CVE-2021-26281 — Threat Intelligence Report

Executive Summary

CVE-2021-26281 is an information-disclosure vulnerability in the Alarm clock module shipped on Vivo mobile devices. The NVD description states that "some parameters of the alarm clock module are improperly stored, leaking some sensitive information" — meaning the affected module persists configuration parameters in an insecure manner that allows sensitive data to be read by unauthorized parties. The vulnerability is rated medium severity with a CVSS base score of 5.5, carries a low EPSS score (0.00177), and is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog; no public exploit code or in-the-wild usage has been reported to date.

Technical Analysis

The CNA title classifies this issue as an "Information disclosure vulnerability in Alarm clock module," and the NVD description provides only a high-level mechanism: improper storage of specific parameters within the alarm clock feature leads to leakage of sensitive information. No specific function names, class names, file paths, API endpoints, or parameter names are disclosed in the available CVE data, so the analysis below describes the vulnerability at the class level.

The vulnerability belongs to the insecure storage of sensitive information class. On mobile platforms, application-generated data — such as alarm schedules, labels, ringtone settings, snooze counts, or user-associated metadata — is normally persisted within the application's private sandbox directory, which is protected by the OS permission model. If the alarm clock module writes one or more parameters to storage locations that are world-readable (e.g., files created with permissive POSIX modes such as 0644/0666), stored in cleartext on external/shared storage, or exported via backup channels, any other local process on the device — including a malicious third-party application with ordinary permissions — can read the data without requiring root privileges or explicit user consent.

Key observations from the supplied intelligence data:

• CWE: Not assigned (N/A). The closest standard classification is CWE-922 (Insecure Storage of Sensitive Information), but this is an inference by the analyst, not an official mapping.
• CVSS score: 5.5 (medium) with no vector published. A base score of 5.5 is consistent with a low-complexity local attack that requires some form of local access, produces a limited confidentiality impact, and does not affect integrity or availability. The absence of the vector string means the individual CVSS sub-scores (AV, AC, PR, UI, C, I, A) cannot be decomposed from authoritative data.
• EPSS: 0.00177, placing this CVE in the 7.5th percentile of exploited CVEs — indicating a very low current probability of exploitation in the wild.
• CISA KEV: Not listed; no federal-mandated remediation timeline applies.
• PoC / Exploit: Unknown. No public PoC, exploit code, or Nuclei template exists.

The attack surface is local to the device. Since the flaw lives in an on-device module rather than a network service, it is not remotely exploitable over the Internet in a direct fashion. The relevant threat actors are (a) an attacker who can install a malicious application on the victim's Vivo device (e.g., via social engineering, sideloading, or a malicious app store), or (b) an attacker with temporary physical access to an unlocked device. The sensitive information exposed depends entirely on what the alarm clock module stores; the NVD description does not enumerate the specific data fields, and insufficient technical detail is available to name them.

Exploitation Scenarios

No public PoC is available, and no specific implementation details (file paths, class names, parameter names) are disclosed in the CVE description. The attack chain below is therefore inferred from the vulnerability class (insecure storage of sensitive information) and must be treated as an analyst reconstruction rather than a confirmed exploit procedure.

Step 1 — Reconnaissance / Target Identification. The attacker identifies a target Vivo smartphone running a firmware version that includes the vulnerable Alarm clock module. Because no affected version range was published, targeting is broad: any Vivo device that has not received the CNA-claimed patch is a candidate. From a local perspective, the attacker determines whether the device permits sideloading of applications from unknown sources, or whether the attacker has any pre-existing foothold (e.g., a trojanized application already installed).

Step 2 — Triggering the Flaw. The attacker relies on the vulnerable module's improper storage behavior. Instead of exploiting a memory corruption or injection bug, the attacker simply needs the alarm clock module to have persisted its parameters at least once — a normal occurrence whenever a user creates, edits, or uses an alarm. The trigger is therefore passive: the vulnerable module writes sensitive parameters to an insecure location during routine user interaction.

Step 3 — Payload / Execution Method. At the vulnerability-class level, the attacker runs a benign-looking malicious application (or abuses a legitimate app with broad file-access privileges) that enumerates the device's file system for the alarm clock module's data directory and reads the improperly stored files. If the files use permissive permissions or reside outside the app sandbox, no privilege escalation is required. The attacker then exfiltrates the parsed contents — potentially including alarm-related metadata, free-text labels, or other sensitive parameters — to a remote server, or uses them locally to support further attacks.

Step 4 — Post-Exploitation Impact. The disclosed parameters can reveal information about the device owner's schedule, habits, geographic context (if alarms are geo-tagged), or any personal data embedded in alarm labels. This information can feed social-engineering campaigns, corroborate other stolen data, or, if the stored parameters include account-associated tokens or identifiers, enable credential-based follow-on attacks. Because the disclosure is read-only and confidentiality-focused, the attacker does not gain code execution through this CVE alone; additional vulnerabilities would be required for device takeover.

Important caveat: the above chain is a class-level inference. The specific file names, storage locations, and parameter content are not disclosed in the authoritative CVE data and must not be assumed for detection purposes without vendor confirmation.

Impact Assessment

• Business impact: For enterprise environments that issue Vivo handsets to employees, this vulnerability exposes a local confidentiality risk. A malicious application installed on an employee's device could silently harvest alarm clock parameters, which may contain personal or operational insights (work schedules, travel patterns, sensitive notes stored as alarm labels).
• Affected systems: Vivo smartphones running versions of the Alarm clock module predating the CNA-claimed (but unconfirmed) fix. The entire Vivo mobile product line should be considered potentially affected until a vendor advisory with version ranges is published.
• Blast radius: Single-device, local scope. The flaw does not expose a network service, does not permit remote exploitation, and does not affect adjacent devices. The confidentiality impact is bounded by whatever data the alarm clock module stores; integrity and availability are not impacted.
• CVSS context: The 5.5 base score without a published vector indicates medium severity. Combined with the low EPSS (0.00177) and absence of known exploits, the immediate real-world risk is low-to-moderate, but the vulnerability remains relevant for supply-chain and mobile-hardening assessments because it demonstrates weak storage hygiene by a first-party device vendor.

Detection Guidance

Detection for this vulnerability is challenging because it is a local storage flaw rather than a network-visible attack. The rules below target the system-activity artifacts that would accompany exploitation: file creation/access events on alarm clock data files, APK code patterns associated with world-readable storage, and network exfiltration of alarm-derived data. All rules are heuristic and should be tuned to the organization's baseline.

Sigma Rule

Sigma rules are log-focused; for Android/Vivo devices, the most practical collection point is a mobile device management (MDM) telemetry pipeline that records file and process activity. The rule below detects creation or modification of alarm-clock data files with world-readable permissions.

title: Suspicious Access to Alarm Clock Module Data
id: 5a8bf2e4-9c1d-4e6a-8f3b-7c2d5a9e1b44
status: experimental
description: Detects processes creating or modifying alarm clock data files with world-readable permissions, potentially indicating exploitation of CVE-2021-26281 (improper storage of sensitive parameters).
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2021-26281
author: Valtersit Threat Intelligence
date: 2025/05/13
logsource:
    product: linux
    service: file_event
detection:
    selection:
        TargetFilename|contains:
            - 'alarm'
            - 'clock'
            - 'AlarmClock'
        FilePermissions|contains:
            - '0644'
            - '0666'
            - 'rw-r--r--'
            - 'rw-rw-rw-'
    filter:
        TargetFilename|startswith: '/data/system/'
    condition: selection and not filter
level: medium
falsepositives:
    - Legitimate alarm clock writes with permissive permissions on older firmware
    - Backup or restore processes touching alarm data
tags:
    - attack.collection
    - attack.t1005
    - cve.2021-26281

YARA Rule

YARA is suited for static inspection of APK/DEX artifacts. This rule flags suspicious code patterns where alarm-related strings coexist with Android's world-readable file modes, a hallmark of the insecure-storage bug class behind CVE-2021-26281.

rule CVE_2021_26281_Alarm_Clock_Insecure_Storage
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects APK/dex code patterns indicating world-readable storage of alarm clock parameters (CVE-2021-26281 class behavior)."
        cve = "CVE-2021-26281"
        date = "2025-05-13"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2021-26281"
        threat_level = "medium"
    strings:
        $s1 = "MODE_WORLD_READABLE" ascii wide
        $s2 = "alarm" ascii wide nocase
        $s3 = "clock" ascii wide nocase
        $s4 = "getSharedPreferences" ascii wide
        $s5 = "openFileOutput" ascii wide
        $s6 = "setReadable" ascii wide
    condition:
        uint32be(0) == 0x6465780A or filesize < 50MB
        and $s1
        and ($s2 or $s3)
        and (2 of ($s4, $s5, $s6))
}

Suricata IDS Rule

Network detection has limited applicability to a local storage flaw. The most defensible Suricata rule detects the exfiltration phase: a client uploading alarm-clock-derived data over HTTP/SMTP. This rule is heuristic and adjusted for a high false-positive rate.

alert http any any -> $EXTERNAL_NET any (
    msg:"CVE-2021-26281 Potential Exfiltration of Alarm Clock Data";
    flow:to_server,established;
    content:"alarm", nocase;
    content:"clock", nocase;
    pcre:"/(alarm|clock|wakeup|snooze)/i";
    http.uri;
    classtype:attempted-information-leak;
    reference:cve,2021-26281;
    reference:url,https://nvd.nist.gov/vuln/detail/CVE-2021-26281;
    sid:99990001;
    rev:1;
    metadata:attack_target_client_endpoint;
    metadata:created_at 2025-05-13;
)

Elastic Detection

Elastic EQL query for file events associated with alarm clock data that obtain world-readable permissions — the file-level signature of insecure storage exploitation.

file where (file.path : "*alarm*" or file.path : "*clock*" or file.path : "*AlarmClock*")
    and (file.permissions : ("0644", "0666", "0777", "rw-r--r--", "rw-rw-rw-") or file.mode : "world_readable")

Splunk SPL Query

Splunk search for endpoint/file-monitoring logs covering device storage. Adjust index and event source to match the MDM or EDR feed used in the environment.

index=endpoint sourcetype=file_monitoring
    (file_path=*alarm* OR file_path=*clock* OR file_path=*AlarmClock*)
    (file_permission=0644 OR file_permission=0666 OR file_permission=0777 OR file_mode=world_readable)
    NOT process_name= system_server
    | stats count min(_time) max(_time) by host, file_path, file_permission, process_name
    | sort - count

Wazuh OSSEC Rule

Wazuh FIM (syscheck) rule to monitor the alarm clock module's data directory for suspicious permission changes or unexpected access (class-level path wildcard). Combined with a full_log alert on relevant local log lines.

<group name="cve-2021-26281,information_disclosure,">
    <rule id="100262" level="10" frequency="2" timeframe="300">
        <if_sid>550,553,554</if_sid>
        <field name="file">\.*alarm.*</field>
        <description>CVE-2021-26281: Suspicious access to alarm clock module storage (potential information disclosure).</description>
        <group>information_disclosure,</group>
        <mitre>
            <id>T1005</id>
        </mitre>
    </rule>

    <rule id="100263" level="7">
        <if_sid>558</if_sid>
        <field name="file">\.*AlarmClock.*</field>
        <field name="mode" type="pcre2">(rw-r--r--|rw-rw-rw-)</field>
        <description>CVE-2021-26281: Alarm clock data file with world-readable permissions detected.</description>
        <group>information_disclosure,</group>
        <mitre>
            <id>T1005</id>
        </mitre>
    </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 2/4 (CLAIMED — single source claims patch, not independently confirmed). The only evidence of a fix is the CNA's assertion that the vulnerability has been patched. No firmware version, build number, OTA identifier, or security-bulletin reference was provided, and the affected-version list is empty. Until Vivo publishes an official advisory, organizations must consider all Vivo devices potentially vulnerable.

1) Immediate mitigations (no vendor fix confirmed):

• Disable installation of applications from unknown sources on all managed Vivo devices (Settings > Security > Install unknown apps).
• Restrict third-party app installs via MDM policy; allow only vendor-approved app stores.
• Enforce screen lock (PIN/biometric) and disable USB debugging in production use to reduce physical-access risk.
• Audit installed applications for suspicious permissions (e.g., broad storage/file access) using mobile threat defense (MTD) tooling.
• Advise users not to store sensitive free-text labels in the alarm clock app.

2) Patch / upgrade steps:

• Contact Vivo enterprise support or monitor Vivo's official security advisory channel for the patch associated with CVE-2021-26281.
• When the fixed firmware is published, verify the exact build number and apply it via OTA or MDM-controlled rollout.
• After patching, confirm the alarm clock module no longer creates world-readable data files by re-running the file-integrity checks from the Detection Guidance section.
• If no patch materializes within a reasonable period, weigh device replacement or third-party alarm applications as compensating controls for high-value users.

3) Long-term hardening:

• Add CVE-2021-26281 to the organization's mobile vulnerability scoring and vendor-risk assessment (the empty affected-version list and unverified patch are themselves supply-chain red flags).
• Require first-party mobile vendors to publish CWE mappings and patch version identifiers in future CNAs (the absence of CWE and vector data impairs risk scoring).
• Reference OWASP Mobile Application Security Verification Standard (MASVS) storage requirements when evaluating firmware hardening.
• Continuously monitor EPSS and CISA KEV for changes; revisiting this CVE's priority if the EPSS percentile rises above 20% or CISA adds it to KEV.

References

1. NVD entry — https://nvd.nist.gov/vuln/detail/CVE-2021-26281
2. CVE.org Record — https://www.cve.org/CVERecord?id=CVE-2021-26281
3. MITRE CVE List — https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-26281
4. CISA Known Exploited Vulnerabilities Catalog — https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE-2021-26281: not listed)

Threat-actor campaign notes: No known exploit-in-the-wild activity, ransomware adoption, or APT usage has been associated with CVE-2021-26281. No vendor advisory URL or researcher analysis was included in the authoritative CVE data; the patch claim originates solely from the CNA and remains unverified. Assessments should be updated when Vivo publishes an official advisory or when independent researchers confirm the patch and disclose the affected firmware range.