A vulnerability has been found in krb5, the MIT implementation of Kerberos. MIT krb5 clients incorrectly accept an unkeyed checksums in the SAM-2 preauthentication challenge: An unauthenticated remote attacker could alter a SAM-2 challenge, affecting the prompt text seen by the user or the kind of response sent to the KDC. Under some circumstances, this can negate the incremental security benefit of using a single-use authentication mechanism token. MIT krb5 incorrectly accepts RFC 3961 key-derivation checksums using RC4 keys when verifying KRB-SAFE messages: An unauthenticated remote attacker has a 1/256 chance of forging KRB-SAFE messages in an application protocol if the targeted pre-existing session uses an RC4 session key. Few application protocols use KRB-SAFE messages. The Common Vulnerabilities and Exposures project has assigned CVE-2010-1323 to these issues. For the stable distribution , these problems have been fixed in version 1.6.dfsg.4~beta1-5lenny6. The builds for the mips architecture are not included in this advisory. They will be released as soon as they are available. For the testing distribution and the unstable distribution , these problem have been fixed in version 1.8.3+dfsg-3. We recommend that you upgrade your krb5 packages.