This is the first official acknowledgement that the ASD might not disclose all of the vulnerabilities it discovers. However, knowledge of secret vulnerabilities would have always been an essential part the agency's toolkit for offensive cyber operations.
The policy stresses that the agency's starting position for when it finds a weakness is to disclose it and work with vendors to ensure that patches are available before it is made public.
"Occasionally, however, a security weakness will present a novel opportunity to obtain foreign intelligence that will help protect Australians. In these circumstances, the national interest might be better served by not disclosing the vulnerability," the policy reads.
"The decision to retain a vulnerability is never taken lightly. It is only made after a careful multi-stage expert analysis, and is subject to rigorous review and oversight."
ZDNet understands this isn't a new decision-making framework, but one that has been in operation in various forms for quite some time. It's being made public as part of ASD director-general Mike Burgess' strategy to bring the agency "out from the shadows" and to dispel the notion that it warehouses large numbers of zero-day exploits.
The key decision-making principle is that the national interest to keep a vulnerability secret must strongly outweigh the national interest of disclosing it, based on the existence of a "critical intelligence requirement".
"This might happen if the weakness allows us to gather foreign intelligence that will prevent a terrorist attack, for example," the policy reads.
The ASD also considers whether retaining the vulnerability runs the risk of a malicious actor taking advantage of the weakness, as well as what preventative measures might be needed to protect Australian interests.
Newly-discovered vulnerabilities are first assessed by the Equity Steering Group consisting of working-level technical experts. ZDNet understands that both the cybersecurity and offensive cyber operations sides of the ASD are represented, and that the discussions can be robust.
If that group recommends a vulnerability should be retained, it is then considered by the Equity Board made up of officers at the Senior Executive Service pay grades.
Decisions to retain vulnerabilities are reviewed quarterly by the director-general, and annually by the independent Inspector General of Intelligence and Security (IGIS). Briefing IGIS to her satisfaction is understood to be a daunting experience.
The retention of each individual vulnerability is also reviewed after 12 months.
ZDNet understands that at the end of this assessment and review process, the number of vulnerabilities retained for ASD use is very small, an amount that would not be characterised as "warehousing".