CVE-2026-27820 is a low severity vulnerability with a CVSS score of 0.0. No known exploits currently, and patches are available.
Very low probability of exploitation
EPSS predicts the probability of exploitation in the next 30 days based on real-world threat data, complementing CVSS severity scores with actual risk assessment.
A buffer overflow vulnerability exists in Zlib::GzipReader. This vulnerability has been assigned the CVE identifier CVE-2026-27820. We recommend upgrading the zlib gem.
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.
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:
zlib gem 3.2.2 or lower
Thanks to calysteon for reporting this issue. Also thanks to nobu for creating the patch.
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