CVE-2026-33800
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 · uneditedAn Unchecked Input for Loop Condition vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS on MX Series allows an unauthenticated, adjacent attacker to cause a Denial-of-Service (DoS).Micro-BFD session flaps generate respective up/down events which are queued by PFEMAN for processing. Especially in a Virtual-Chassis (VC) scenario with locality‑bias configured, processing takes a significant amount of time for each event. If these sessions keep flapping, new events are constantly added, and in turn PFEMAN never completes processing these events. This results in the PFEMAN watchdog timer expiring, which causes the FPC to crash and restart, representing a complete service outage. This issue only affects MX series FPCs up to and including MPC9, and LC2101/2103 and LC480. It does not affect MPC10/11, LC4800/9600, and MX304. This issue affects Junos OS on MX Series: * all versions before 23.2R2-S7, * 23.4 versions before 23.4R2-S8, * 24.2 versions before 24.2R2-S4, * 24.4 versions before 24.4R2-S3, * 25.2 versions before 25.2R2.
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 confidenceIn Juniper Junos OS on MX Series, the PFEMAN process that handles Micro-BFD session up/down events has an unchecked input loop condition. When BFD sessions flap rapidly, especially in Virtual-Chassis setups with locality-bias configured, events accumulate faster than PFEMAN can process them. The resulting backlog triggers the PFEMAN watchdog timer expiration, causing the FPC to crash and restart.
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
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:A/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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Confirm device is MX SeriesRun 'show system hardware' or 'show chassis hardware' and verify the device model is from the MX Series product lineAffected if Device is not an MX Series router - this vulnerability only affects MX Series
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Check Junos OS versionRun 'show version' and note the exact Junos OS version installedAffected if Version is older than 23.2R2-S7, 23.4R2-S8, 24.2R2-S4, 24.4R2-S3, or 25.2R1, or falls between these patch levels without the fix
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Verify BFD is configuredRun 'show configuration | display set | match bfd' to check for Bidirectional Forwarding Detection session configurationAffected if No BFD sessions are configured - the vulnerability requires active BFD sessions to trigger the flaw
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Check Virtual-Chassis configurationRun 'show virtual-chassis status' to determine if the device is part of a Virtual-Chassis setupAffected if Not in a Virtual-Chassis configuration - the flaw specifically mentions Virtual-Chassis setups with locality-bias as the trigger condition
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Check locality-bias settingsRun 'show configuration | match locality-bias' to see if locality-bias is configured in the BFD or protocol settingsAffected if Locality-bias is not configured - this setting in Virtual-Chassis environments contributes to the event accumulation that triggers the flaw
An affected system is an MX Series router running a vulnerable Junos version with active BFD sessions in a Virtual-Chassis setup that has locality-bias configured, creating conditions for rapid BFD session flaps to trigger the PFEMAN watchdog expiration and FPC crash.
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 · scopedUpgrade Junos OS to a patched version (23.2R2-S7 or later, 23.4R2-S8 or later, 24.2R2-S4 or later, 24.4R2-S3 or later, or 25.2R2 or later) on affected MX Series devices. Consider adjusting BFD session parameters or locality-bias settings as a temporary workaround to reduce flap frequency.
Upgrade to Junos OS 23.2R2-S7 or later, 23.4R2-S8 or later, 24.2R2-S4 or later, 24.4R2-S3 or later, or 25.2R2 or later
- 1. Identify the exact Junos OS version currently running on the MX Series device using the command: show version
- 2. Verify the FPC/MPC type in use (confirm it's MPC9 or earlier, or LC2101/2103/LC480) using: show chassis fpc pic-status
- 3. If the current version is before 23.2R2-S7, 23.4R2-S8, 24.2R2-S4, 24.4R2-S3, or 25.2R2, plan an upgrade to one of these fixed releases or later
- 4. Schedule a maintenance window as this upgrade will require a system reboot
- 5. Download the appropriate Junos OS upgrade image from the Juniper Networks support portal
- 6. Upload the image to the device and verify integrity using: file checksum md5 <image> (compare with published checksum)
- 7. Install the upgrade using: request system software add <image> reboot
- 8. After reboot, verify the new version is running: show version
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA4.0 h
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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 sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-33800 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
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