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A zero-day is a vulnerability attackers know about when its vendor has no patch available. It is not defined by who discovered the flaw or whether it has been publicly announced. A researcher, a software maker, or an attacker may uncover a weakness; the risk becomes especially urgent when attackers exploit it before a fix reaches users.
What “zero-day” means—and what it does not
Google Project Zero defines a zero-day as “a vulnerability that attackers know about, and there is no patch available from the vendor.” The definition describes attacker awareness and patch availability, not a particular discovery method or a fixed number of days.
- Vulnerability: the underlying weakness in software, hardware, or a service.
- Exploit: a technique or code that takes advantage of a vulnerability.
- Patch: a vendor-provided fix for the weakness. A mitigation may instead reduce exposure or risk without removing the underlying flaw.
These are separate things. A vulnerability can be found privately, exploited before the public hears about it, and patched before technical details are disclosed. Public disclosure does not itself mean a patch is available, and a patch release does not mean every affected system has installed it.
How zero-day vulnerabilities are discovered
There is no single route. A researcher outside the affected organization may identify a flaw and report it; a vendor’s own security team may find it; or an attacker may discover and keep it for private use. Project Zero says its research covers widely used software including mobile operating systems, browsers, and open-source libraries.
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Finding a suspicious behavior is not always the same as confirming a vulnerability. The affected vendor or project needs enough information to assess whether there is a security flaw, which products and versions may be affected, and what response is appropriate. The available evidence establishes these broad discovery routes, but does not establish a specific technical recipe for finding flaws or building exploits.
What happens from discovery to public disclosure
- Identify the possible weakness. A researcher, product team, or attacker notices behavior that may expose a security flaw.
- Report and assess it. A reporter may send a technical description privately to the vendor or project so it can validate the issue and assess affected products. NIST Special Publication 800-216 recommends a formal process for accepting, assessing, and managing vulnerability reports, then communicating mitigation or remediation. Its guidance is for systems under federal control; it is not a universal vendor deadline.
- Determine whether exploitation is underway. A flaw may be used against real targets before public disclosure or before a patch is ready. CISA, the FBI, and the NSA reported in a 2024 advisory that malicious cyber actors exploited more zero-day vulnerabilities to compromise enterprise networks in 2023 than in 2022.
- Develop a fix or mitigation. The vendor investigates and works on a patch or another way to reduce risk. The specific engineering process varies; the sources here do not establish a universal sequence.
- Release and deploy protection. The vendor makes a fix or mitigation available, but protection depends on affected organizations and users applying it to their systems.
- Disclose technical details. The researcher and vendor may publish information according to their policy and circumstances. Disclosure timing can be separate from both patch availability and the start of exploitation.
How attackers exploit a zero-day
At a high level, an attacker uses an exploit to take advantage of the weakness in an affected product or service. Successful exploitation can let an attacker compromise a system or network, depending on the flaw and the circumstances. The term “zero-day” does not tell you how an exploit works, how reliable it is, or how many people are affected.
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Attackers are not the only people who find zero-days, and not every discovered vulnerability is exploited. But when exploitation begins before defenders have a patch, organizations may have to respond with mitigations and prioritization while a vendor works on a fix.
The same CISA, FBI, and NSA advisory said that a majority of the most frequently exploited vulnerabilities in its set were initially exploited as zero-days in 2023, compared with less than half in 2022. Those findings apply to the advisory’s specified years and vulnerability set, not to all cyberattacks or later years.
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How vendors and defenders respond
For the vendor or project
The response is to assess the report, identify affected products, and develop a patch or mitigation. A patch addresses the weakness; a mitigation can reduce the risk while a patch is unavailable or not yet deployed. The sources do not establish one universal patch-development workflow or deadline.
For an organization
Organizations can use CISA’s Known Exploited Vulnerabilities (KEV) Catalog as one input when prioritizing vulnerability management. CISA describes KEV as an authoritative source of vulnerabilities exploited in the wild and says organizations should use it as an input to prioritization. It is not an exhaustive inventory of every flaw, nor does a catalog entry by itself establish that a particular organization is affected.
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For an individual user
When a vendor releases a security update for a product you use, install it through that product’s normal update mechanism as soon as practical, following the vendor’s instructions. If a vendor or your organization provides a mitigation for an affected product, follow that guidance while the patch is unavailable or awaiting deployment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How disclosure deadlines differ
Disclosure policies set expectations between a reporter and the affected vendor; they are not all the same. Google Project Zero’s policy is one specific example. NIST SP 800-216 is a federal vulnerability-disclosure framework, not a competing fixed-day countdown.
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| Policy or guidance | Scope and ordinary timing | Active exploitation or post-patch timing | What becomes public |
|---|---|---|---|
| Google Project Zero policy | For a report to a vendor, the policy gives 90 days after notification for a patch to be made available. | For vulnerabilities Project Zero finds actively exploited against real users, the policy uses a 7-day deadline instead. If a patch arrives during the ordinary 90-day period, Project Zero generally waits 30 days after it is available to users before publishing technical details. The 30-day post-patch window also applies when a patch meets the 7-day active-exploitation deadline. The policy describes a possible 14-day grace period when a vendor commits to a near-term fix. | If no patch is available by day 90, Project Zero says it publishes details at the deadline. In a trial announced in July 2025, it also described sharing limited report metadata at about one week—recipient, affected product, report date, and deadline—while withholding technical details or information it believes could materially assist discovery until the deadline. |
| NIST SP 800-216 | Published May 24, 2023, it recommends a federal framework for accepting, assessing, and managing vulnerability reports for software, hardware, and digital services under federal control. A universal fixed-day patch deadline is not stated. | A Project Zero-style 90-, 30-, or 7-day schedule is not stated. | It recommends communicating mitigation or remediation; a universal technical-disclosure timetable is not stated. |
Project Zero’s tracked figures show what its own policy has covered, not an industry-wide success rate: as of July 29, 2025, it reported 2,131 vulnerabilities in New or Fixed status under its 90-day deadline, 95 disclosures without a patch made available to users, and a 95.5% lifetime under-deadline fix rate. These are figures for Project Zero’s tracked issue population only.
Quick Recap
What to remember when a zero-day is reported
- The label means attackers know about a vulnerability and the vendor has no patch available; it does not identify who found it.
- An exploit is the means of taking advantage of a flaw, while a patch is a vendor fix. A mitigation may reduce risk without being a full fix.
- Exploitation can precede public disclosure and patch availability, so defenders may need to act on mitigations before an update exists.
- Deadlines depend on the policy or framework involved. Project Zero’s schedule is its own policy, not an industry-wide rule.
- A released patch is only protective once it is applied to affected systems.
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