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Fencer Vulns
medium 5.9

CVE-2026-18040 json

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

published 2026-10-03 36 d ago modified 2026-10-06 33 d ago also EUVD-2026-92004

cvss
5.9
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.1%
EPSS 0.1% chance of exploitation in 30 days, percentile 1%
public exploits
0
none known

Summary

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.

Patch / fix

  • Legion of the Bouncy Castle Inc. BC-JAVA 1.86
  • bouncycastle Debian 14 1.86-1

debian advisories DEBIAN-CVE-2026-18040

  • bouncycastle SLES for SAP applications 16.0, SLES 16.0 1.86-160000.1.1
  • bouncycastle SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES for SAP 15 SP4 +8 more 1.86-150200.3.41.1

suse advisories SUSE-SU-2026:24461-1SUSE-SU-2026:4797-1

  • bouncycastle openSUSE Leap 16.0 1.86-160000.1.1
  • bouncycastle openSUSE Tumbleweed 1.86-1.1

opensuse advisories openSUSE-SU-2026:22169-1openSUSE-SU-2026:12090-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-18040 updated cvss 5.1

bouncycastle: bcprov-jdk18on: bouncycastle: Information disclosure via cryptographic side channels in HQC

  • Red Hat OpenShift Developer Tools and Services jenkins, jenkins-2-plugins, ocp-tools-4/jenkins-rhel8, ocp-tools-4/jenkins-rhel9 Red Hat AMQ Broker 7 bcprov-jdk18on Red Hat Ansible Automation Platform 2 ansible-automation-platform-27/de-minimal-rhel9, ansible-automation-platform-27/de-supported-rhel9 Red Hat build of Apache Camel 4 for Quarkus 3 bcprov-jdk18on Red Hat build of Apache Camel for Spring Boot 4 bcprov-jdk18on Red Hat build of Apicurio Registry 3 bcprov-lts8on
    +10 more products
    Red Hat Build of Keycloak bcprov-jdk18on, rhbk/keycloak-rhel9 Red Hat build of Quarkus bcprov-jdk18on Red Hat Fuse 7 bcprov-jdk18on Red Hat JBoss Enterprise Application Platform 7 bcprov-jdk18on, jboss-eap-7/eap74-els-openjdk8-openshift-rhel8, jboss-eap-7/eap74-els-openjdk11-openshift-rhel8, jboss-eap-7/eap74-els-openjdk17-openshift-rhel8 Red Hat JBoss Enterprise Application Platform 8 bcprov-jdk18on Red Hat JBoss Enterprise Application Platform Expansion Pack bcprov-jdk18on Red Hat OpenShift AI (RHOAI) rhoai/odh-modelmesh-rhel9, rhoai/odh-spark-operator-rhel9, rhoai/odh-th06-cpu-torch210-py312-rhel9, rhoai/odh-th06-cpu-torch291-py312-rhel9, rhoai/odh-th06-rocm64-torch291-py312-rhel9, rhoai/odh-th06-cuda130-torch210-py312-rhel9, +1 more Red Hat OpenShift Dev Spaces devspaces/openvsx-rhel9, devspaces/jetbrains-ide-rhel9, devspaces/pluginregistry-rhel9 Red Hat Satellite 6 candlepin, candlepin-selinux Red Hat Single Sign-On 7 bcprov-jdk18on
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
34 d ago

Exploit prediction

0.11% chance of exploitation in the next 30 days, higher than 1% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 5.9 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
CNA 4.0 5.9 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

Affected products

  • Legion of the Bouncy Castle Inc. BC-JAVA

Classification

Weakness
  • CWE-208 Observable Timing Discrepancy

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.