Summary
Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
Full technical description
Details
A buffer overflow vulnerability exists in 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.
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
References
Impact
CVE-2026-27820 has a CVSS score of 9.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.2.3, 3.1.2, 3.0.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
zlib to 3.2.3 or later; zlib to 3.1.2 or later; zlib to 3.0.1 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-27820? CVE-2026-27820 is a medium-severity security vulnerability in zlib (rubygems), affecting versions >= 3.2.0, < 3.2.3. It is fixed in 3.2.3, 3.1.2, 3.0.1.
- How severe is CVE-2026-27820? CVE-2026-27820 has a CVSS score of 9.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of zlib are affected by CVE-2026-27820? zlib (rubygems) versions >= 3.2.0, < 3.2.3 is affected.
- Is there a fix for CVE-2026-27820? Yes. CVE-2026-27820 is fixed in 3.2.3, 3.1.2, 3.0.1. Upgrade to this version or later.
- Is CVE-2026-27820 exploitable, and should I be worried? Whether CVE-2026-27820 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-27820 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-27820?
- Upgrade
zlibto 3.2.3 or later - Upgrade
zlibto 3.1.2 or later - Upgrade
zlibto 3.0.1 or later
- Upgrade