Security readout for executives and security teams
Plain-English summary
This flaw can let a logged-in remote attacker consume all IP addresses in a Cisco VPN local address pool. When the pool is exhausted, legitimate users cannot log in, creating a denial-of-service outage. The source bundle rates it high severity with CVSS 7.7.
Executive priority
Treat this as a high-priority remote access availability issue where Cisco VPN services support business operations. It is not described as data theft or privilege escalation, but it can block legitimate access.
Technical view
Cisco IOS and IOS XE IKEv2 AutoReconnect may fail to release allocated IP addresses under specific failure conditions. An authenticated attacker using a non-AnyConnect client can repeatedly trigger allocation failures until the assigned local pool is depleted, causing availability impact only.
Likely exposure
Exposure is most likely on Cisco IOS or IOS XE devices using IKEv2 AutoReconnect with an assigned local IP pool for VPN users. The bundle does not provide exact affected or fixed version ranges.
Exploitation context
The bundle says exploitation requires remote authentication and no user interaction. It does not show CISA KEV listing or other cited evidence of active exploitation in the wild.
Researcher notes
Key uncertainty is version scope: the bundle names Cisco IOS and IOS XE but lists affected versions as n/a. Validation should focus on feature exposure, pool behavior, and vendor advisory mapping rather than assumptions about all deployments.
Mitigation direction
- Check Cisco advisory for fixed releases, workarounds, and upgrade guidance.
- Prioritize internet-facing VPN gateways using IKEv2 AutoReconnect.
- Monitor local pool utilization for unexpected depletion or abnormal allocation churn.
- Review operational procedures for safely restoring exhausted address pools.
- Track remediation as an availability risk for remote access services.
Validation and detection
- Inventory Cisco IOS and IOS XE devices that terminate IKEv2 VPN sessions.
- Confirm whether IKEv2 AutoReconnect and assigned local IP pools are enabled.
- Compare running software against Cisco advisory guidance for CVE-2021-1620.
- Review authentication logs for repeated non-AnyConnect connection attempts.
- Alert on local pool exhaustion or sharp drops in free VPN addresses.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-563: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-1620 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.7 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H3.14Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.7HighVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- 20210922 Cisco IOS and IOS XE Software IKEv2 AutoReconnect Feature Denial of Service VulnerabilityCVE reference · vendor-advisory, x_refsource_CISCO
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Assignment to Variable without Use
Assignment to Variable without Use represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.