directory traversal attack