VulnerabilityPublished 5 Dec 2025

CVE-2025-66566: yawkat lz4-java · Insertion of Sensitive Information Into Sent Data

Affects yawkat lz4-javaSource CVE Database V5

Not known to be exploited · CVSS 8.2 High : upgrade to 1.10.1 soon.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 4.0 base score

8.2High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.6% chance of exploitation in 30 days – higher than 48% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.10.1

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: None
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: High: There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 5 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade com.github.yawkat/lz4-java to 1.10.1.

Affected software

Product	Affected	Fixed in
Mavencom.github.yawkat/lz4-java`pkg:maven/com.github.yawkat/lz4-java`Other threats	Affected <1.10.1	Fixed in 1.10.1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

European organizations should immediately assess their software inventory to identify usage of yawkat lz4-java versions prior to 1.10.1. The primary mitigation is to upgrade all instances of lz4-java to version 1.10.1 or later, where the vulnerability is fixed. For applications where upgrading is not immediately feasible, implement compensating controls such as ensuring output buffers are explicitly cleared before reuse in custom decompression logic. Conduct code reviews to verify that decompression buffers do not retain sensitive data. Network-level protections such as input validation and filtering of compressed data from untrusted sources can reduce exposure. Employ runtime application self-protection (RASP) or intrusion detection systems to monitor for anomalous decompression requests. Additionally, review logging and monitoring to detect unusual decompression activity. Finally, update incident response plans to include scenarios involving data leakage via compression libraries.

Description

CVE-2025-66566 is a high-severity vulnerability in yawkat lz4-java versions prior to 1.10.1, where insufficient clearing of the output buffer in Java-based decompressor implementations allows remote attackers to read residual data from previous decompression operations. This can lead to disclosure of sensitive information if the output buffer is reused without proper clearing. The vulnerability does not affect JNI-based implementations and requires no authentication or user interaction to exploit. Although no known exploits are currently reported in the wild, the vulnerability poses a significant confidentiality risk. The issue is fixed in version 1.10.1 of lz4-java. European organizations using vulnerable versions in their Java applications, especially those handling sensitive data, should prioritize upgrading to mitigate potential data leakage.

Impact

For European organizations, the primary impact of this vulnerability is the potential unauthorized disclosure of sensitive data processed by applications using vulnerable versions of lz4-java. This can include personal data, credentials, or proprietary information, depending on the application's context. Data leakage can lead to regulatory non-compliance under GDPR, reputational damage, and potential financial losses. Since the vulnerability can be exploited remotely without authentication, any exposed service accepting compressed input using the vulnerable library is at risk. The impact is particularly critical for sectors such as finance, healthcare, and government, where sensitive data confidentiality is paramount. Additionally, organizations relying on Java-based microservices or middleware that incorporate lz4-java for compression may inadvertently expose internal data. The lack of user interaction or privileges required for exploitation increases the threat surface. However, the absence of known exploits in the wild suggests that immediate widespread attacks may not be occurring, but proactive mitigation is essential to prevent future exploitation.

CWE-201Insertion of Sensitive Information Into Sent Data (opens in a new tab)
Base weaknessOWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The code transmits data to another actor, but a portion of the data includes sensitive information that should not be accessible to that actor.
Requirements
Specify which data in the software should be regarded as sensitive. Consider which types of users should have access to which types of data.
Implementation
Ensure that any possibly sensitive data specified in the requirements is verified with designers to ensure that it is either a calculated risk or mitigated elsewhere.
System Configuration
Setup default error messages so that unexpected errors do not disclose sensitive information.
OWASP Cheat Sheets
Error Handling (opens in a new tab)
Logging (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V8.3.4 (opens in a new tab)
V14.3.3 (opens in a new tab)
V7.3.3 (opens in a new tab)
Attack patterns
Choosing Message Identifier (opens in a new tab)
Exploiting Incorrectly Configured SSL/TLS (opens in a new tab)
WiFi MAC Address Tracking (opens in a new tab)
WiFi SSID Tracking (opens in a new tab)
Cellular Broadcast Message Request (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Dec 2025, 19:16 UTC

WeaknessesCWE-201

Technical details

References

Community reviews

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