Ios XrOperating system · Cisco
CVE-2021-34737
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedA vulnerability in the DHCP version 4 (DHCPv4) server feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to trigger a crash of the dhcpd process, resulting in a denial of service (DoS) condition. This vulnerability exists because certain DHCPv4 messages are improperly validated when they are processed by an affected device. An attacker could exploit this vulnerability by sending a malformed DHCPv4 message to an affected device. A successful exploit could allow the attacker to cause a NULL pointer dereference, resulting in a crash of the dhcpd process. While the dhcpd process is restarting, which may take up to approximately two minutes, DHCPv4 server services are unavailable on the affected device. This could temporarily prevent network access to clients that join the network during that time period. Note: Only the dhcpd process crashes and eventually restarts automatically. The router does not reload.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA vulnerability in the DHCPv4 server of Cisco IOS XR Software allows an unauthenticated remote attacker to send malformed DHCPv4 messages that trigger improper validation, leading to a NULL pointer dereference and crash of the dhcpd process. The process auto-restarts within approximately two minutes, causing temporary DHCPv4 service unavailability but not a full router reload.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Ios XrOperating system
< 7.3.2>= 7.4.0, <= 7.4.1>= 7.4.0, < 7.4.1CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify the Cisco IOS XR software versionExecute 'show version' on the IOS XR device and locate the running software version string (typically displayed at the top of the output)Affected if The version falls below 7.3.2, or is 7.4.0 through 7.4.1 (inclusive)
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Verify DHCPv4 server is configuredRun 'show running-config | include dhcp' or 'show dhcp v4' to determine if DHCPv4 server features are enabled on the deviceAffected if DHCPv4 server configuration exists and the service is active (the vulnerability only triggers when DHCPv4 server is running)
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Check for dhcpd process restarts or crashesReview system logs using 'show log' or 'show logging' and search for dhcpd-related messages, particularly entries indicating process termination, restart, or null pointer dereference errorsAffected if Recent dhcpd process crashes or automatic restarts are recorded in the logs, especially around the time of suspicious network activity on UDP port 67
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Confirm DHCPv4 service availabilityTest DHCPv4 service by observing client lease renewals or using 'show dhcp v4 server statistics' if available, and note any service interruptionsAffected if DHCPv4 service shows unexpected unavailability or gaps in lease activity corresponding to potential exploit attempts
You are affected if your Cisco IOS XR version is below 7.3.2 or between 7.4.0 and 7.4.1 inclusive, AND the DHCPv4 server feature is enabled on the device.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped7.3.27.4.1
Apply the Cisco IOS XR software patch for CVE-2021-34737 once available. Consider implementing DHCPv4 message rate limiting or ACL filtering on untrusted interfaces as a temporary compensating control until the patch can be applied.
Upgrade to Cisco IOS XR 7.3.2 or later, or 7.4.2 or later
- 1. Identify the current Cisco IOS XR version running on the affected device using 'show version'
- 2. Confirm the DHCPv4 server is enabled on the device using 'show running-config | include dhcp'
- 3. Schedule a maintenance window for the upgrade (process restart causes brief service interruption)
- 4. Download the fixed IOS XR release from Cisco (7.3.2 or later, or 7.4.2 or later) from tools.cisco.com
- 5. Upload the new software package to the device using 'tftp' or other file transfer method
- 6. Install the new software package using 'install add' followed by 'install activate'
- 7. Verify the installation completed successfully using 'install commit'
- 8. Confirm the new version is running with 'show version' and DHCPv4 server is functional
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Ios Xr
Scoped from the published advisory
- Consultation3.0 h
- Implementation3.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,120.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-34737 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-34737 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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