Memory corruption via missing ungetc buffer expansion
Published Apr 16, 2026 · Updated Apr 16, 2026
Buffer overflow in Ruby zlib 3.0.0 and earlier, 3.1.0 through 3.1.1, and 3.2.0 through 3.2.2 allows attackers to corrupt memory. The zstream_buffer_ungets function prepends caller-provided bytes, then uses memmove without first ensuring that the backing Ruby string has enough capacity. Reachability requires caller-controlled bytes and previously buffered output; the documented consequence is corruption of the reader process's memory.
Summary
What happened
Buffer overflow in Ruby zlib 3.0.0 and earlier, 3.1.0 through 3.1.1, and 3.2.0 through 3.2.2 allows attackers to corrupt memory. The zstream_buffer_ungets function prepends caller-provided bytes, then uses memmove without first ensuring that the backing Ruby string has enough capacity. Reachability requires caller-controlled bytes and previously buffered output; the documented consequence is corruption of the reader process's memory.
The record
- CVE
- CVE-2026-27820
- Published
- Apr 16, 2026
- Updated
- Apr 16, 2026
- Vendor
- libwww-perl
- Product
- zlib
- Classifications
- CWE-131, CWE-120, T1499
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 16, 2026CVE publishedPublication date reported by the CVE source.
- Apr 16, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 3.0.1
- Affected versionversion=>= 3.1.0, < 3.1.2
- Affected versionversion=>= 3.2.0, < 3.2.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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