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

High 8.2 Published 2 Oct 2026 Updated 2 Oct 2026

Timing side-channel in Bouncy Castle NTRU allows attackers to recover private key information via non-constant-time integer divisions.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-18036?

CVE-2026-18036 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-208

Which versions are affected by CVE-2026-18036?

How do I fix CVE-2026-18036?

Upgrade to a fixed version.

Is CVE-2026-18036 being exploited?

There is no known exploitation of CVE-2026-18036 in the wild and no public exploit code.

Sources

Also known as CLEANSTART-2026-AC10204, CLEANSTART-2026-NB89107, CLEANSTART-2026-RL38047, DEBIAN-CVE-2026-18036, GHSA-frjj-v5w4-68pc, UBUNTU-CVE-2026-18036.