CVE-2024-23083

Executive Summary

CVE-2024-23083 is a disputed NullPointerException (NPE) vulnerability reported in Time4J Base v5.9.3, a Java date/time calculation library. The flaw is triggered through the net.time4j.format.internal.FormatUtils::useDefaultWeekmodel(Locale) component during locale-specific week model initialization. Multiple third parties dispute the finding, suggesting the submission may have originated from a tool insufficiently robust for vulnerability identification. The reported CVSS score is 5.3 (Medium), though exploitation requires a non-standard locale configuration and successful reproduction remains unverified. Enterprises using Time4J for calendar, chronology, or formatting operations should monitor this finding but prioritize validated patches from the vendor.

Technical Analysis

The vulnerability is reported to manifest as a NullPointerException in the useDefaultWeekmodel(Locale) method of the net.time4j.format.internal.FormatUtils class. This method is responsible for resolving the default week model (defining week numbering rules such as first day-of-week and minimal days in first week) based on the provided Locale object.

Mechanism: The NPE likely occurs when the method receives a locale for which the underlying java.util.Calendar or internal locale-data provider returns a null reference during week-model property lookup. In Java's globalization framework, locales with incomplete CLDR (Common Locale Data Repository) data can trigger null returns when querying week-related attributes such as Calendar.MIN_DAYS_IN_FIRST_WEEK or Calendar.DAY_OF_WEEK. Time4J's default week model resolution may not adequately null-check these values before dereferencing.

Attack Surface: The attack surface is local — a client-side deserialization or application logic path that passes attacker-controllable locale data to Time4J formatting or parsing APIs. Example entry points:

• REST endpoints that accept Locale parameters (e.g., Accept-Language headers, locale query strings)
• Batch jobs processing locale-specific date inputs
• Applications that serialize/deserialize locale metadata from untrusted sources

Code-Level Explanation (reconstructed):

// net.time4j.format.internal.FormatUtils
public static Weekmodel useDefaultWeekmodel(Locale locale) {
    Calendar calendar = Calendar.getInstance(locale);
    int firstDay = calendar.getFirstDayOfWeek();        // may throw NPE if calendar is null
    int minDays = calendar.getMinimalDaysInFirstWeek(); // unreachable if above fails
    return Weekmodel.of(firstDay, minDays);
}

If Calendar.getInstance(locale) returns null for a malformed or unsupported locale, the subsequent method calls dereference a null object, throwing NullPointerException. This aligns with the reported component path.

Dispute Context: The submission lacks a reproducible proof-of-concept, and the NPE may stem from a caller passing a null locale directly rather than an inherent library defect. Java's own Calendar.getInstance(Locale) does not accept null and would throw NullPointerException as well — a behavior consistent with standard Java semantics rather than a Time4J-specific vulnerability.

Exploitation Scenarios

Despite the disputed nature, if an attacker can trigger the NPE in a vulnerable context, the following attack chain applies:

1. Reconnaissance/Identification: The attacker identifies a Java application using Time4J 5.9.3. They probe endpoint behavior by sending requests with locale variations such as ?locale=xx_YY or manipulating Accept-Language headers to include exotic locale tags (e.g., de_DE_#Latn or sr_Latn_RS).

2. Trigger/Weaponization: The attacker sends a request with a carefully crafted locale that triggers the null-dereference path. Example payloads:

   • GET /api/format?locale=zz_ZZ&date=2024-12-31 (unknown locale)
   • GET /api/parse?locale=@proleptic (malformed locale tag)
   • HTTP header: Accept-Language: en_US;q=0.8, xx_YY;q=0.1

3. Execution/Payload: The application passes the attacker-controlled locale to a Time4J formatting call such as ChronoFormatter.ofPattern("MM/dd/yyyy", Locale.fromString(localeParam)).format(date). The useDefaultWeekmodel method throws an uncaught NullPointerException.

4. Post-Exploitation Impact: Exploitation results in:

   • Denial of Service (DoS): Repeated requests targeting the vulnerable endpoint can exhaust worker threads, degrade application performance, or crash the JVM if the exception propagates to an unhealthy state.
   • Information Disclosure (potential): In debug-enabled environments, the stack trace may leak internal class names and file paths useful for further targeted attacks.
   • Data Integrity (low): If the NPE occurs during a transactional date-processing pipeline, partial writes may occur if exception handling is insufficient.

Note: There is no evidence of remote code execution, privilege escalation, or data exfiltration via this vulnerability.

Impact Assessment

CVSS Score: 5.3 (Medium). With an unknown vector, the practical severity depends on exposure. The CIA triad impact per the score's typical allocation suggests:

• Confidentiality: Low (stack traces may leak minimal system details)
• Integrity: Low (no data corruption expected)
• Availability: Medium (Denial of Service via exception-triggering locales)

Business Impact:

• Affected Systems: Java applications using Time4J 5.9.3 (or earlier versions without the claimed fix) for date/time formatting, parsing, or calendar arithmetic with user-supplied locale inputs.
• Blast Radius: Restricted to applications that accept external locale parameters. Backend services with hardcoded locales are not exposed.
• Financial Loss: Low — DoS attacks may require sustained request volume, and the disputed nature minimizes urgency.

CWE: Unknown — the closest classification would be CWE-476 (NULL Pointer Dereference), but this has not been formally assigned.

Detection Guidance

Sigma Rule

title: Time4J NullPointerException from useDefaultWeekmodel
id: 3c7b5f9a-1e4a-4f6c-9b2e-8d0a1f3c5e7d
status: experimental
description: Detects Java application logs containing NullPointerException referencing Time4J FormatUtils useDefaultWeekmodel
logsource:
  category: application
  product: java
detection:
  selection:
    message|contains:
      - 'NullPointerException'
      - 'net.time4j.format.internal.FormatUtils'
      - 'useDefaultWeekmodel'
  condition: selection
level: medium
fields:
  - src_ip
  - user_agent
  - request_path
  - message
falsepositives:
  - Legitimate null locale passed by developer
  - Other NPEs in the same class
tags:
  - attack.t1499
  - cve.2024.23083

YARA Rule

rule CVE_2024_23083_Time4J_NPE
{
    meta:
        author = "ValterSIT Threat Intel"
        date = "2024-02-12"
        description = "Detects Time4J NullPointerException strings in logs or crash dumps"
        cve = "CVE-2024-23083"
        severity = "medium"
    strings:
        $s1 = "net.time4j.format.internal.FormatUtils" ascii wide
        $s2 = "useDefaultWeekmodel" ascii wide
        $s3 = "NullPointerException" ascii wide
        $s4 = "Weekmodel.of" ascii wide
    condition:
        (uint16(0) == 0x5A4D or uint16(0) == 0x4D5A) and 2 of them and $s1 and $s2
        or (filesize < 10MB and $s1 and $s2 and $s3)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2024-23083 Time4J useDefaultWeekmodel locale attempt"; flow:established,to_server; content:"locale="; nocase; pcre:"/locale=[^\&\"'\s]{2,}/"; content:"format"; nocase; pcre:"/\b(?:format|parse|display)\b/i"; classtype:attempted-dos; sid:2024230830; rev:1; reference:cve,2024-23083;)

Elastic Detection

sequence by process.name
  [any where process.name == "java" and
   stringContains(message, "NullPointerException") and
   stringContains(message, "net.time4j.format.internal.FormatUtils") and
   stringContains(message, "useDefaultWeekmodel")] with maxspan=5m

Splunk SPL Query

index=java sourcetype=java_stacktrace
  "NullPointerException" "net.time4j.format.internal.FormatUtils" "useDefaultWeekmodel"
  | table _time host src_ip user_agent request_path
  | stats count by host src_ip user_agent request_path
  | where count > 5

Wazuh OSSEC Rule

<group name="time4j_npe,">
  <rule id="123456" level="10">
    <decoded>java</decoded>
    <match>NullPointerException</match>
    <match>net.time4j.format.internal.FormatUtils</match>
    <match>useDefaultWeekmodel</match>
    <description>Time4J CVE-2024-23083 null pointer in useDefaultWeekmodel</description>
    <group>attack.dos,</group>
    <mitre>
      <id>T1499</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED — single source claims patch (not independently confirmed)).

1. Immediate Mitigations (if unpatched):

   • Validate all locale inputs server-side. Reject locale strings not matching ^[a-zA-Z]{2}(-[a-zA-Z]{2})?$ or using a strict allowlist.
   • Wrap Time4J formatting calls in try-catch blocks that handle NullPointerException gracefully, returning HTTP 400 responses.
   • Set unit tests to run Time4J operations with locales like zz_ZZ to verify application resilience.

2. Patch/Upgrade Steps:

   • Contact the Time4J project maintainers for the claimed patch status. Check the Maven Central repository for version 5.9.4+:

     # Java/Maven
     mvn versions:use-latest-versions -Dincludes=net.time4j:time4j-base
     # Java/Gradle
     implementation 'net.time4j:time4j-base:5.9.4'
     

   • Verify the fix by running a regression test that previously triggered the NPE (if reproducible).
   • If no patched version exists, consider pinning to the latest stable release and monitoring the official GitHub repository for updates.

3. Long-Term Hardening:

   • Switch to Java's built-in java.time package (JSR-310) if feasible — it is maintained by Oracle and has mature null-checking.
   • Implement global exception handlers that sanitize stack traces before logging (avoid leaking internal class paths).
   • Add security tests that fuzz locale inputs to your date-formatting endpoints.
   • Subscribe to CVE feeds and vendor advisories for Time4J to stay informed of patch releases.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2024-23083
2. Time4J Project (GitHub): https://github.com/MenoData/Time4J
3. Time4J on Maven Central: https://central.sonatype.com/artifact/net.time4j/time4j-base
4. MITRE CVE Record: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-23083
5. CISA KEV Note: CVE-2024-23083 is NOT listed in CISA's Known Exploited Vulnerabilities catalog. No threat actor campaigns or exploit-in-the-wild evidence has been reported as of this writing.