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HighVulnerabilityupdated 25 Oct 2026, 15:22 UTC

zlib compression library can crash or be overwritten by attackers

zlib is a common library that many programs use to shrink and unpack data. A flaw in it can let an attacker crash a program or, in some cases, run their own code on the machine.

Official, 2 sources
SeverityHighVulnerability
Being exploited?UnclearNo reports either way yet
Fix available?UnclearNo fixed version published in the sources yet
How sure is this?Official, 2 sourcesBacked by an official source

How the attack works simplified

  1. 1Attacker sends crafted dataTargets non-blocking write operations
  2. 2Data reaches zlibVulnerable library processes it
  3. 3Heap buffer overflowMemory beyond the buffer is overwritten
  4. 4Program crashes or is hijackedPossible denial of service or code execution
  5. ResultService disrupted or attacker runs code

What happened

A flaw in how zlib handles certain non-blocking write operations can cause a heap buffer overflow, allowing memory to be overwritten.

Why it is dangerous: A successful attack could cause a denial of service (programs or servers stop working) or arbitrary code execution. Anyone running software that uses the affected zlib version is at risk.

Are you affected?

You are affected if you run a Linux system or application that uses a vulnerable version of zlib, as listed in the Ubuntu security notice USN-8954-1.

What to do

  1. Apply the zlib security update from your operating system vendor (for Ubuntu, see USN-8954-1)
  2. Restart services and applications that use zlib after updating, so the fixed library is loaded
  3. Check the CERT-Bund (BSI) advisory for further guidance on affected systems

Timeline

  1. 22 Oct 2026, 19:54 UTC
    Publicly disclosed
  2. 23 Oct 2026, 10:22 UTC
    Reported by CERT-Bund (BSI) · CERT-Bund (BSI)
  3. 25 Oct 2026, 13:32 UTC
    Picked up by Zero Radar
  4. 25 Oct 2026, 15:22 UTC
    Ubuntu USN reported on it · Ubuntu USN

Sources 2 reports, oldest first

Official vendor, government CERT or advisory database

Technical details

ID
ZR-334c0c5d · ZR-f9c49274
Disclosed
22 Oct 2026, 19:54 UTC
On Zero Radar
25 Oct 2026, 13:32 UTC (3 days after disclosure)
Last change
25 Oct 2026, 15:22 UTC

The plain-language parts and the attack diagram were written by an AI model (anthropic/claude-haiku-5.5) from the sources above. Facts like versions, exploitation and dates come from the sources themselves. How Zero Radar works

zlib compression library can crash or be overwritten by attackers · Zero Radar