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158 Web Application Penetration Testing it may contain information on internal workings of the application such as relative paths of the point where the application is installed or how objects are referenced internally. How to Test Black Box testing There are a variety of techniques that will cause exception messages to be sent in an HTTP response. Note that in most cases this will be an HTML page, but exceptions can be sent as part of SOAP or REST responses too. Some tests to try include: • invalid input (such as input that is not consistent with application logic. • input that contains non alphanumeric characters or query syn tax. • empty inputs. • inputs that are too long. • access to internal pages without authentication. • bypassing application flow. All the above tests could lead to application errors that may contain stack traces. It is recommended to use a fuzzer in addition to any manual testing. Some tools, such as OWASP ZAP and Burp proxy will automatically detect these exceptions in the response stream as you are doing other penetration and testing work. Gray Box Testing Search the code for the calls that cause an exception to be rendered to a String or output stream. For example, in Java this might be code in a JSP that looks like: In some cases, the stack trace will be specifically formatted into HTML, so be careful of accesses to stack trace elements. Search the configuration to verify error handling configuration and the use of default error pages. For example, in Java this configuration can be found in web.xml. Tools • ZAP Proxy - https://www.owasp.org/index.php/OWASP_Zed_ Attack_Proxy_Project References • [RFC2616] Hypertext Transfer Protocol - HTTP/1.1 Testing for Weak SSL/TLS Ciphers, Insufficient Transport Layer Protection (OTG-CRYPST-001) Summary Sensitive data must be protected when it is transmitted through the network. Such data can include user credentials and credit cards. As a rule of thumb, if data must be protected when it is stored, it must be protected also during transmission. HTTP is a clear-text protocol and it is normally secured via an SSL/TLS tunnel, resulting in HTTPS traffic [1]. The use of this protocol ensures not only confidentiality, but also authentication. Servers are authenticated using digital certificates and it is also possible to use client certificate for mutual authentication. Even if high grade ciphers are today supported and normally used, some misconfiguration in the server can be used to force the use of a weak cipher - or at worst no encryption - permitting to an attacker to gain access to the supposed secure communication channel. Other misconfiguration can be used for a Denial of Service attack. Common Issues A vulnerability occurs if the HTTP protocol is used to transmit sensitive information [2] (e.g. credentials transmitted over HTTP [3]). When the SSL/TLS service is present it is good but it increments the attack surface and the following vulnerabilities exist: • SSL/TLS protocols, ciphers, keys and renegotiation must be properly configured. • Certificate validity must be ensured. Other vulnerabilities linked to this are: • Software exposed must be updated due to possibility of known vulnerabilities [4]. • Usage of Secure flag for Session Cookies [5]. • Usage of HTTP Strict Transport Security (HSTS) [6]. • The presence of HTTP and HTTPS both, which can be used to intercept traffic [7], [8]. • The presence of mixed HTTPS and HTTP content in the same page, which can be used to Leak information. Sensitive data transmitted in clear-text The application should not transmit sensitive information via unencrypted channels. Typically it is possible to find basic authentication over HTTP, input password or session cookie sent via HTTP and, in general, other information considered by regulations, laws or organization policy. Weak SSL/TLS Ciphers/Protocols/Keys Historically, there have been limitations set in place by the U.S. government to allow cryptosystems to be exported only for key sizes of at most 40 bits, a key length which could be broken and would allow the decryption of communications. Since then cryptographic export regulations have been relaxed the maximum key size is 128 bits. It is important to check the SSL configuration being used to avoid putting in place cryptographic support which could be easily defeated. To reach this goal SSL-based services should not offer the possibility to choose weak cipher suite. A cipher suite is specified by an encryption protocol (e.g. DES, RC4, AES), the encryption key length (e.g. 40, 56, or 128 bits), and a hash algorithm (e.g. SHA, MD5) used for integrity checking. Briefly, the key points for the cipher suite determination are the following: [1] The client sends to the server a ClientHello message
159 Web Application Penetration Testing specifying, among other information, the protocol and the cipher suites that it is able to handle. Note that a client is usually a web browser (most popular SSL client nowadays), but not necessarily, since it can be any SSL-enabled application; the same holds for the server, which needs not to be a web server, though this is the most common case [9]. [2] The server responds with a ServerHello message, containing the chosen protocol and cipher suite that will be used for that session (in general the server selects the strongest protocol and cipher suite supported by both the client and server). It is possible (for example, by means of configuration directives) to specify which cipher suites the server will honor. In this way you may control whether or not conversations with clients will support 40-bit encryption only. [1] The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client. [2] The server sends a ServerHelloDone message and waits for a client response. [3] Upon receipt of the ServerHelloDone message, the client verifies the validity of the server’s digital certificate. SSL certificate validity – client and server When accessing a web application via the HTTPS protocol, a secure channel is established between the client and the server. The identity of one (the server) or both parties (client and server) is then established by means of digital certificates. So, once the cipher suite is determined, the “SSL Handshake” continues with the exchange of the certificates: [1] The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client. [2] The server sends a ServerHelloDone message and waits for a client response. [3] Upon receipt of the ServerHelloDone message, the client verifies the validity of the server’s digital certificate. In order for the communication to be set up, a number of checks on the certificates must be passed. While discussing SSL and certificate based authentication is beyond the scope of this guide, this section will focus on the main criteria involved in ascertaining certificate validity: • Checking if the Certificate Authority (CA) is a known one (meaning one considered trusted); • Checking that the certificate is currently valid; • Checking that the name of the site and the name reported in the certificate match. Let’s examine each check more in detail. • Each browser comes with a pre-loaded list of trusted CAs, against which the certificate signing CA is compared (this list can be customized and expanded at will). During the initial negotiations with an HTTPS server, if the server certificate relates to a CA unknown to the browser, a warning is usually raised. This happens most often because a web application relies on a certificate signed by a self-established CA. Whether this is to be considered a concern depends on several factors. For example, this may be fine for an Intranet environment (think of corporate web email being provided via HTTPS; here, obviously all users recognize the internal CA as a trusted CA). When a service is provided to the general public via the Internet, however (i.e. when it is important to positively verify the identity of the server we are talking to), it is usually imperative to rely on a trusted CA, one which is recognized by all the user base (and here we stop with our considerations; we won’t delve deeper in the implications of the trust model being used by digital certificates). • Certificates have an associated period of validity, therefore they may expire. Again, we are warned by the browser about this. A public service needs a temporally valid certificate; otherwise, it means we are talking with a server whose certificate was issued by someone we trust, but has expired without being renewed. • What if the name on the certificate and the name of the server do not match? If this happens, it might sound suspicious. For a number of reasons, this is not so rare to see. A system may host a number of name-based virtual hosts, which share the same IP address and are identified by means of the HTTP 1.1 Host: header information. In this case, since the SSL handshake checks the server certificate before the HTTP request is processed, it is not possible to assign different certificates to each virtual server. Therefore, if the name of the site and the name reported in the certificate do not match, we have a condition which is typically signaled by the browser. To avoid this, IP-based virtual servers must be used. [33] and [34] describe techniques to deal with this problem and allow name-based virtual hosts to be correctly referenced. Other vulnerabilities The presence of a new service, listening in a separate tcp port may introduce vulnerabilities such as infrastructure vulnerabilities if the software is not up to date [4]. Furthermore, for the correct protection of data during transmission the Session Cookie must use the Secure flag [5] and some directives should be sent to the browser to accept only secure traffic (e.g. HSTS [6], CSP). Also there are some attacks that can be used to intercept traffic if the web server exposes the application on both HTTP and HTTPS [6], [7] or in case of mixed HTTP and HTTPS resources in the same page. How to Test Testing for sensitive data transmitted in clear-text Various types of information which must be protected can be also transmitted in clear text. It is possible to check if this information is transmitted over HTTP instead of HTTPS. Please refer to specific tests for full details, for credentials [3] and other kind of data [2]. Example 1. Basic Authentication over HTTP A typical example is the usage of Basic Authentication over HTTP because with Basic Authentication, after log in, credentials are encoded - and not encrypted - into HTTP Headers.
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