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CWE-807: Reliance on Untrusted Inputs in a Security Decision
 DescriptionDescription Summary The application uses a protection mechanism that relies on the existence or values of an input, but the input can be modified by an untrusted actor in a way that bypasses the protection mechanism.
Extended Description Developers may assume that inputs such as cookies, environment variables, and hidden form fields cannot be modified. However, an attacker could change these inputs using customized clients or other attacks. This change might not be detected. When security decisions such as authentication and authorization are made based on the values of these inputs, attackers can bypass the security of the software. Without sufficient encryption, integrity checking, or other mechanism, any input that originates from an outsider cannot be trusted. Time of Introduction
Applicable PlatformsLanguages Language-independent Common Consequences
Likelihood of ExploitMedium to High Detection Methods
Demonstrative ExamplesExample 1 The following code excerpt reads a value from a browser cookie to determine the role of the user. (Bad Code) Example
Language: Java Cookie[] cookies = request.getCookies(); for (int i =0; i< cookies.length; i++) { Cookie c = cookies[i];
if (c.getName().equals("role")) {
userRole = c.getValue();
}
} Example 2 The following code could be for a medical records application. It performs authentication by checking if a cookie has been set. (Bad Code) Example
Language: PHP $auth = $_COOKIES['authenticated']; if (! $auth) { if (AuthenticateUser($_POST['user'], $_POST['password']) ==
"success") {
// save the cookie to send out in future responses
setcookie("authenticated", "1", time()+60*60*2);
}
else {
ShowLoginScreen();
die("\n");
}
} DisplayMedicalHistory($_POST['patient_ID']); The programmer expects that the AuthenticateUser() check will always be applied, and the "authenticated" cookie will only be set when authentication succeeds. The programmer even diligently specifies a 2-hour expiration for the cookie. However, the attacker can set the "authenticated" cookie to a non-zero value such as 1. As a result, the $auth variable is 1, and the AuthenticateUser() check is not even performed. The attacker has bypassed the authentication. Example 3 In the following example, an authentication flag is read from a browser cookie, thus allowing for external control of user state data. (Bad Code) Example
Language: Java Cookie[] cookies = request.getCookies(); for (int i =0; i< cookies.length; i++) { Cookie c = cookies[i];
if (c.getName().equals("authenticated") &&
Boolean.TRUE.equals(c.getValue())) {
authenticated = true;
}
} Example 4 The following code samples use a DNS lookup in order to decide whether or not an inbound request is from a trusted host. If an attacker can poison the DNS cache, they can gain trusted status. (Bad Code) Example
Language: C struct hostent *hp;struct in_addr myaddr; char* tHost = "trustme.example.com"; myaddr.s_addr=inet_addr(ip_addr_string); hp = gethostbyaddr((char *) &myaddr, sizeof(struct
in_addr), AF_INET); if (hp && !strncmp(hp->h_name, tHost,
sizeof(tHost))) { trusted = true;
} else { trusted = false;
} (Bad Code) Example
Language: Java String ip = request.getRemoteAddr(); InetAddress addr = InetAddress.getByName(ip); if (addr.getCanonicalHostName().endsWith("trustme.com")) { trusted = true;
} (Bad Code) Example
Language: C# IPAddress hostIPAddress = IPAddress.Parse(RemoteIpAddress); IPHostEntry hostInfo = Dns.GetHostByAddress(hostIPAddress); if (hostInfo.HostName.EndsWith("trustme.com")) { trusted = true;
} IP addresses are more reliable than DNS names, but they can also be spoofed. Attackers can easily forge the source IP address of the packets they send, but response packets will return to the forged IP address. To see the response packets, the attacker has to sniff the traffic between the victim machine and the forged IP address. In order to accomplish the required sniffing, attackers typically attempt to locate themselves on the same subnet as the victim machine. Attackers may be able to circumvent this requirement by using source routing, but source routing is disabled across much of the Internet today. In summary, IP address verification can be a useful part of an authentication scheme, but it should not be the single factor required for authentication. Observed Examples
Potential Mitigations
Relationships
Taxonomy Mappings
Related Attack Patterns
References
Content History
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Page Last Updated:
February 20, 2013
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