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iOSOperating system · Cisco

CVE-2017-3857

HIGH · 7.5 CVSS v3.1 Published 2017-03-22
Fix available
A fix is available. Upgrade to after 15.6 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

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 · unedited
A vulnerability in the Layer 2 Tunneling Protocol (L2TP) parsing function of Cisco IOS (12.0 through 12.4 and 15.0 through 15.6) and Cisco IOS XE (3.1 through 3.18) could allow an unauthenticated, remote attacker to cause an affected device to reload. The vulnerability is due to insufficient validation of L2TP packets. An attacker could exploit this vulnerability by sending a crafted L2TP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition. This vulnerability affects Cisco devices that are running a vulnerable release of Cisco IOS or Cisco IOS XE Software if the L2TP feature is enabled for the device and the device is configured as an L2TP Version 2 (L2TPv2) or L2TP Version 3 (L2TPv3) endpoint. By default, the L2TP feature is not enabled. Cisco Bug IDs: CSCuy82078.

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 confidence

A vulnerability in the L2TP packet parsing function of Cisco IOS (12.0-12.4, 15.0-15.6) and IOS XE (3.1-3.18) allows an unauthenticated remote attacker to send a crafted L2TP packet to cause the affected device to reload, resulting in denial of service. The vulnerability stems from insufficient validation of L2TP packets.

MitigationIf L2TP is not required, disable the L2TP feature on affected devices. If L2TP is required, apply the Cisco vendor patch by upgrading to a fixed IOS/IOS XE version. The default state (L2TP disabled) is not vulnerable.

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
iOSOperating system
Affected:>= 12.0, <= 12.4>= 15.0, <= 15.6
Ios XeOperating system
Affected:>= 3.1.0, <= 3.18.0

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
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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify device platform and OS type
    Run 'show version' and look for 'Cisco IOS' or 'Cisco IOS XE' in the output
    Affected if Device is running Cisco IOS or IOS XE software
  2. Determine the IOS version number
    Run 'show version' and locate the version string (for example, 'Version 15.4(3)T' or similar). Compare it to the affected ranges: 12.0 through 12.4, or 15.0 through 15.6 for IOS; 3.1.0 through 3.18.0 for IOS XE
    Affected if The installed version falls within the affected ranges (12.0-12.4, 15.0-15.6 for IOS, or 3.1.0-3.18.0 for IOS XE)
  3. Verify if L2TP is enabled on the device
    Run 'show l2tun session' or 'show l2tp session' and check for active L2TP sessions. Also run 'show running-config | include l2tp' to search for any L2TP-related configuration lines
    Affected if L2TP is actively enabled, configured, or has active sessions (the vulnerability only affects devices where L2TP feature is in use)
  4. Check for L2TP interface bindings
    Run 'show interfaces' and look for any interface references to L2TP, or check the configuration for 'l2tp' or 'l2tp-group' settings
    Affected if L2TP interfaces or L2TP groups are configured and active on the device

A device is affected only if it runs a vulnerable IOS or IOS XE version AND has L2TP enabled or in active use; devices with L2TP disabled are not vulnerable.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 data
Upgrade available Upgrade to a release after 15.6
Interim mitigation

If L2TP is not required, disable the L2TP feature on affected devices. If L2TP is required, apply the Cisco vendor patch by upgrading to a fixed IOS/IOS XE version. The default state (L2TP disabled) is not vulnerable.

Fix this in iOS Scoped from the published advisory
  • Consultation4.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,840
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2017-3857 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data