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Fix NoMethodError in Kerberos client decoding by adding ASN.1 structural validation #21104

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Fix NoMethodError in Kerberos client decoding by adding ASN.1 structural validation #21104

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29 changes: 26 additions & 3 deletions lib/rex/proto/kerberos/client.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -219,8 +219,31 @@ def recv_response_udp
# @return [<Rex::Proto::Kerberos::Model::KrbError, Rex::Proto::Kerberos::Model::KdcResponse, Rex::Proto::Kerberos::Model::KrbError>] the kerberos message response
# @raise [Rex::Proto::Kerberos::Model::Error::KerberosDecodingError] if the response can't be processed
def decode_kerb_response(data)
asn1 = OpenSSL::ASN1.decode(data)
msg_type = asn1.value[0].value[1].value[0].value
begin
asn1 = OpenSSL::ASN1.decode(data)
rescue OpenSSL::ASN1::ASN1Error => e
raise ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError, "Kerberos Client: Failed to decode ASN.1: #{e.message}"
end

# Safely navigate the ASN.1 structure to extract the message type.
# The structure is expected to be an Application Tag wrapper around a Sequence,
# where the element with context-specific tag [1] contains the msg-type integer.
# Reference: RFC 4120 sections 5.2.1, 5.4.1, 5.9.1
msg_type = begin
if asn1.is_a?(OpenSSL::ASN1::ASN1Data) && asn1.value.is_a?(Array) && asn1.value[0].is_a?(OpenSSL::ASN1::Sequence)
seq_value = asn1.value[0].value
# Find the element with context-specific tag 1 (msg-type)
msg_type_tag = seq_value.find { |elem| elem.is_a?(OpenSSL::ASN1::ASN1Data) && elem.tag == 1 }
# Extract the Integer value from the tagged element
if msg_type_tag && msg_type_tag.value.is_a?(Array) && msg_type_tag.value[0].is_a?(OpenSSL::ASN1::Integer)
msg_type_tag.value[0].value
end
end
rescue StandardError
nil
end

raise ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError, 'Kerberos Client: Invalid or truncated ASN.1 response structure' if msg_type.nil?

case msg_type
when Rex::Proto::Kerberos::Model::KRB_ERROR
Expand All @@ -230,7 +253,7 @@ def decode_kerb_response(data)
when Rex::Proto::Kerberos::Model::AP_REP
res = Rex::Proto::Kerberos::Model::ApRep.decode(asn1)
else
raise ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError, 'Kerberos Client: Unknown response'
raise ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError, "Kerberos Client: Unknown response type: #{msg_type}"
end

res
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