Private-key recovery via data-dependent X25519 reductions
Published Sep 1, 2026 · Updated Sep 3, 2026
Observable timing discrepancy in phpseclib 3.0.56 and 4.0.0 allows local users to recover reused X25519 private keys. PrimeField Integer add() and subtract() conditionally reduce field values, making each Montgomery-ladder step reveal information about the secret scalar prefix. Exploitation requires the pure-PHP path, a known public key, a reused private key, and per-step timing or libgmp call-count observation; recovery permanently compromises that key.
Summary
What happened
Observable timing discrepancy in phpseclib 3.0.56 and 4.0.0 allows local users to recover reused X25519 private keys. PrimeField Integer add() and subtract() conditionally reduce field values, making each Montgomery-ladder step reveal information about the secret scalar prefix. Exploitation requires the pure-PHP path, a known public key, a reused private key, and per-step timing or libgmp call-count observation; recovery permanently compromises that key.
The record
- CVE
- CVE-2026-84308
- Published
- Sep 1, 2026
- Updated
- Sep 3, 2026
- Vendor
- phpseclib
- Product
- phpseclib
- Classifications
- CWE-208, CWE-385
- Attack vector
- local
- Privileges
- authenticated
Timeline
How it unfolded
- Sep 1, 2026CVE publishedPublication date reported by the CVE source.
- Sep 3, 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=< 3.0.57
- Affected versionversion=>= 4.0.0, < 4.0.1
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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Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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