Hostname validation bypass via truncated DNS label length
Published Aug 27, 2026 · Updated Aug 27, 2026
Numeric truncation in Ruby Resolv 0.3.1 and earlier and 0.4.0 through 0.7.1 allows remote attackers to bypass hostname validation. The MessageEncoder put_string path packs an unchecked label length into one octet, so oversized labels alter the DNS name placed on the wire. Applications are exposed only when attacker-controlled hostnames reach Resolv directly, through resolv-replace, or through a dependency and length validation is absent.
Summary
What happened
Numeric truncation in Ruby Resolv 0.3.1 and earlier and 0.4.0 through 0.7.1 allows remote attackers to bypass hostname validation. The MessageEncoder put_string path packs an unchecked label length into one octet, so oversized labels alter the DNS name placed on the wire. Applications are exposed only when attacker-controlled hostnames reach Resolv directly, through resolv-replace, or through a dependency and length validation is absent.
The record
- CVE
- CVE-2026-80213
- Published
- Aug 27, 2026
- Updated
- Aug 27, 2026
- Vendor
- libwww-perl
- Product
- Resolv
- Classifications
- CWE-197, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 27, 2026CVE publishedPublication date reported by the CVE source.
- Aug 27, 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=0 <0.3.2
- Affected versionversion=0.4.0 <0.7.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Public exploit references
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Technologies
Your stack
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Your stack
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