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CVE.org (no NVD analysis yet)· CNA bcorg

CVE-2026-18040: HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler

Noroxi assessment

Priority: Monitor (action score 23/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.10% (FIRST EPSS).
  • Someone with local access to the system can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
  • Fixed version: Legion of the Bouncy Castle Inc. BC-JAVA 1.86.

Low priority for now. Upgrade to Legion of the Bouncy Castle Inc. BC-JAVA 1.86 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, making the cache line touched a function of the operand, and its fixed-weight support sampler left its duplicate scan as soon as a collision was found and stored accepted positions at a secret index. Both run on secret inputs during encapsulation and decapsulation, and the sampler re-expands the secret key from its seed on every decapsulation, so an attacker able to observe cache behaviour or decapsulation timing can recover information about the HQC private key. The field arithmetic is now table-free and the sampler branch-free within a batch of candidates, with output and randomness consumption unchanged.

MediumCVSS 5.9 · v4.0—No exploit Fix available
Published
Oct 3, 2026
Updated
Oct 6, 2026
EPSS
0.1% · 1th percentile

Report tools

JSON

Action score

23

Monitor

Low priority for now.

CVSS
23 / 40 · 5.9 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.1%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · bcorg
5.9
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-18040

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (bcorg). Independent of NVD's CPE analysis and usually ahead of it.

  • Legion of the Bouncy Castle Inc. BC-JAVA

    • 1.73 and later · before 1.86affected · maven

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:12bouncycastlefrom 0—
Debian:14bouncycastlebefore 1.86-11.86-1
openSUSE:Leap 16.0bouncycastlebefore 1.86-160000.1.11.86-160000.1.1
openSUSE:Tumbleweedbouncycastlebefore 1.86-1.11.86-1.1
SUSE:Linux Enterprise Module for Development Tools 15 SP7bouncycastlebefore 1.86-150200.3.41.11.86-150200.3.41.1
SUSE:Linux Enterprise Server 16.0bouncycastlebefore 1.86-160000.1.11.86-160000.1.1

Same product

legion of the bouncy castle inc.: all records

Other highest-scoring records for the same primary product.

  • CVE-2025-14813GOSTCTR implementation unable to process more than 255 blocks correctly
    37Monitor
  • CVE-2026-71885MLS X.509 credential not bound to the LeafNode signature key
    36Monitor
  • CVE-2026-5598Non-constant time comparisons risk private key leakage in FrodoKEM.
    35Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2026-17508Oct 2, 2026 · Password-based KDF cost parameters honoured unbounded from untrusted input across the remaining PBE entry points
    21Monitor
  • ← EarlierCVE-2026-18036Oct 2, 2026 · NTRU leaks private key information by reducing secret values with a non-constant-time integer division
    32Monitor
  • Later →CVE-2026-71886Oct 3, 2026 · OpenPGP certification accepted from a subkey without certification authority
    32Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Legion of the Bouncy Castle Inc. BC-JAVA1.86Vendor (CNA)
debian:bouncycastle1.86-1 · Debian:14Package registry (OSV)
opensuse:bouncycastle1.86-1.1 · openSUSE:TumbleweedPackage registry (OSV)
suse:bouncycastle1.86-150200.3.41.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7Package registry (OSV)
suse:bouncycastle1.86-160000.1.1 · SUSE:Linux Enterprise Server 16.0Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Published
  2. Fix recordedsame day
  3. TodayNov 8, 2026+36 d
EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-180361 days apartNTRU leaks private key information by reducing secret values with a non-constant-time integer division
    32Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Someone with local access to the system can trigger it.
  • No account or password is required.
  • No user action is required.
  • Special conditions such as timing or configuration are required.

If successful

Confidentiality
Integrity
Availability
Attack vector
Local
Attack complexity
High
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-208 · Observable Timing Discrepancy

CVSS vector

CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. SSVC exploitation— → none
  2. Exploit maturity— → none
  3. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records