CVE-2026-27171: zlib before 1.3.2 allows CPU consumption via crc32_combine64 and crc32_combine_gen64 because x2nmodp can do ri

Routine

LOW 2.9UPGRADE

CVE-2026-27171 is a low-severity flaw (CVSS 2.9) affecting zlib. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

zlib before 1.3.2 allows CPU consumption via crc32_combine64 and crc32_combine_gen64 because x2nmodp can do right shifts within a loop that has no termination condition.

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	2.9 (Low)
Attack vector	Local
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 84%)
CISA exploited list	No
Weakness	CWE-1284
Published	2026-02-18
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: zlib

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-02-18 04:16 UTCPublished CVE record published
2026-06-17 10:26 UTCUpdated Record last modified

zlib zlib UPGRADE

Affected: >= 1.2.12 and < 1.3.2

Fixed in: 1.3.2

Safe to downgrade to: a release before 1.2.12

1. Check installed version

apt-cache policy zlib || rpm -q zlib || zlib --version

2. UPGRADE to 1.3.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade zlib

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade zlib

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.2.12 (Debian / Ubuntu)

apt-cache madison zlib sudo apt install zlib=<VERSION_BEFORE_1.2.12> sudo apt-mark hold zlib

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list zlib sudo dnf downgrade zlib-<VERSION_BEFORE_1.2.12> sudo dnf versionlock add zlib

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-27171?

zlib: upgrade to 1.3.2. Exact commands for each OS are on this page.

Is CVE-2026-27171 being exploited?

CVE-2026-27171 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-27171 affect?

CVE-2026-27171 affects zlib. Severity is low (CVSS 2.9). zlib before 1.3.2 allows CPU consumption via crc32_combine64 and crc32_combine_gen64 because x2nmodp can do right shifts within a loop that has no termination condition.

How urgent is CVE-2026-27171?

Priority 19/100. Public exploit reference

Software pages

zlib vulnerabilities

Related CVEs

CVE-2026-27820 CRITICAL 9.8 UPGRADE zlib is a Ruby interface for the zlib compression/decompression library
CVE-2026-22184 HIGH 7.8 UPGRADE zlib versions up to and including 1.3.1.2 include a global buffer overflow in the untgz utility located under

References

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