CVE-2021-0251
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 NULL Pointer Dereference vulnerability in the Captive Portal Content Delivery (CPCD) services daemon (cpcd) of Juniper Networks Junos OS on MX Series with MS-PIC, MS-SPC3, MS-MIC or MS-MPC allows an attacker to send malformed HTTP packets to the device thereby causing a Denial of Service (DoS), crashing the Multiservices PIC Management Daemon (mspmand) process thereby denying users the ability to login, while concurrently impacting other mspmand services and traffic through the device. Continued receipt and processing of these malformed packets will create a sustained Denial of Service (DoS) condition. While the Services PIC is restarting, all PIC services will be bypassed until the Services PIC completes its boot process. An attacker sending these malformed HTTP packets to the device who is not part of the Captive Portal experience is not able to exploit this issue. This issue is not applicable to MX RE-based CPCD platforms. This issue affects: Juniper Networks Junos OS on MX Series 17.3 version 17.3R1 and later versions prior to 17.4 versions 17.4R2-S9, 17.4R3-S2; 18.1 versions prior to 18.1R3-S9; 18.2 versions prior to 18.2R3-S3; 18.3 versions prior to 18.3R3-S1; 18.4 versions prior to 18.4R3; 19.1 versions prior to 19.1R2-S2, 19.1R3; 19.2 versions prior to 19.2R2; 19.3 versions prior to 19.3R3. This issue does not affect: Juniper Networks Junos OS versions prior to 17.3R1.
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 NULL pointer dereference vulnerability exists in the Captive Portal Content Delivery (CPCD) services daemon (cpcd) on Juniper MX Series devices with MS-PIC, MS-SPC3, MS-MIC, or MS-MPC. An attacker who is part of the Captive Portal experience can send malformed HTTP packets causing cpcd to crash, which terminates the mspmand process and creates a sustained DoS condition affecting login capabilities and traffic through the device.
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= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3CVSS 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
- Changed
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/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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Identify hardware platformCheck if the device is a Juniper MX Series with an MS-PIC, MS-SPC3, MS-MIC, or MS-MPC card installed using 'show chassis hardware' or 'show pic' commandsAffected if The device is an MX Series with one of the listed MSC cards (MS-PIC, MS-SPC3, MS-MIC, or MS-MPC)
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Check Junos OS versionRun 'show version' or 'request system version' to obtain the installed Junos OS versionAffected if The installed version matches 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, or 19.3 (exact versions)
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Verify CPCD service statusCheck if Captive Portal Content Delivery services are enabled by reviewing the configuration with 'show configuration services captive-portal' or checking cpcd process status with 'show services captive-portal status'Affected if CPCD/captive-portal services are configured and running on the device
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Check mspmand process healthRun 'show system processes extensive | match mspmand' or check for recent crashes in 'show log messages' for mspmand-related entriesAffected if The mspmand process is absent, crashed, or restart loops are logged
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Review crash logs for cpcdExamine system logs with 'show log messages' for cpcd crash events or NULL pointer dereference errors related to the captive portal daemonAffected if Recent crash logs show cpcd termination or NULL pointer dereference errors in captive portal context
The device is affected if it is an MX Series with MS-PIC/MS-SPC3/MS-MIC/MS-MPC hardware, runs an affected Junos version (17.3-19.3), and has CPCD/captive-portal services enabled.
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 dataUpgrade Junos OS to the fixed versions (17.4R2-S9, 17.4R3-S2, 18.1R3-S9, 18.2R3-S3, 18.3R3-S1, 18.4R3, 19.1R2-S2, 19.1R3, 19.2R2, or 19.3R3) as specified in the Juniper advisory. As an interim measure, consider disabling CPCD services if not required, or implement firewall filters to limit CPCD access to legitimate captive portal users only.
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- Implementation2.0 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-2021-0251 — 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-2021-0251 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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