Affected: Ubuntu:Pro:14.04:LTS: krb5, Ubuntu:Pro:16.04:LTS: krb5, Ubuntu:Pro:18.04:LTS: krb5, Ubuntu:20.04:LTS: krb5, Ubuntu:22.04:LTS: krb5, Ubuntu:24.04:LTS: krb5, Ubuntu:25.10: krb5, Ubuntu:26.04:LTS: krb5

An integer underflow vulnerability was found in MIT krb5 in the berval2tl_data() function in plugins/kdb/ldap/libkdb_ldap/ldap_principal2.c. The function performs an unsigned subtraction (bv_len - 2) without a prior bounds check. When bv_len is 0 or 1, the subtraction wraps to a large value which is then truncated to uint16_t, yielding 0xFFFE (65534) or 0xFFFF (65535). The subsequent malloc succeeds and memcpy reads up to 65534 bytes from a 0-1 byte buffer, resulting in a heap out-of-bounds read. The attack vector involves a malicious or compromised LDAP KDB backend returning a krbExtraData attribute with bv_len < 2, triggering the underflow when the KDC or kadmind reads principal data.