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

Quick assessment

Affected
Legion of the Bouncy Castle Inc. BC-JAVA
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 Bouncy Castle for Java 1.86 之前的版本中，NTRU 算法在三个辅助函数中使用取模运算符（%）来缩减秘密值，而这些辅助函数的参考实现刻意避免了除法操作。然而，实际实现中却使用了整数除法，其延迟时间依赖于秘密操作数，从而引入了时序侧信道泄露风险。 具体而言： 方法使用变量除数进行除法运算。由于除数是变量，编译器无法像优化常量除法那样，通过乘法指令进行“强度削减”（strength-reduce），因此在每次调用时都会执行除法操作，包括在解封装（decapsulation）路径中——该路径

CVSS 8.2 · High EPSS 0.28% · P19

Possible ATT&CK Techniques 1 AI

T1564.008 · Email Hiding Rules

Affected Version Matrix 1

VendorProduct Version RangeStatus
Legion of the Bouncy Castle Inc. BC-JAVA 1.73< 1.86 affected
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I. Basic Information for CVE-2026-18036

Vulnerability Information
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Vulnerability Title
NTRU leaks private key information by reducing secret values with a non-constant-time integer division
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber
Source: CVE Program / CVE List V5
Vulnerability Type
通过时间差异性导致的信息暴露
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Legion of the Bouncy Castle Inc. BC-JAVA 1.73 ~ 1.86 -

II. Public POCs for CVE-2026-18036

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III. Intelligence Information for CVE-2026-18036

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Patches & Fixes for CVE-2026-18036 (1)

Proof of Concept for CVE-2026-18036 (1)

Same Patch Batch · Legion of the Bouncy Castle Inc. · 2026-10-02 · 24 CVEs total

CVE-2026-63569 9.1 CRITICAL MTI/A0 DHAgreement does not validate the peer's ephemeral value
CVE-2026-16000 8.7 HIGH KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used
CVE-2026-63571 8.7 HIGH Attribute certificate path validation does not verify the attribute certificate's signatur
CVE-2026-63566 8.7 HIGH DTLS handshake reassembler allocates buffer from unchecked 24-bit length
CVE-2026-63574 8.7 HIGH Unbounded allocation from OpenPGP signature and user attribute subpacket lengths
CVE-2026-63568 8.7 HIGH Unbounded CMP/CRMF password-based MAC iteration count allows CPU exhaustion
CVE-2026-103600 8.7 HIGH Unbounded ASN.1 nesting depth causes process-terminating stack overflow
CVE-2026-103604 8.7 HIGH Quadratic-time escaping when converting X.509 distinguished names to strings
CVE-2026-103603 8.7 HIGH Unbounded HSS public key level count allows huge array allocation during signature verific
CVE-2026-17507 8.7 HIGH MLS membership checks compare a uint32 leaf_index as signed, admitting an out-of-range sen
CVE-2026-16001 8.2 HIGH IesEngine stream-mode MAC forgery via length-dependent KDF split
CVE-2026-63577 8.2 HIGH Name Constraints bypass: directoryName constraint matched at any position in the DN instea
CVE-2026-63576 8.2 HIGH URI name constraints checked against a mis-parsed host
CVE-2026-63573 8.2 HIGH Bleichenbacher padding oracle in CMS RSA PKCS#1 v1.5 key-transport unwrap
CVE-2026-103602 8.2 HIGH Name constraints bypass via trailing dot in rfc822Name, dNSName and URI hosts
CVE-2026-103601 8.2 HIGH CcmBlockCipher and KCcmBlockCipher leave unverified plaintext in the output buffer after a
CVE-2026-63567 8.2 HIGH IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)
CVE-2026-15999 8.2 HIGH AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication
CVE-2026-63575 7.1 HIGH PKCS#12 key derivation loops about 2^32 times on a zero or negative iteration count
CVE-2026-63578 7.1 HIGH Unbounded PBE iteration count when decrypting PKCS#8 private keys

Showing top 20 of 24 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: BC-JAVA
Same vendor: Legion of the Bouncy Castle Inc.
Same weakness: CWE-208

V. Comments for CVE-2026-18036

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