CVE•Published 2026-04-16•Modified 2026-05-21•1 article on news•6 live references•NVD data

CVE-2026-27820Ruby-lang · Zlib

Vulnerability data via NVD (ingested)

CVSS v3.1

9.8

CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS percentile

Exploit Prediction Scoring System · top 98% of all CVEs

Weaknesses (CWE)

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')CWE-131Incorrect Calculation of Buffer Size

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.

Timeline

Published 2026-04-16

Modified 2026-05-21

External references

NVD MITRE Exploit-DB GitHub PoC Sploitus VulnCheck

Search for exposed instances

Shodan + Censys queries derived from NVD's CPE data. The vuln tag catches assets Shodan has explicitly linked to this CVE; the product / banner fingerprints find exposed instances even when the vuln tag was never applied (which is common).

Shodan · vuln tag0 hosts

vuln:CVE-2026-27820

Hosts Shodan has explicitly fingerprinted as vulnerable.

Shodan · product

product:"Ruby-lang Zlib"

All exposed Ruby-lang Zlib instances — cross-reference with the CVE's affected-version range.

Shodan · banner/body mention

http.html:"Zlib"

HTTP body or banner mentions "Zlib" — catches deploys Shodan didn't identify as a product.

More intel sources (5)

Shodan report

vuln:CVE-2026-27820