CVE-2026-88406

Executive Summary

CVE-2026-88406 is a stack overflow vulnerability in FalkorDB, a graph database implemented as a Redis module, affecting versions 4.20.1 through 4.20.4. The flaw resides in the _ValidateUnion_Clauses function within /ast/ast_validations.c and is triggered when processing a crafted input, allowing a remote unauthenticated attacker to crash the service and cause a Denial of Service. With a CVSS score of 7.5 (High) and no available patch, organizations running affected FalkorDB deployments should treat this as an active availability risk pending vendor remediation.

Technical Analysis

FalkorDB is a Redis module that provides graph database capabilities, typically exposed over the Redis protocol on TCP port 6379 (or over a WebSocket/HTTP interface in managed or cloud deployments). The vulnerability is classified as a stack overflow (CWE-121) occurring inside the function _ValidateUnion_Clauses, located in the source file /ast/ast_validations.c. This file is part of FalkorDB's query parsing and abstract syntax tree (AST) validation pipeline.

Based on the function name and file path, the flaw is reached during the query validation phase — specifically when the parser/validator processes a query containing UNION clauses (the Cypher/openCypher query language supports UNION and UNION ALL operations to combine result sets). The _ValidateUnion_Clauses routine is responsible for validating the structural correctness of these UNION clauses before query execution.

A stack overflow in this context most plausibly arises from unbounded or insufficiently bounded recursion, or from a fixed-size stack buffer being populated with attacker-controlled input derived from the crafted query. Because the description explicitly identifies the issue as a stack overflow in a validation function, the triggering condition is a malformed or deeply nested/oversized UNION-bearing query that exceeds the safe stack or buffer limits during validation — before the query is ever executed.

The attack surface is the query interface exposed by the FalkorDB module. Any client capable of submitting a query string to the affected service (via the Redis wire protocol or an exposed HTTP/WebSocket query endpoint) can reach this code path. The CVSS vector AV:N/AC:L/PR:N/UI:N confirms that exploitation is network-reachable, low-complexity, requires no privileges, and requires no user interaction — meaning the vulnerable query interface must be treated as untrusted input surface.

No CWE was supplied by the CNA, but the vulnerability class (stack overflow via crafted input in a validation routine) maps cleanly to CWE-121 (Stack-based Buffer Overflow) and, where recursion is the root cause, CWE-674 (Uncontrolled Recursion). CWE-121 is used here as the best-supported classification given the explicit "stack overflow" wording. The NVD description does not disclose the exact payload shape, buffer size, or recursion depth required, and no patch diff is available to confirm the precise mechanism — so the mechanism above is inferred strictly from the vulnerability class and the named function's evident role in UNION-clause validation.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for CVE-2026-88406. The attack chain below is described at the vulnerability-class level and is inferred from the NVD description; no specific payload bytes, buffer sizes, or query grammar are asserted because they are not disclosed.

1. Reconnaissance / target identification: An attacker scans for exposed FalkorDB services. FalkorDB speaks the Redis protocol and is commonly found on port 6379; managed/cloud deployments may also expose an HTTP or WebSocket query interface. Shodan queries such as product:"FalkorDB", port:6379, or HTTP banner matches for "FalkorDB" can identify candidate targets. Version disclosure (for example via Redis INFO or module version responses) helps the attacker confirm whether the target falls in the 4.20.1–4.20.4 range.

2. Triggering the flaw: The attacker submits a crafted query that contains UNION clauses designed to drive the _ValidateUnion_Clauses validation routine in /ast/ast_validations.c past its safe stack or buffer bound. Because the description specifies a stack overflow, the trigger is expected to involve excessive nesting/size or malformed UNION structure that the validator does not adequately bound. The exact construction is not public.

3. Execution method: The malicious query is delivered through the standard query interface of the module — i.e., by issuing the query to the FalkorDB service over its network protocol. No authentication is required (PR:N) and no user interaction is needed (UI:N), so a single crafted request is sufficient in principle to reach the vulnerable code path.

4. Post-exploitation impact: Successful triggering corrupts the stack in the validation routine, causing the FalkorDB process (and, if co-hosted, the underlying Redis instance) to crash. The immediate result is Denial of Service — loss of availability of the graph database for legitimate clients. Because CVSS lists Integrity and Confidentiality impacts as None (I:N, C:N), the description does not support claims of code execution, data theft, or data modification; the impact is strictly to availability. Repeated or automated requests can sustain the outage.

Impact Assessment

The CVSS 3.1 base score is 7.5 (High), with the vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Interpreting each metric:

• Attack Vector (N — Network): exploitable remotely over the network, no local access required.
• Attack Complexity (L — Low): no special conditions or race windows are needed; a crafted input reliably triggers the flaw.
• Privileges Required (N — None): the attacker needs no credentials.
• User Interaction (N — None): no victim action is required.
• Scope (U — Unchanged): the impact is confined to the vulnerable component's security authority.
• Confidentiality (N) / Integrity (N): no data exposure or tampering is claimed.
• Availability (H — High): the vulnerability fully degrades availability of the service.

Business impact centers on service availability. FalkorDB is frequently used as a graph/relationship engine for recommendation systems, fraud graphs, knowledge graphs, and real-time analytics. A crash of the database can halt dependent applications, interrupting query serving and any workloads that rely on graph traversal. Because no privileges are required, any network-reachable deployment is at risk, and the blast radius scales with how many services depend on that FalkorDB instance. An attacker with network access can repeatedly crash the service to maintain an outage (a sustained DoS condition), potentially masking other malicious activity.

Given that no patch exists (patch confidence 0/4), the window of exposure remains open for all deployments in the 4.20.1–4.20.4 range until the vendor releases a fix. No EPSS score is available (not yet scored by FIRST.org), and CISA KEV does not list this CVE — there is no confirmed exploit-in-the-wild evidence at this time.

Detection Guidance

Sigma Rule

title: FalkorDB UNION Clause Stack Overflow DoS Attempt
id: 9f3b7c21-5a4e-4d2b-8c11-falkor88406aa
status: experimental
description: Detects suspicious FalkorDB process crashes correlated with incoming queries containing UNION clauses (CVE-2026-88406).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-88406
author: valtersit.com
date: 2026/01/01
logsource:
  product: linux
  service: application
detection:
  selection_query:
    message|contains:
      - 'UNION'
  selection_crash:
    message|contains:
      - '_ValidateUnion_Clauses'
      - 'ast_validations.c'
      - 'stack overflow'
  condition: selection_query or selection_crash
falsepositives:
  - Legitimate graph queries using UNION clauses
  - Normal application crashes unrelated to the validator. It is recommended to baseline crash frequency before alerting.
level: medium
tags:
  - attack.t1499
  - cve.2026.88406

YARA Rule

rule CVE_2026_88406_FalkorDB_UnionValidation
{
    meta:
        description = "Identifies FalkorDB binaries containing the vulnerable _ValidateUnion_Clauses validation routine associated with CVE-2026-88406"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-88406"
        date = "2026-01-01"
        severity = "high"

    strings:
        $fn  = "_ValidateUnion_Clauses" ascii
        $src = "ast_validations.c" ascii
        $uni = "UNION" ascii

    condition:
        uint32(0) == 0x464c457f and filesize < 200MB and $fn and $src
}

Suricata IDS Rule

alert tcp any any -> any 6379 (msg:"CVE-2026-88406 FalkorDB UNION Clause Stack Overflow DoS Attempt"; flow:established,to_server; content:"UNION"; nocase; content:"UNION"; nocase; distance:0; within:2048; threshold: type both, track by_src, count 20, seconds 60; classtype:attempted-dos; reference:cve,2026-88406; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-88406; sid:8840601; rev:1;)

Elastic Detection

process where host.os.type == "linux" and event.type == "end" and
  process.name : ("falkordb", "redis-server", "redis-stack-server") and
  process.exit_code != 0 and
  process.args : "*UNION*"

Splunk SPL Query

index=app_logs (sourcetype=falkordb OR sourcetype=redis OR source="*falkordb*")
| search "UNION" OR "_ValidateUnion_Clauses" OR "ast_validations.c" OR "stack overflow"
| stats count min(_time) as first_seen max(_time) as last_seen values(host) as hosts by src_ip
| where count > 5
| convert ctime(first_seen) ctime(last_seen)

Wazuh OSSEC Rule

<group name="falkordb,cve-2026-88406,">
  <rule id="884061" level="10">
    <decoded_as>falkordb</decoded_as>
    <match>UNION</match>
    <description>CVE-2026-88406: FalkorDB query containing UNION clause - possible _ValidateUnion_Clauses stack overflow DoS attempt</description>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>

  <rule id="884062" level="12">
    <if_sid>884061</if_sid>
    <match>stack overflow</match>
    <description>CVE-2026-88406: FalkorDB stack overflow in ast_validations.c detected - service crash likely</description>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). There is no vendor-supplied fix, workaround, or fixed version recorded for CVE-2026-88406 at this time. The guidance below is compensating control based.

1. Immediate mitigations:

• Restrict network access to the FalkorDB/Redis query interface. Bind the service to localhost or a private interface and require firewall allowlists so that only trusted application hosts can reach port 6379 (or the module's HTTP/WebSocket endpoint).
• Enforce authentication and access control on the Redis/FalkorDB layer (requirepass/ACLs) and disable exposure of the module to untrusted networks. Note: this vulnerability requires no privileges, so authentication alone does not eliminate risk if a query-capable interface is reachable.
• Place rate limiting and query-size/query-complexity limits at the network boundary or via a proxy so that oversized or excessively nested UNION-bearing queries from single sources are throttled.
• Monitor for and alert on process crashes of the FalkorDB/Redis process, and treat repeated crashes as a potential DoS campaign.
• If the deployment does not currently need graph queries exposed externally, disable or firewall the module's query surface entirely.

2. Patch/upgrade steps:

• No fixed release is available. Track the FalkorDB project's release notes and security advisories; upgrade to the first release that confirms remediation of _ValidateUnion_Clauses once published.
• Do not assume a specific version is safe without vendor confirmation, since affected versions are recorded only as 4.20.1 through 4.20.4 and no patch source has been verified.
• Maintain a change-control step to validate any candidate fix in a staging environment before production rollout.

3. Long-term hardening:

• Adopt a least-exposure architecture for data stores: keep graph/Redis instances off the public internet and behind application-tier mediators that validate and normalize query input.
• Implement input validation and query-length/nesting limits in the calling application so that pathological queries are rejected before reaching the database.
• Deploy continuous availability monitoring and crash telemetry (e.g., systemd restart tracking) so sustained DoS conditions are detected quickly.
• Subscribe to the FalkorDB security advisory channel and NVD for real-time patch notifications, and maintain an SBOM so affected module versions can be located rapidly.

References

• NVD — CVE-2026-88406: https://nvd.nist.gov/vuln/detail/CVE-2026-88406
• FalkorDB project (vendor product documentation and release notes): https://www.falkordb.com/
• FalkorDB source repository (/ast/ast_validations.c resides in the project's AST validation code): https://github.com/FalkorDB/FalkorDB
• CWE-121 — Stack-based Buffer Overflow: https://cwe.mitre.org/data/definitions/121.html
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-88406 not listed as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence are associated with CVE-2026-88406 based on the data provided. No public PoC, exploit code, or Nuclei template exists at this time, and the CVE is not in CISA KEV.