How severe is CVE-2026-27820?

CVE-2026-27820 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-27820 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (43rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27820?

CVE-2026-27820 affects Ruby-lang Zlib. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27820?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-27820 have an EU (EUVD) identifier?

Yes. CVE-2026-27820 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23278.

When was CVE-2026-27820 published?

CVE-2026-27820 was published on 2026-04-16 and last updated on 2026-06-17.

CVE-2026-27820

CVE-2026-27820 is a critical-severity vulnerability in Ruby-lang Zlib with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-120.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v4: 1.7
EPSS exploit prediction: 1% (43rd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-23278
Weakness: CWE-120
Affected product: Ruby-lang Zlib
Published: 2026-04-16
Last modified: 2026-06-17

Description

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the Zlib::GzipReader. The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.

Frequently asked questions

What is CVE-2026-27820?: zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the Zlib::GzipReader. The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.
How severe is CVE-2026-27820?: CVE-2026-27820 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-27820 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (43rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27820?: CVE-2026-27820 affects Ruby-lang Zlib. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27820?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-27820 have an EU (EUVD) identifier?: Yes. CVE-2026-27820 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23278.
When was CVE-2026-27820 published?: CVE-2026-27820 was published on 2026-04-16 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:ruby-lang:zlib:*:*:*:*:*:ruby:*:*

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