External risk intelligence

LZ4 Integer Overflow Allows Out-of-Bounds Write and Denial of Service

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2021-3520

The vulnerability exists in the lz4 compression library, a low-level component used by diverse products including storage appliances, management utilities, and data forwarders. While these products are often deployed in internal or backend environments, they are sometimes positioned in roles that could be reached via network traffic or public-facing interfaces, making reachability possible but not inherently default.

Integer Overflow

Lz4 Project Lz4

1.8.3 to before 1.9.41.14.08.88.2.0 to before 8.2.129.0.0 to before 9.0.69.1.0

Halo Surface Signal: 3 out of 5 — possibly public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in the lz4 data compression library could allow an attacker to cause a system crash or potentially impact data confidentiality and integrity by submitting a specially crafted file to an affected application. This issue is significant due to the widespread use of lz4 in various software products.

Flaw in lz4 library impacts data integrity and availability.
Widespread use of lz4 requires understanding potential exposure.
Confirm relevance and exposure of lz4 component usage.

Attack Path

How an attacker could exploit the issue

An attacker could send a specially crafted file to an application that uses the lz4 library. This malicious file could trigger an integer overflow, causing the application to mismanage memory, potentially leading to a crash or unauthorized data access.

Entry condition: Network access to an affected application.
Trigger point: Submitting a crafted file.
Resulting risk: System instability and data compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could affect the availability of applications linked with lz4 by causing them to crash. When supported by the advisory, there's also potential impact to the confidentiality and integrity of system data due to an out-of-bounds write.

Application availability.
Crafted files could trigger crashes.
System instability and data corruption.

Operational Fix

Recommended remediation, mitigation, and detection steps

Determine which teams are responsible for the lz4 component and its affected products, such as NetApp, Oracle, or Splunk deployments. Begin by identifying all instances of the affected technology, assessing their reachability and business criticality, and locating the accountable owner for each. Remediation planning should then be prioritized based on this risk assessment.

Own the issue: Infrastructure or Application Teams.
Verify first: Asset reachability and business criticality.
Action: Plan remediation based on risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the lz4 library and why is it in so many different software products?

lz4 is a widely used, high-performance compression library designed for speed. Because it efficiently reduces data size without heavy processor overhead, many developers integrate it into products like storage appliances, data management utilities, and log forwarders to help them handle large volumes of information quickly.

How does an integer overflow cause the vulnerability in CVE-2021-3520?

This flaw involves an integer overflow, which is a calculation error. When the library processes a specifically crafted file, the math results in a negative number for memory operations. This causes the software to misinterpret the data size, leading to an out-of-bounds write—a type of memory corruption where data is written where it should not be, often causing crashes.

When is an application vulnerable to this lz4 issue?

An application is only at risk if it processes untrusted or malicious input using the affected version of the lz4 library. The vulnerability is not triggered by normal, valid file compression tasks; it specifically requires a crafted file designed to exploit how the library manages memory during the decompression or processing sequence.

Do I need to worry if my systems using lz4 are internal?

According to Halo Surface Signal, this vulnerability is classified as external because it can be reached via network traffic. Even if a product seems internal, you should assess if it has any interfaces accessible to broader network segments, as that connectivity can serve as a potential path for an attacker to reach the vulnerable component.

What should I do first to manage this CVE-2021-3520 risk?

Start by auditing your environment to locate applications that incorporate lz4, such as specific NetApp, Oracle, or Splunk tools. Prioritize identifying which of these assets are business-critical and have network exposure. Once you have a clear inventory, work with your infrastructure teams to plan updates or patches for the software versions identified.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.