CVE-2026-9754: Information Disclosure Vulnerability

CVE-2026-9754 Overview

CVE-2026-9754 is an information disclosure vulnerability affecting MongoDB Server. An authenticated user holding only the read role can issue a specially-crafted filemd5 command to read limited amounts of uninitialized stack memory from the server process. The flaw is tracked as [CWE-457] Use of Uninitialized Variable. Because the attack requires only network reachability and standard read privileges, any account that can query a database becomes a potential vector for harvesting residual memory contents.

Critical Impact
Authenticated low-privilege users can extract fragments of server-process stack memory over the network, potentially exposing secrets, query data, or pointers useful for further exploitation.

Affected Products

MongoDB Server (tracked via MongoDB issue SERVER-122207)
Deployments where users are granted the built-in read role
Sharded and replica set deployments exposing the filemd5 command

Discovery Timeline

2026-06-09 - CVE-2026-9754 published to NVD
2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-9754

Vulnerability Analysis

The filemd5 command computes an MD5 checksum over the chunks of a GridFS file. When invoked with crafted arguments, the server returns response fields populated from a stack buffer that was not fully initialized before use. The unread bytes leak back to the client inside command replies. Because MongoDB authenticates a session before the command dispatcher runs, the attacker must hold a valid account, but no administrative or write privileges are required. The exposure is confidentiality-focused: integrity and availability remain unaffected.

Root Cause

The defect maps to [CWE-457] Use of Uninitialized Variable. A code path in the filemd5 handler allocates a stack-resident structure or buffer and emits its contents in the command reply without first zeroing or fully populating every field. Compiler-dependent layout means the leaked bytes contain whatever residue the prior stack frame left behind, including pointers, fragments of prior query payloads, or transient secrets.

Attack Vector

Exploitation is remote and authenticated. An attacker with the read role connects to the MongoDB instance over its normal wire protocol port and issues repeated filemd5 commands with parameters that trigger the leaking response path. Each invocation returns a small, bounded slice of uninitialized stack memory. By iterating, the attacker can accumulate sufficient bytes to reconstruct sensitive material or to defeat memory-layout protections in preparation for chained attacks. See the MongoDB Server Issue Tracker entry SERVER-122207 for upstream tracking.

No public proof-of-concept is referenced in NVD, and verified exploit code is not available. The vulnerability mechanism is therefore described in prose only.

Detection Methods for CVE-2026-9754

Indicators of Compromise

Repeated filemd5 command invocations from a single authenticated session, especially against non-existent or unusual GridFS collections
Sessions issuing filemd5 without performing any associated GridFS file upload or chunk-write activity
Command replies of anomalous size or with unexpected binary fields returned to read-only accounts

Detection Strategies

Enable MongoDB audit logging and alert on filemd5 calls originating from accounts whose normal workflow does not include GridFS operations
Baseline per-user command frequency and flag spikes of filemd5 invocations from read-only roles
Correlate authentication events with subsequent low-privilege command bursts to surface credential abuse

Monitoring Recommendations

Forward MongoDB audit and diagnostic logs to a central analytics platform for retention and query
Track network egress volume from database hosts to client subnets to detect slow data-harvesting patterns
Review role assignments quarterly to confirm the read role is granted only where required

How to Mitigate CVE-2026-9754

Immediate Actions Required

Apply the MongoDB security release that addresses SERVER-122207 as soon as it is available for your deployment channel
Audit all database users and revoke the read role from accounts that do not require it
Restrict network reachability of MongoDB instances to known application hosts using firewall or security-group rules

Patch Information

Refer to the MongoDB Server Issue Tracker entry SERVER-122207 for fixed-version details and upgrade guidance. Operators should plan a rolling upgrade across replica set members and mongos routers to remediate the leak without downtime.

Workarounds

Restrict the filemd5 command via role-based access control by avoiding the built-in read role for clients that do not need GridFS
Place MongoDB behind a network proxy or service mesh that can enforce per-command policy and rate limits
Rotate any credentials, tokens, or keys that may have been resident in server memory if exploitation is suspected

bash

# Example: remove broad read access and grant a least-privilege custom role
use admin
db.createRole({
  role: "appReadNoGridFS",
  privileges: [
    { resource: { db: "app", collection: "" }, actions: [ "find" ] }
  ],
  roles: []
})
db.revokeRolesFromUser("appUser", [ { role: "read", db: "app" } ])
db.grantRolesToUser("appUser", [ { role: "appReadNoGridFS", db: "admin" } ])