Bug 2544959 (CVE-2026-18036)

Summary: CVE-2026-18036 bouncycastle: bouncycastle: Information disclosure via timing side-channel in NTRU
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in Bouncy Castle. A timing side-channel vulnerability in the NTRU algorithm can lead to information disclosure. The implementation performs variable-time division operations on secret data instead of constant-time calculations, causing processing time to vary depending on the secret values. An attacker capable of measuring these timing differences during cryptographic operations can exploit this flaw to recover private keys.
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Bug Depends On: 2545143, 2545144    
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Description OSIDB Bzimport 2026-10-02 08:01:12 UTC 
In Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose reference implementations are deliberately division-free, so each reduction was carried out by an integer division whose latency depends on the secret operand. Polynomial.modQ divided by a variable divisor, which a compiler cannot strength-reduce to a multiply the way it can a constant one, so it emitted a division on every call including on the decapsulation path where the dividend derives from the private key; Polynomial.mod3 and NTRUSampling.mod3 divided the secret key polynomials f and g during key generation, the message polynomials r and m during encapsulation, and coefficients recovered during decapsulation. An attacker able to measure that timing can recover information about the NTRU private key. modQ now masks, which is exact because q is always a power of two, and mod3 uses the reference implementation's division-free fold and select; the results are unchanged.