CVE-2026-9748: MongoDB Denial of Service via Pipeline Signal Misuse

Technical summary

The $_internalConvertBucketIndexStats aggregation stage incorrectly invokes PauseExecution to skip documents when index stats conversion fails. PauseExecution is a TeeBuffer-internal protocol signal used exclusively to coordinate sub-pipelines within the $facet operator. When $_internalConvertBucketIndexStats precedes $facet in an aggregation pipeline, the upstream PauseExecution signal violates TeeBuffer's invariant assumptions, causing an assertion failure and mongod process termination.

Business impact

Authenticated attackers can crash MongoDB database instances by crafting specific aggregation pipeline queries, resulting in service unavailability. Recovery requires manual restart of the affected mongod process. In applications with limited MongoDB replicas or single-instance deployments, this creates immediate operational disruption. Multi-replica deployments may experience failover delays and temporary query unavailability.

Affected systems

MongoDB is affected. Organizations running MongoDB instances accessible to authenticated users should assess exposure. The vulnerability requires valid database credentials and the ability to execute aggregation pipeline queries, limiting the attack surface to authenticated application users and database administrators.

Exploitability

Exploitation requires valid MongoDB authentication credentials and the ability to submit custom aggregation pipeline queries. No remote unauthenticated exploitation is possible. The attack complexity is low once authentication is achieved—a specific pipeline structure triggers the crash deterministically. Exploitation does not require unusual system configurations or user interaction.

Remediation

Apply the security patch released by MongoDB for CVE-2026-9748. Verify the specific patched version number against MongoDB's official security advisory. Until patched, restrict aggregation pipeline query submission to highly trusted application accounts and monitor for unexpected pipeline structures in query logs.

Patch guidance

Consult MongoDB's official security advisory for CVE-2026-9748 to identify the patched version applicable to your MongoDB deployment (Community, Enterprise, or Atlas). Test the patch in a staging environment before production rollout to ensure compatibility with active application workloads and aggregation pipelines. Verify patch application by confirming the installed MongoDB version matches the advisory's recommended release.

Detection guidance

Monitor MongoDB logs for aggregation pipeline queries containing both $_internalConvertBucketIndexStats and $facet operators in proximity. Alert on any unexpected hard assertions or process crashes involving TeeBuffer during pipeline execution. In production environments, establish a baseline of legitimate aggregation patterns and flag deviations that combine internal stages with $facet operators.

Why prioritize this

CVE-2026-9748 carries a CVSS score of 6.5 (MEDIUM) reflecting authenticated-only access requirements and limited impact scope (denial of service only, no data confidentiality or integrity breach). However, the deterministic crash mechanism and ease of exploitation via crafted queries warrant relatively prompt patching in environments where untrusted users or developers have aggregation pipeline access. Organizations with strict query governance or read-only application roles may deprioritize relative to higher-severity issues.

Risk score, explained

The CVSS 3.1 score of 6.5 reflects: network-based attack vector (AV:N), low attack complexity (AC:L), requirement for low-level authentication privilege (PR:L), no user interaction (UI:N), unchanged scope (S:U), no confidentiality impact (C:N), no integrity impact (I:N), and high availability impact (A:H). The score appropriately captures the service disruption potential balanced against the authentication barrier and absence of data exposure.

Frequently asked questions