The quarterly external scan is one of the most mechanical requirements in PCI DSS, and one of the most common reasons a compliance year goes wrong. A missed quarter cannot be scanned after the fact. A failing scan that was never followed by a passing rescan leaves a gap no amount of documentation fills.
The rules for what passes come from the Approved Scanning Vendor (ASV) Program Guide, which the PCI Security Standards Council publishes alongside the standard. Most teams never read it. They learn its rules one failed scan at a time.
This article covers what the requirement asks, who decides what is scanned, what makes a scan fail, how disputes work, and how to run the quarter so the evidence is there at the end of the year. Internal scanning and penetration testing belong to the wider Requirement 11 and are left to a separate article.
What the requirement asks for
Requirement 11.3.2 asks for external vulnerability scans at least once every three months. The scans must be performed by an ASV qualified by the Council. Vulnerabilities must be resolved and the ASV Program Guide requirements for a passing scan met, with rescans as needed to confirm.
Requirement 11.3.2.1 adds external scans after any significant change. Those do not have to be performed by an ASV. They can be run by qualified personnel with organizational independence, and vulnerabilities scored 4.0 or higher under CVSS must be resolved.
The usual evidence is four passing quarterly scans across the 12 months before your assessment. For your first assessment, the standard does not require four passing scans if your assessor verifies three things: your most recent scan passed, you have documented policies and procedures requiring scans at least once every three months, and vulnerabilities found in the scan were corrected as shown in a rescan.
Who decides what gets scanned
You do. The ASV Program Guide puts responsibility for scope on the scan customer. You give the ASV every internet-facing IP address, range and domain that is in scope or provides a path to the cardholder data environment, and you attest that the list is complete.
Scope errors are harder to spot than scan failures, because a scan of the wrong list can pass cleanly. Check for:
Addresses that change. Cloud load balancers, autoscaling groups and ephemeral public addresses can move between quarters. Scan by domain where the ASV supports it, and refresh the list before each scan.
Load balancers. If several servers sit behind one address, the scan may reach only one of them. The Program Guide expects you to document how load balancing works so the ASV can confirm every server is covered.
Forgotten hosts. Old marketing sites, staging environments and admin interfaces that share a network path with the CDE.
Third-party hosted components. If a provider hosts part of your payment path, agree in writing who scans it.
Your ASV may also find components you did not list during discovery. Treat that as a scope finding, not an annoyance. The work of defining scope is covered more fully in reducing your PCI DSS scope, and segmentation decides which internal paths matter, as network segmentation for PCI DSS explains.
What makes a scan pass or fail
The core rule in the Program Guide is a CVSS threshold. A component fails if the scan finds a vulnerability with a CVSS base score of 4.0 or higher. A scan passes only when every component in scope passes.
The Guide also lists conditions that fail a scan regardless of score. Among them are vulnerabilities that could lead to SQL injection or cross-site scripting, and operating systems or software no longer supported by their vendor. Read the current Guide for the full list, because it decides more failures than the threshold does for some teams.
Separately, the Guide defines special notes. These cover findings that are not necessarily vulnerabilities but carry risk, such as remote access software or directory browsing on a web server. A special note does not fail the scan by itself, but you must give the ASV a business justification and describe the security measures in place. An unanswered special note can hold up the report.
The finished report has three parts: the Attestation of Scan Compliance, signed by you and the ASV; a summary by component; and the vulnerability details. The attestation is the page your assessor and acquirer look at first.
Disputing a finding
Not every finding is real. Scanners infer versions from banners and responses, and backported security fixes are a frequent source of false positives. The Program Guide lets you dispute a finding with your ASV, and the ASV decides.
A dispute that succeeds usually contains:
The exact finding. The component, port and vulnerability identifier from the report.
The reason it does not apply. For a false positive, the evidence that the vulnerable version is not present, such as the vendor's advisory for the backported fix and the installed package version.
For a compensating control, how it addresses the risk. Describe the control and show it is in place. Your ASV assesses it.
Evidence from the system. Command output or configuration exports, dated, rather than a statement that it has been checked.
Disputes take time. Run scans early in the quarter so there is room for a dispute, a fix and a rescan before the quarter ends.
Scan interference
The ASV must be able to scan without active protection systems blocking it. If an intrusion prevention system or web application firewall shuts the scan out, the result does not reflect what an attacker would see, and the Program Guide treats that as interference. Arrange with your ASV how its scanning addresses will be handled for the duration of the scan, so that it reaches the systems behind those protections. The protections stay in place for everyone else.
Running the quarter
A quarter that ends with a passing scan in the file looks like this:
Refresh scope. Update the list of addresses and domains, and record what changed since last quarter.
Scan early. Aim for the first weeks of the quarter.
Triage the results. Assign each failing finding to an owner with a deadline, and decide early which ones to dispute.
Remediate and rescan. Repeat until every component passes.
File the report. Keep the full report, including any failed scans and disputes, not only the final passing attestation.
Treat ASV findings as part of your normal vulnerability process, with the same severities and deadlines. The approach in vulnerability management SLAs auditors accept applies, with the quarter end as a hard stop.
What an ASV scan does not tell you
A passing ASV scan says that your internet-facing components showed no vulnerabilities above the threshold on the day of the scan. It does not cover your internal network, which needs its own quarterly scans under 11.3.1. It is not a penetration test, which 11.4 requires separately. And it does not detect changes to scripts on your payment pages, which 11.6.1 addresses.
The PCI DSS framework page shows where external scanning sits among the other testing requirements.
How TrustLynce helps
TrustLynce stores each quarter's ASV reports, rescans and dispute records as timestamped evidence through evidence collection, so the year's record is complete when your assessor asks. Missing or overdue scans show up in continuous monitoring as a failed control routed to its owner.