CVE-2026-18695 Overview
CVE-2026-18695 is a denial-of-service vulnerability in MongoDB Server's query engine. The flaw affects how MongoDB processes certain query predicates against time-series collections that use a metaField. An authenticated user with write access can trigger a reachable assertion [CWE-617] that causes the mongod server process to terminate unexpectedly. The issue is tracked in the MongoDB Jira as SERVER-129460. Successful exploitation crashes the database process, disrupting availability for every application depending on that instance.
Critical Impact
An authenticated attacker with write privileges to a time-series collection can crash the MongoDB server process, producing a denial of service across all connected workloads.
Affected Products
- MongoDB Server deployments exposing time-series collections configured with a metaField
- Environments where authenticated users hold write access to those collections
- Clustered or sharded MongoDB deployments where a crash of mongod propagates operational impact
Discovery Timeline
- 2026-08-11 - CVE-2026-18695 published to the National Vulnerability Database
- 2026-08-11 - Last updated in NVD database
Technical Details for CVE-2026-18695
Vulnerability Analysis
The vulnerability resides in MongoDB Server's query planner and execution logic for time-series collections. Time-series collections use an internal bucketed storage layout, and queries against the metaField are rewritten to operate on that layout. Certain query predicates trigger an unexpected internal state that the server treats as unrecoverable. The process then terminates via a fatal assertion rather than returning a query error to the client. Because the crash occurs at the server layer, all in-flight sessions and replica traffic are interrupted at once. The MongoDB issue tracker entry SERVER-129460 is the primary technical reference.
Root Cause
The root cause is a reachable assertion classified as [CWE-617] (Reachable Assertion). The query engine assumes an invariant about the shape of predicates evaluated against a time-series metaField. Malformed or unexpected predicate structures violate that invariant. Instead of degrading gracefully, mongod aborts. The bug is a logic flaw in input handling rather than a memory-safety issue.
Attack Vector
The attack vector is network-based and requires authentication with write privileges on a time-series collection. No user interaction is required. An attacker submits a crafted query containing predicates against the collection's metaField. The server processes the predicate, hits the assertion, and terminates. Repeating the query after restart turns the flaw into a persistent denial-of-service primitive against production clusters.
The technical details are described in prose because no verified proof-of-concept code has been published. Refer to the MongoDB Server Issue Tracker entry for engineering context.
Detection Methods for CVE-2026-18695
Indicators of Compromise
- Unexpected mongod process terminations correlated with fatal assertion messages in server logs
- Log entries referencing time-series bucket unpacking or metaField predicate evaluation immediately before a crash
- Repeated client connection resets from a single authenticated principal targeting a time-series collection
- Cluster failover events with no corresponding infrastructure or resource-exhaustion cause
Detection Strategies
- Alert on mongod exit codes and abnormal restart counts reported by systemd, Kubernetes, or the MongoDB Ops Manager
- Correlate database audit log entries for queries against time-series collections with subsequent process termination
- Baseline query patterns per service account and flag anomalous predicate structures against metaField
- Monitor replica set elections that follow shortly after query traffic from a specific authenticated user
Monitoring Recommendations
- Enable MongoDB auditing for authenticate, command, and slowOp events on collections with timeseries options
- Forward mongod logs to a centralized data lake and parse for Fatal Assertion and invariant strings
- Track write-privileged accounts that can query time-series collections and review their access regularly
How to Mitigate CVE-2026-18695
Immediate Actions Required
- Inventory all MongoDB deployments and identify collections created with the timeseries option using a metaField
- Restrict write access on affected collections to only the service accounts that require it
- Apply the fixed MongoDB Server release referenced in SERVER-129460 once available for your deployment channel
- Ensure replica sets and sharded clusters have healthy secondaries so a single crash does not cause a full outage
Patch Information
Refer to the MongoDB Server Issue Tracker for the fixed versions and backport information for supported release branches. Apply the patched build for your MongoDB Server major version and restart the process during a maintenance window. Verify the fix by confirming the build version reported by db.version() matches the advisory.
Workarounds
- Revoke write privileges on time-series collections from users that do not require them, using role-based access control
- Route application queries through a controlled service layer that validates predicate structure before forwarding to MongoDB
- Disable or migrate non-critical time-series collections that use a metaField until the patch is deployed
- Configure automatic process supervision so mongod restarts quickly if a crash occurs, limiting outage duration
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
