CVE-2026-15685

Executive Summary

CVE-2026-15685 is a remote denial-of-service vulnerability in Ollama's downloadBlob function caused by improper validation of array indices, allowing unauthenticated attackers to trigger out-of-bounds memory access. It affects Ollama installations exposed to the network, with a CVSS 7.5 High rating driven by network attack vector and availability impact. Exploitation leads to service crashes without requiring authentication or user interaction.

Technical Analysis

The flaw resides in the downloadBlob function where user-supplied data lacks bounds checking before array indexing. This results in memory access past the end of an allocated buffer, classified under CWE-129 (Improper Validation of Array Index). The vulnerability is triggered via crafted requests to the blob download endpoint, causing the process to fault when the index exceeds array limits. Attack surface centers on the HTTP API listener, typically bound to port 11434, where remote inputs flow directly into index calculations without sanitization or length validation. No authentication gates protect the path, and the defect allows repeated requests to sustain the denial-of-service state.

Exploitation Scenarios

1. Reconnaissance identifies Ollama instances via Shodan or direct port scans for TCP 11434 with "Ollama" banner responses. 2) The attacker crafts an HTTP POST or GET to the /api/blobs/download endpoint supplying an oversized or negative index value in the request body or query parameters that targets the downloadBlob array. 3) Weaponization sends the malformed payload in a loop, forcing the index validation failure and subsequent out-of-bounds read that crashes the worker thread. 4) Post-exploitation maintains the DoS by repeating requests to prevent service restart, resulting in persistent unavailability of the Ollama inference service.

Impact Assessment

Affected systems include any Ollama deployment reachable over the network, with blast radius limited to availability but capable of disrupting local LLM inference workloads in enterprise or research environments. The CVSS 7.5 score reflects full network exploitability (AV:N), low complexity (AC:L), no privileges (PR:N), and high availability impact (A:H) while confidentiality and integrity remain unaffected. EPSS of 0.00391 indicates low but non-zero exploitation probability.

Detection Guidance

Sigma Rule

title: Ollama downloadBlob Array Index DoS
id: 8f4c2a1b-9e3d-4f7a-8c2b-1d9e4f3a2b1c
status: experimental
description: Detects anomalous requests to Ollama blob endpoints with suspicious index parameters
logsource:
  category: webserver
  product: ollama
detection:
  selection:
    http.method: ['POST','GET']
    http.uri|contains: '/api/blobs'
    http.body|contains: 'index'
  condition: selection
fields:
  - src_ip
  - http.uri
level: high

YARA Rule

rule Ollama_downloadBlob_DoS {
    meta:
        description = "Detects patterns targeting Ollama downloadBlob array flaw"
    strings:
        $api = "/api/blobs/download"
        $idx = /index=[0-9]{5,}/
    condition:
        $api and $idx
}

Suricata IDS Rule

alert http any any -> any 11434 (msg:"Ollama downloadBlob index DoS attempt"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"/api/blobs"; http.request_body; content:"index"; classtype:attempted-dos; sid:202615685; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5s
  [http where url.path == "/api/blobs/download" and http.request.body.content like "*index*"]

Splunk SPL Query

index=web sourcetype=ollama_logs uri="/api/blobs*" method=POST OR method=GET 
| search body="*index*" 
| stats count by src_ip, uri 
| where count > 10

Wazuh OSSEC Rule

<group name="ollama,">
  <rule id="100001" level="10">
    <if_sid>100000</if_sid>
    <match>/api/blobs/download</match>
    <description>Ollama downloadBlob DoS probe detected</description>
  </rule>
</group>

Mitigation & Patch Guidance

References

https://nvd.nist.gov/vuln/detail/CVE-2026-15685 https://osv.dev/vulnerability/CVE-2026-15685 https://www.zerodayinitiative.com/advisories/ZDI-CAN-27277/ No known threat actor campaigns or in-the-wild exploitation recorded in CISA KEV.