CVE-2026-18036: NTRU leaks private key information by reducing secret values with a non-constant-time integer division

DescriptionCVE.org

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.

• Legion of the Bouncy Castle Inc. · BC-JAVAfrom 1.73 before 1.86

Exploitation

Unlikely (under 1%). EPSS under 1%, or no score yet.

CISA KEV

Not listed

Public check

No public nuclei template

Weakness

Observable Timing Discrepancy · Information Disclosure

Vendor

Legion of the Bouncy Castle Inc.

Published

October 2026

Band

Unlikely (under 1%), October 2026

• NVD entrynvd.nist.gov
• CVE recordcve.org
• Weakness CWE-208cwe.mitre.org
• Vendor advisorygithub.com
• Patchgithub.com