CVE-2021-3520Public exploitNetwork attack vector

Application crash via negative output-size argument

Published Jun 2, 2021 · Updated Aug 3, 2024

Improper input validation in LZ4 1.8.3 allows attackers to crash linked applications through a negative output-size argument. LZ4_decompress_generic accepts a negative outputSize from its caller, lets it influence length, and passes the converted value to memmove as an extremely large size_t. The upstream maintainer states that a crafted file cannot directly set this parameter and that correctly invoking callers are unaffected, so exposure requires caller code that supplies an invalid negative size.

CVSS severity9.8
Critical
EPSS probability3.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in LZ4 1.8.3 allows attackers to crash linked applications through a negative output-size argument. LZ4_decompress_generic accepts a negative outputSize from its caller, lets it influence length, and passes the converted value to memmove as an extremely large size_t. The upstream maintainer states that a crafted file cannot directly set this parameter and that correctly invoking callers are unaffected, so exposure requires caller code that supplies an invalid negative size.

The record

CVE
CVE-2021-3520
Published
Jun 2, 2021
Updated
Aug 3, 2024
Vendor
LZ4
Product
LZ4
Classifications
CWE-190, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=lz4-1.8.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

LZ4 · LZ4version=lz4-1.8.3

Published CVSS scores

9.8Canonical UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-190Integer Overflow or Wraparound
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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