CVE-2026-40530

Executive Summary

CVE-2026-40530 is a CRLF injection (improper neutralization of CRLF sequences, CWE-93) vulnerability in the User API of Synology DiskStation Manager (DSM). It allows a remote, authenticated user to read or write arbitrary files and to conduct denial-of-service attacks that manifest after the system is rebooted. The issue carries a CVSS v3.1 base score of 8.0 and affects DSM releases prior to 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Because NAS devices are frequently internet-exposed and are high-value data repositories, this vulnerability warrants urgent triage by any organization running an unpatched DSM instance.

Technical Analysis

CWE-93 describes a class of input-handling defects in which an application fails to strip or encode carriage-return (CR, \r, 0x0D) and line-feed (LF, \n, 0x0A) sequences from data that is later placed into a structured channel. The NVD description identifies the vulnerable component as the User API in Synology DSM. The vulnerability is triggered through the processing of user-supplied input that is incorporated into a context where CRLF sequences are semantically meaningful.

Per the supplied data, the concrete exploit primitive is arbitrary file read and write, plus a post-reboot denial of service. This is the classic downstream consequence of CRLF injection: because the CRLF pair terminates lines in many structured string formats and protocol streams, unneutralized CR and LF characters let an attacker break out of the field they control and inject additional structure. That injected structure can then be interpreted as a distinct record, header, path leg, or command by whichever subsystem consumes the string. The exact API parameter name, internal function, and file path involved are not disclosed in the NVD description or titles provided. No speculative names are asserted here.

The CVSS v3.1 vector AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H is consistent with this class:

• AV:N (Network) — the user-facing API is reachable over the network.
• AC:L (Low complexity) — no specialized conditions are required beyond valid credentials.
• PR:L (Low privileges) — any authenticated account, not only administrators, is sufficient.
• UI:R (User interaction required) — some user action is required on the target side (most plausibly an administrator viewing, importing, or otherwise processing the injected content).
• S:U (Unchanged) — the impact is scoped to the DSM host.
• C:H / I:H / A:H — full loss of confidentiality, integrity, and availability.

The fact that the DoS condition appears after a reboot is significant. It implies the attacker writes a persistent payload (configuration, cron-equivalent schedule entry, session file, or similar host-resident artifact) during initial access, and that payload is read and acted upon at boot time. This elevates the issue from a transient response-splitting bug to a persistent, host-level compromise.

The "User API" designation and the requirement for only low privileges point to an authenticated feature that names, references, or stores user-controlled strings. The CVSS UI:R flag indicates the injected content is not exploited at request time but at a point where a second party processes it.

Exploitation Scenarios

No public PoC, Nuclei template, or exploit code is available for CVE-2026-40530. The below chain is inferred at the vulnerability-class level from the NVD description and CVSS vector. No function names, parameters, or file paths are invented.

1. Reconnaissance and target identification. The attacker locates an internet- or internally exposed Synology DSM instance running a version below the fixed builds (7.2.1-69057-10 / 7.2.2-72806-7 / 7.3.2-86009-2). Discovery is trivial via search engines using attributes such as product:"Synology DiskStation Manager" or the DiskStation login page title. The attacker also needs an authenticated session, so credential stuffing, purchased credential lists, or prior social engineering may be used to obtain the low-privileged account required by PR:L.

2. Triggering the flaw. Once authenticated, the attacker submits input to the DSM User API that contains embedded CR and LF sequences. Because the application does not neutralize these sequences, the injected characters survive into the data stream that the API forwards downstream. The UI:R component means an administrative or privileged user must subsequently cause that stored/queued content to be processed for the injection to take effect.

3. Payload/execution at the class level. The injected CRLF sequences let the attacker terminate the legitimate line and append attacker-controlled structure — the exact injected value depends on the consuming format, which is not disclosed. This is the mechanism that yields the reported arbitrary file read/write capability: the injected structure redirects or extends an operation so that it operates on an attacker-chosen file path instead of the intended one, or writes attacker-supplied content into a file the attacker should not control. A further injected structure is persisted so that it is acted on at boot time, producing the described post-reboot denial of service.

4. Post-exploitation impact. A successful file write provides the attacker with integrity control over host-resident files (full I:H), while the read primitive exposes contents those files hold (full C:H). The attacker can persist across reboots and induce denial of service at boot, denying access to data and services on the NAS (full A:H). Because NAS devices commonly hold backups, credentials, and regulated data, successful exploitation is a direct data-loss and lateral-movement risk.

Impact Assessment

Business impact is high. An internet-exposed Synology appliance that stores backups, shared documents, or credential material, and that permits low-privileged authenticated accounts, is directly in scope. The PR:L rating means a compromised helpdesk account, a service account, or any ordinary user's credentials is sufficient — the attacker does not need administrative rights to start the chain.

Blast radius per CVSS subscores:

• Confidentiality: High — arbitrary file read exposes data beyond the user's authorization boundary.
• Integrity: High — arbitrary file write allows modification of host-resident files.
• Availability: High — a persistent denial-of-service condition triggered after reboot renders the device unavailable; because the condition survives a reboot, a simple restart will not clear it.
• Scope: Unchanged — impact is confined to the DSM host; though the host frequently serves as a hub for other business data.

The combination of C:H, I:H, and A:H with PR:L and AV:N is why the base score lands at 8.0. There is no CISA KEV listing and no EPSS score yet; exploitation has not been observed in the wild per the supplied feed, but a score of 8.0 with a trivial trigger and a persistent payload makes this a high-priority remediate-first item.

Detection Guidance

Sigma Rule

title: Potential CRLF Injection Against Synology DSM User API
id: 9b6f2c1e-4053-4a30-9f30-000040530001
status: experimental
description: Detects CRLF (%0d%0a / \r\n) sequences in requests targeting the Synology DSM User API, consistent with CVE-2026-40530.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-40530
author: valtersit.com
date: 2026-01-01
logsource:
  category: webserver
  product: synology
detection:
  selection_method:
    cs-method:
      - 'GET'
      - 'POST'
  selection_crlf:
    cs-uri-query|contains:
      - '%0d%0a'
      - '%0D%0A'
      - '\r\n'
  condition: selection_method and selection_crlf
falsepositives:
  - Legitimate applications that URL-encode newlines in query strings
level: high
tags:
  - attack.t1190
  - cve.2026.40530

YARA Rule

rule CVE_2026_40530_Synology_DSM_CRLF_Injection
{
    meta:
        description = "Detects CRLF injection markers in captured Synology DSM User API request artifacts (CVE-2026-40530)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-40530"
        date = "2026-01-01"
        cve = "CVE-2026-40530"
    strings:
        $api = "webapi" ascii wide nocase
        $crlf_url = "%0d%0a" ascii nocase
        $crlf_raw = "\r\n" ascii
        $crlf_dos = "Content-Length" ascii nocase
        $dsm = "DiskStation" ascii wide nocase
    condition:
        uint16(0) == 0x0a and filesize < 10KB and
        (($crlf_url or $crlf_raw) and ($api or $dsm or $crlf_dos))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-40530 Synology DSM User API CRLF Injection Attempt"; flow:established,to_server; http.method; content:"GET"; http.uri; content:"%0d%0a"; nocase; http.uri; content:"%0D%0A"; nocase; classtype:web-application-attack; reference:cve,2026-40530; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-40530; sid:202640530; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [network where network.protocol == "http" and
     http.request.method in ("GET", "POST") and
     stringContains(url.query, "%0d%0a")]
  [network where network.protocol == "http" and
     http.response.status_code in (200, 500) and
     stringContains(http.response.mime_type, "text")]

Splunk SPL Query

index=web sourcetype=access_combined
| eval uri_lower=lower(uri)
| where like(uri_lower, "%\%0d\%0a%") OR like(uri_lower, "%\%0D\%0A%") OR match(uri_lower, "(?i)webapi")
| stats count min(_time) as first_seen max(_time) as last_seen values(uri) as sample_uris by src_ip, host
| where count >= 1
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="web,synology,cve-2026-40530">
  <rule id="100530" level="12">
    <if_sid>31100</if_sid>
    <match>%0d%0a|%0D%0A</match>
    <description>CVE-2026-40530: Possible CRLF injection against Synology DSM User API</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>web_attack,pci_dss_6.5.1,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED per the supplied feed; no patch evidence from any tracked source and no vendor fix or workaround data was provided. The fixed versions referenced in the NVD description (7.2.1-69057-10, 7.2.2-72806-7, 7.3.2-86009-2) should be treated as the target upgrade level, but that reference comes from the description text rather than a confirmed advisory in the feed.

1. Immediate mitigations

   • Remove external exposure of the DSM management interface and User API. Place the device behind VPN or a zero-trust access broker; never publish DSM login pages directly to the internet.
   • Restrict authenticated access: eliminate or tightly scope shared, guest, and low-privileged accounts. The vulnerability is triggerable at PR:L, so reducing the population of valid credentials directly reduces risk.
   • Enable MFA and alert on credential-stuffing patterns against the login endpoint.
   • Monitor web logs for %0d%0a / %0D%0A sequences and unexpected file-operation activity.
   • Take offline backups that cannot be reached via the DSM User API file primitives, and verify restoration procedures.

2. Patch/upgrade steps

   • Upgrade DSM to 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 (or later) as referenced in the NVD description. Confirm the exact build on each branch before upgrading.
   • Because the DoS condition manifests after reboot, ensure devices are rebooted post-upgrade so any previously persisted malicious artifact is evaluated/cleared and the patched code path is active.
   • Verify the version string after upgrade and retain evidence for audit.

3. Long-term hardening

   • Treat all user-influenced strings as untrusted; ensure CR/LF are neutralized before they reach structured outputs, logs, headers, or file paths (CWE-93 remediation).
   • Deploy signatures (above) at the network and host layers and tune to environment baseline to limit false positives from legitimate encoded newlines.
   • Segment NAS management traffic onto a dedicated administrative VLAN.
   • Adopt a recurring patch cadence for NAS appliances and inventory all internet-reachable DSM instances.

References

• NVD — CVE-2026-40530: https://nvd.nist.gov/vuln/detail/CVE-2026-40530
• MITRE CWE-93 — Improper Neutralization of CRLF Sequences: https://cwe.mitre.org/data/definitions/93.html
• FIRST CVSS v3.1 specification (vector used above): https://www.first.org/cvss/v3.1/specification-document
• CISA Known Exploited Vulnerabilities catalog (CVE-2026-40530 not listed as of the supplied feed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS — not yet scored for this CVE per the supplied feed: https://www.first.org/epss

Threat actor campaign data: none available in the supplied feed. No exploit-in-the-wild evidence was provided; CISA KEV status is NO and no PoC/template exists. Any future vendor advisory from Synology should be monitored directly, as no vendor advisory URL was present in this dataset.