Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.9 · CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U · score source

EPSS

• CVE-2026-27820: 0.56% estimated exploitation activity in the next 30 days; percentile 46.70 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Details A buffer overflow vulnerability exists in Zlib::GzipReader. The zstreambufferungets 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. Recommended action We recommend to update the zlib gem to version 3.2.3 or later. In order to ensure compatibility with bundled version in older Ruby series, you may update as follows instead: For Ruby 3.2 users: Update to zlib 3.0.1 For Ruby 3.3 users: Update to zlib 3.1.2 You can use gem update zlib to update it. If you are using bundler, please add gem "zlib", "= 3.2.3" to your Gemfile. Affected versions zlib gem 3.2.2 or lower Credits calysteon References https://hackerone.com/reports/3467067

Published by the source

16 Apr 2026, 20:40 UTC

Last modified by the source

8 Jun 2026, 23:12 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g857-hhfv-j68w

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# Patch Next advisory brief: CVE-2026-27820

Advisory: Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
IDs: CVE-2026-27820, GHSA-g857-hhfv-j68w
Record: https://patchnext.com/advisories/cve-2026-27820 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: zlib 3.2.3, 3.1.2, 3.0.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems zlib: affected >= 3.2.0, < 3.2.3; >= 3.1.0, < 3.1.2; < 3.0.1; fixed in 3.0.1, 3.1.2, 3.2.3

## Suggested inspection (read-only)
- RubyGems zlib: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on zlib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g857-hhfv-j68w (GitHub-reviewed advisory GHSA-g857-hhfv-j68w lists affected ranges: RubyGems zlib >=3.2.0 =3.1.0 =0 <3.0.1)

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