Executive Summary

This vulnerability involves the NSD software's handling of secondary servers requesting zone transfers over TLS or TCP. When a provide-xfr rule includes a tls-auth-name, the secondary server is supposed to provide a client certificate with that name to authenticate the transfer.

However, the vulnerability arises because if the transfer request comes over TLS on the regular tls-port (not the tls-auth-port) or over TCP on the regular port, no client certificate is required even if the tls-auth-name is specified. This bypasses the intended client certificate verification.

This means an attacker who meets other access control conditions can bypass the client certificate requirement and potentially perform unauthorized zone transfers.

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Impact Analysis

This vulnerability can allow an attacker to bypass client certificate authentication when requesting zone transfers from an NSD server.

As a result, unauthorized parties could obtain DNS zone data that should be protected, potentially exposing sensitive network information.

This could lead to further attacks such as DNS spoofing, reconnaissance, or other security breaches based on the leaked zone information.

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Detection Guidance

This vulnerability involves secondary servers transferring zones without providing the required client certificate when using certain ports and protocols. Detection would involve monitoring zone transfer requests to identify transfers occurring over TLS on the regular tls-port or over TCP on the regular port without client certificate authentication.

Specifically, you should look for zone transfer requests that match the "provide-xfr" rule conditions but do not present a client certificate, especially if the transfer is over TLS on the regular tls-port or over TCP on the regular port.

While no explicit commands are provided in the resources, network monitoring tools or packet capture utilities (such as tcpdump or Wireshark) can be used to capture and analyze DNS zone transfer traffic on the relevant ports to check for the presence or absence of client certificates during TLS handshakes.

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Mitigation Strategies

To mitigate this vulnerability, the recommended immediate step is to upgrade to NSD version 4.14.3, which includes the fix for this issue.

If upgrading is not immediately possible, a manual patch is available for NSD 4.14.2. This patch can be applied to the NSD source directory using the provided diff file, followed by running 'make install' to install the updated version.

Additionally, configuring the "tls-auth-xfr-only" option explicitly to "yes" can help enforce client certificate authentication for transfers, reducing the risk of bypass.

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