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Fix krb5_read_message handling [CVE-2014-5355]
In recvauth_common, do not use strcmp against the data fields of krb5_data objects populated by krb5_read_message(), as there is no guarantee that they are C strings. Instead, create an expected krb5_data value and use data_eq(). In the sample user-to-user server application, check that the received client principal name is null-terminated before using it with printf and krb5_parse_name. CVE-2014-5355: In MIT krb5, when a server process uses the krb5_recvauth function, an unauthenticated remote attacker can cause a NULL dereference by sending a zero-byte version string, or a read beyond the end of allocated storage by sending a non-null-terminated version string. The example user-to-user server application (uuserver) is similarly vulnerable to a zero-length or non-null-terminated principal name string. The krb5_recvauth function reads two version strings from the client using krb5_read_message(), which produces a krb5_data structure containing a length and a pointer to an octet sequence. krb5_recvauth assumes that the data pointer is a valid C string and passes it to strcmp() to verify the versions. If the client sends an empty octet sequence, the data pointer will be NULL and strcmp() will dereference a NULL pointer, causing the process to crash. If the client sends a non-null-terminated octet sequence, strcmp() will read beyond the end of the allocated storage, possibly causing the process to crash. uuserver similarly uses krb5_read_message() to read a client principal name, and then passes it to printf() and krb5_parse_name() without verifying that it is a valid C string. The krb5_recvauth function is used by kpropd and the Kerberized versions of the BSD rlogin and rsh daemons. These daemons are usually run out of inetd or in a mode which forks before processing incoming connections, so a process crash will generally not result in a complete denial of service. Thanks to Tim Uglow for discovering this issue. CVSSv2: AV:N/AC:L/Au:N/C:N/I:N/A:P/E:POC/RL:OF/RC:C [tlyu@mit.edu: CVSS score] ticket: 8050 (new) target_version: 1.13.1 tags: pullup
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