Security readout for executives and security teams
Plain-English summary
This flaw can let a remote, unauthenticated attacker crash the DHCPv4 server process on affected Cisco IOS XR devices. The router does not reload, but DHCPv4 service may be unavailable for about two minutes while the process restarts, affecting clients trying to join during that window.
Executive priority
Treat this as a moderate infrastructure availability issue. It is unlikely to take routers offline, but it can disrupt client onboarding where IOS XR provides DHCPv4 service. Prioritize exposed network segments with high dependency on DHCP availability.
Technical view
Cisco describes improper validation of certain DHCPv4 messages in IOS XR DHCPv4 server handling. A malformed DHCPv4 message can trigger a NULL pointer dereference and crash dhcpd. CVSS 3.1 is 5.8, with network attack vector, low complexity, no privileges, no user interaction, and low availability impact.
Likely exposure
Exposure is most likely on Cisco IOS XR devices that run the DHCPv4 server feature. Devices not using that feature are not indicated as exposed by the provided description. The affected version range is not included in the supplied bundle, so release validation must rely on Cisco’s advisory.
Exploitation context
The bundle does not indicate known active exploitation, and this CVE is not marked KEV. The described attack is remote and unauthenticated, but impact is limited to temporary DHCPv4 server unavailability; Cisco notes the router itself does not reload.
Researcher notes
The root issue is CWE-476 NULL pointer dereference caused by improper DHCPv4 message validation. Evidence supports DoS against dhcpd only, with automatic restart. The supplied bundle does not provide affected release tables, fixed versions, workarounds, or exploitation reports.
Mitigation direction
- Review Cisco’s advisory for affected IOS XR releases and fixed software guidance.
- Inventory IOS XR devices and identify where DHCPv4 server is enabled.
- Prioritize updates or vendor-recommended workarounds for exposed DHCPv4 server deployments.
- Monitor for repeated dhcpd crashes or DHCP service interruptions.
- Avoid assuming a workaround exists unless Cisco documents it.
Validation and detection
- Confirm whether Cisco IOS XR is deployed on routing infrastructure.
- Check whether the DHCPv4 server feature is enabled on those devices.
- Map installed IOS XR versions against Cisco’s advisory.
- Review operational logs for dhcpd restarts or DHCP service gaps.
- Confirm remediation status after applying Cisco-recommended updates or guidance.
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-476: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-34737 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
- Medium
- CVSS
- 5.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L
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:N/UI:N/S:C/C:N/I:N/A:L3.91.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.8MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L
Source materials
- CVE List V5 sourceCVE List V5
- 20210908 Cisco IOS XR Software DHCP Version 4 Server 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.
NULL Pointer Dereference
NULL Pointer Dereference represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.