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CyberDudeBivash ThreatWire • JENKINS CRISIS • Unauthenticated DoS • Build Pipeline Availability • December 2025

JENKINS CRISIS: Unauthenticated Flaw Lets Anyone Crash Your Build Server, Halting ALL Software Delivery (Urgent Fix Required)

Issue: Denial of Service (DoS) via HTTP-based CLI connection handling
Tracking: SECURITY-3630 / CVE-2025-67635
Affected: Jenkins weekly 2.540 and earlier, Jenkins LTS 2.528.2 and earlier
Fixed: Jenkins weekly 2.541, Jenkins LTS 2.528.3
Impact: Request-handling threads can hang indefinitely → controller becomes unresponsive

Official advisory: Jenkins Security Advisory 2025-12-10

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TL;DR — What to Do in the Next 60 Minutes

Upgrade now: Jenkins weekly to 2.541 or Jenkins LTS to 2.528.3 (or newer).
Contain immediately: block untrusted access to Jenkins CLI endpoints at your reverse proxy / load balancer.
Confirm recovery: validate thread pool health, controller responsiveness, and queue processing after patch.
Hunt: check logs for repeated HTTP-based CLI connection attempts and abnormal spikes in stuck requests.

Primary source: Jenkins Security Advisory 2025-12-10 (SECURITY-3630 / CVE-2025-67635). Read vendor bulletin.

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What Is the Vulnerability?

Jenkins disclosed a denial-of-service vulnerability in the HTTP-based CLI subsystem. In affected versions, Jenkins does not properly close HTTP-based CLI connections when the connection stream becomes corrupted. An unauthenticated attacker can create HTTP-based CLI connection requests that cause request-handling threads to wait indefinitely, effectively starving the controller of worker threads and making the service unresponsive. (SECURITY-3630 / CVE-2025-67635) Vendor advisory.

Why This Can “Crash” Your Build Server Fast

In CI/CD reality, a Jenkins controller is a shared scheduling brain: once it becomes sluggish or unresponsive, builds stop starting, web UI times out, webhooks queue up, and developers lose delivery velocity.

Operational impact you will actually see:

Queue builds keep growing but do not execute
UI/API timeouts and “stuck” requests
Thread pool exhaustion (many threads blocked waiting)
Webhook processing delays, SCM polling issues
On-call forced to restart Jenkins repeatedly (until fixed)

Affected Versions and Fixed Versions

Per Jenkins Security Advisory 2025-12-10:

Affected: Jenkins weekly up to and including 2.540; Jenkins LTS up to and including 2.528.2.
Fixed: Jenkins weekly 2.541; Jenkins LTS 2.528.3 properly closes corrupted HTTP-based CLI connections.

Source: Jenkins Security Advisory 2025-12-10.

Patch Guide (Safe, Repeatable, Production-Friendly)

Step-by-step:

Inventory: list every Jenkins controller (prod, DR, staging, “forgotten” internal nodes).
Confirm version: identify weekly vs LTS branch and current version.
Upgrade: update to Jenkins 2.541 (weekly) or 2.528.3 (LTS) or newer.
Restart and verify: validate UI access, build execution, and queue movement after upgrade.
Review access surface: restrict CLI exposure and enforce edge controls (see containment section).

Emergency Containment (If You Cannot Patch Immediately)

Patching is the only full fix. If you need a short window to patch safely, reduce exposure now:

Restrict who can reach Jenkins: allow only VPN / trusted IP ranges (especially for internet-exposed instances).
Block CLI endpoints from untrusted networks at the reverse proxy (HTTP-based CLI is the vulnerable surface).
Prefer safer CLI transport modes where applicable: Jenkins documentation notes HTTP-based CLI behavior can vary behind proxies and recommends WebSocket mode in many cases.
Add rate limits: limit connection attempts and concurrent requests per IP at the edge.

Reference (CLI operational notes): Jenkins CLI documentation.

Detection and Threat Hunting

High-signal indicators of DoS attempts:

Sudden spike in connections or requests related to CLI interactions
Large increase in request-handling threads in a waiting/blocked state
Repeated timeouts on UI/API while CPU is not necessarily maxed
Reverse proxy logs showing repeated long-lived connections to Jenkins

If your controller becomes unresponsive, capture a thread dump during the incident and look for patterns of many threads waiting around CLI-related request handling. Then patch immediately to prevent recurrence. (Root issue described in vendor advisory.) Jenkins Security Advisory 2025-12-10.

If You Are Under Attack: Rapid Response

Contain at the edge: temporary IP allowlist, rate limits, block CLI surface from untrusted traffic.
Stabilize service: if already unresponsive, perform a controlled restart during a safe window.
Patch immediately: upgrade to fixed Jenkins releases (2.541 or 2.528.3+).
Post-incident hardening: restrict Jenkins exposure permanently; segment controller network; review admin access.

Business Impact: Why CI Availability Is Security

CI/CD outages are not “just DevOps problems.” They are business continuity incidents: shipping stops, hotfixes delay, customer outages last longer, and incident response becomes slower. That is why availability bugs like this deserve emergency treatment.

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References (Primary Sources)

Jenkins Security Advisory 2025-12-10 (SECURITY-3630 / CVE-2025-67635): https://www.jenkins.io/security/advisory/2025-12-10/
NVD entry for CVE-2025-67635: https://nvd.nist.gov/vuln/detail/CVE-2025-67635
Jenkins CLI documentation: https://www.jenkins.io/doc/book/managing/cli/

#cyberdudebivash #Jenkins #CICD #DevSecOps #PatchNow #CVE2025 #DoS #BuildSecurity #SRE #IncidentResponse #SoftwareDelivery

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