Password-hash weakening via signed round-count overflow
Published Mar 24, 2026 · Updated Mar 24, 2026
Integer overflow in bcrypt before 3.1.22 allows attackers to test password guesses against JRuby hashes at constant-time cost when cost 31 is used. In BCrypt.java, 1 << log_rounds uses a signed 32-bit integer, so a value of 31 becomes negative and the strengthening loop performs zero iterations. Exposure requires a JRuby application to create or verify a cost-31 hash; the hash remains syntactically valid while password guessing loses the intended work factor.
Summary
What happened
Integer overflow in bcrypt before 3.1.22 allows attackers to test password guesses against JRuby hashes at constant-time cost when cost 31 is used. In BCrypt.java, 1 << log_rounds uses a signed 32-bit integer, so a value of 31 becomes negative and the strengthening loop performs zero iterations. Exposure requires a JRuby application to create or verify a cost-31 hash; the hash remains syntactically valid while password guessing loses the intended work factor.
The record
- CVE
- CVE-2026-33306
- Published
- Mar 24, 2026
- Updated
- Mar 24, 2026
- Vendor
- bcrypt-ruby
- Product
- bcrypt
- Classifications
- CWE-190, T1110.002
- Attack vector
- local
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 24, 2026CVE publishedPublication date reported by the CVE source.
- Mar 24, 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?
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- Affected versionversion=< 3.1.22
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
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Your stack
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