Fix vulnerabilities in GSS message token handling

Fix vulnerabilities in GSS message token handling

In gss_krb5int_unseal_token_v3() and gss_krb5int_unseal_v3_iov(),
verify the Extra Count field of CFX wrap tokens against the encrypted
header.  Reported by Jacob Champion.

In gss_krb5int_unseal_token_v3(), check for a decrypted plaintext
length too short to contain the encrypted header and extra count
bytes.  Reported by Jacob Champion.

In kg_unseal_iov_token(), separately track the header IOV length and
complete token length when parsing the token's ASN.1 wrapper.  This
fix contains modified versions of functions from k5-der.h and
util_token.c; this duplication will be cleaned up in a future commit.

CVE-2024-37370:

In MIT krb5 release 1.3 and later, an attacker can modify the
plaintext Extra Count field of a confidential GSS krb5 wrap token,
causing the unwrapped token to appear truncated to the application.

CVE-2024-37371:

In MIT krb5 release 1.3 and later, an attacker can cause invalid
memory reads by sending message tokens with invalid length fields.

ticket: 9128 (new)
tags: pullup
target_version: 1.21-next