CVE-2021-1285
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 · uneditedMultiple Cisco products are affected by a vulnerability in the Ethernet Frame Decoder of the Snort detection engine that could allow an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition. The vulnerability is due to improper handling of error conditions when processing Ethernet frames. An attacker could exploit this vulnerability by sending malicious Ethernet frames through an affected device. A successful exploit could allow the attacker to exhaust disk space on the affected device, which could result in administrators being unable to log in to the device or the device being unable to boot up correctly.Note: Manual intervention is required to recover from this situation. Customers are advised to contact the Cisco Technical Assistance Center (TAC) to help recover a device in this condition.Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.
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 confidenceThis vulnerability exists in the Ethernet Frame Decoder of Cisco's Snort detection engine across multiple Cisco products. It stems from improper error handling when processing Ethernet frames, allowing an adjacent (local network) attacker to send malicious Ethernet frames that exhaust disk space on the affected device. This causes a denial-of-service condition where administrators cannot log in and the device may fail to boot properly.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS 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
- Adjacent
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Changed
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.0/AV:A/AC:L/PR:N/UI:N/S:C/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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Verify Snort detection engine is runningIdentify if the device runs Cisco Snort-based detection. Check process list for snort/sensort processes, or review installed software packages matching Cisco security products.Affected if Snort-based detection engine is active on the device
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Confirm Ethernet Frame Decoder is enabledReview Snort configuration files (often in /etc/snort/, /usr/local/snort/, or device-specific config paths) for 'decoder' or 'ethernet' settings. Check if the Ethernet Frame Decoder module is loaded and not explicitly disabled.Affected if Ethernet Frame Decoder is loaded and processing traffic
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Check for adjacent network exposureReview network interface configuration and access control lists. Determine if untrusted local network segments can send frames directly to the affected device.Affected if Device is directly accessible from untrusted local network segments or has no network segmentation protecting it from adjacent attackers
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Inspect disk space and log accumulationRun 'df -h' to check disk usage, and review /var/log/ directories for abnormally large Snort-related log files or crash dumps that may indicate exploitation attempts.Affected if Disk space is severely depleted or unusual log growth is observed with Snort as the consuming process
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Check for Snort-related crash logs or errorsSearch /var/log/, /var/crash/, or device-specific log directories for Snort decoder errors, segmentation faults, or Ethernet frame processing failures.Affected if Recent crash logs or decoder error messages related to Ethernet frame handling exist
The environment is affected if Cisco Snort with an active Ethernet Frame Decoder is deployed on a device with direct adjacency to untrusted network segments, as this allows the exploit vector for disk exhaustion.
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.
From vendor dataApply Cisco software updates (patches) that address this vulnerability. No workarounds exist. Manual intervention via Cisco TAC is required to recover compromised devices. Prioritize devices with direct network exposure.
- Consultation3.0 h
- Implementation6.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-1285 — 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-2021-1285 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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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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