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CVE-2026-18040

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

5.9 MediumFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Local
Complexity
High
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
None
Availability
None

Exploit likelihood

0.11%Unchanged since Oct 3, 2026
90%50%10%1%

What each source reports

CVE List Score used 5.9

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

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.

Record
CVE-2026-18040
CVSS 4.0
5.9 MediumCVSS: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
Weakness
CWE-208
Updated
2026-11-02
EUVDEUVD-2026-92004 5.9

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.

Record
EUVD-2026-92004
CVSS 4.0
5.9 MediumCVSS: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
Updated
2026-11-02
GitHub AdvisoriesGHSA-rxw8-22c7-wgq6 5.9

In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels:...

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.

Record
GHSA-rxw8-22c7-wgq6
CVSS 4.0
5.9 MediumCVSS: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
Weakness
CWE-208
Updated
2026-11-02
NVD 5.9

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.

Record
CVE-2026-18040
CVSS 4.0
5.9 MediumCVSS: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
Weakness
CWE-208
Updated
2026-11-02
OSV 5.9

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

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.

Record
CVE-2026-18040
CVSS 4.0
5.9 MediumCVSS: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
Weakness
CWE-208
Updated
2026-11-04
OSVDEBIAN-CVE-2026-18040 5.9

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.

Record
DEBIAN-CVE-2026-18040
CVSS 4.0
5.9 MediumCVSS: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
Updated
2026-11-02
Show 5 moreShow less
OSVUBUNTU-CVE-2026-18040 5.9

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.

Record
UBUNTU-CVE-2026-18040
CVSS 4.0
5.9 MediumCVSS: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
Updated
2026-11-02
OSVopenSUSE-SU-2026:12090-1 No score

bouncycastle-1.86-1.1 on GA media

These are all security issues fixed in the bouncycastle-1.86-1.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2026:12090-1
Updated
2026-11-02
OSVopenSUSE-SU-2026:22169-1 No score

Security update for bouncycastle

This update for bouncycastle fixes the following issues:

Update to 1.86:

  • CVE-2026-17507: negative signed leaf_index values from unsigned wire encodings can cause heap exhaustion (bsc#1284075).
  • CVE-2026-17508: unbounded KDF iteration and memory parameters can cause resource exhaustion (bsc#1284073).
  • CVE-2026-18036: variable-time modulo and division operations in NTRU implementation can allow timing side-channel attacks (bsc#1284065).
  • CVE-2026-18040: table lookups and variable-time duplicate scanning in HQC can cause cache side-channel leaks (bsc#1284205).
  • CVE-2026-71885: unbound X.509 certificates in MLS LeafNode verification can cause identity impersonation and group message interception (bsc#1284165).
  • CVE-2026-71886: unvalidated CERTIFY_OTHER flags in OpenPGP subkeys can cause unauthorized trust delegations and identity spoofing (bsc#1284212).
  • CVE-2026-71887: missing primary key back-signature validation on subkeys can cause signature misattribution and trust context spoofing (bsc#1284213).
  • CVE-2026-71888: disagreement between digestAlgorithm and authAttrs in CMS parsing can cause unauthenticated attribute injection (bsc#1284215).
  • CVE-2026-71889: skipped leaf certificate evaluation in checkNameConstraints can cause X.509 name constraint bypasses (bsc#1284218).
  • CVE-2026-71890: unbounded Remove proposals in external MLS commits can cause arbitrary group member eviction and session takeover (bsc#1284219).
  • CVE-2026-71891: forged ECPoint curves in BLS12-381 key validation can cause aggregate signature verification bypasses (bsc#1284220).
  • CVE-2026-71892: type mismatch in CMS key-transport validation can cause key-size enforcement bypasses for HKDF-derived content keys (bsc#1284221).
  • CVE-2026-85515: laundered EOF exceptions in OpenPGP message stream processing can cause unregistered truncation errors and integrity check bypasses (bsc#1284222).
  • CVE-2026-97873: unbounded iteration counts in legacy PBES1 and PKCS#12 PBE implementations can cause resource exhaustion (bsc#1284224).
Record
openSUSE-SU-2026:22169-1
Updated
2026-11-02
OSVSUSE-SU-2026:24461-1 No score

Security update for bouncycastle

This update for bouncycastle fixes the following issues:

Update to 1.86:

  • CVE-2026-17507: negative signed leaf_index values from unsigned wire encodings can cause heap exhaustion (bsc#1284075).
  • CVE-2026-17508: unbounded KDF iteration and memory parameters can cause resource exhaustion (bsc#1284073).
  • CVE-2026-18036: variable-time modulo and division operations in NTRU implementation can allow timing side-channel attacks (bsc#1284065).
  • CVE-2026-18040: table lookups and variable-time duplicate scanning in HQC can cause cache side-channel leaks (bsc#1284205).
  • CVE-2026-71885: unbound X.509 certificates in MLS LeafNode verification can cause identity impersonation and group message interception (bsc#1284165).
  • CVE-2026-71886: unvalidated CERTIFY_OTHER flags in OpenPGP subkeys can cause unauthorized trust delegations and identity spoofing (bsc#1284212).
  • CVE-2026-71887: missing primary key back-signature validation on subkeys can cause signature misattribution and trust context spoofing (bsc#1284213).
  • CVE-2026-71888: disagreement between digestAlgorithm and authAttrs in CMS parsing can cause unauthenticated attribute injection (bsc#1284215).
  • CVE-2026-71889: skipped leaf certificate evaluation in checkNameConstraints can cause X.509 name constraint bypasses (bsc#1284218).
  • CVE-2026-71890: unbounded Remove proposals in external MLS commits can cause arbitrary group member eviction and session takeover (bsc#1284219).
  • CVE-2026-71891: forged ECPoint curves in BLS12-381 key validation can cause aggregate signature verification bypasses (bsc#1284220).
  • CVE-2026-71892: type mismatch in CMS key-transport validation can cause key-size enforcement bypasses for HKDF-derived content keys (bsc#1284221).
  • CVE-2026-85515: laundered EOF exceptions in OpenPGP message stream processing can cause unregistered truncation errors and integrity check bypasses (bsc#1284222).
  • CVE-2026-97873: unbounded iteration counts in legacy PBES1 and PKCS#12 PBE implementations can cause resource exhaustion (bsc#1284224).
Record
SUSE-SU-2026:24461-1
Updated
2026-11-04
OSVSUSE-SU-2026:4797-1 No score

Security update for bouncycastle

This update for bouncycastle fixes the following issues:

  • CVE-2026-17507: negative signed leaf_index values from unsigned wire encodings can cause heap exhaustion (bsc#1284075).
  • CVE-2026-17508: unbounded KDF iteration and memory parameters can cause resource exhaustion (bsc#1284073).
  • CVE-2026-18036: variable-time modulo and division operations in NTRU implementation can allow timing side-channel attacks (bsc#1284065).
  • CVE-2026-18040: table lookups and variable-time duplicate scanning in HQC can cause cache side-channel leaks (bsc#1284205).
  • CVE-2026-71885: unbound X.509 certificates in MLS LeafNode verification can cause identity impersonation and group message interception (bsc#1284165).
  • CVE-2026-71886: unvalidated CERTIFY_OTHER flags in OpenPGP subkeys can cause unauthorized trust delegations and identity spoofing (bsc#1284212).
  • CVE-2026-71887: missing primary key back-signature validation on subkeys can cause signature misattribution and trust context spoofing (bsc#1284213).
  • CVE-2026-71888: disagreement between digestAlgorithm and authAttrs in CMS parsing can cause unauthenticated attribute injection (bsc#1284215).
  • CVE-2026-71889: skipped leaf certificate evaluation in checkNameConstraints can cause X.509 name constraint bypasses (bsc#1284218).
  • CVE-2026-71890: unbounded Remove proposals in external MLS commits can cause arbitrary group member eviction and session takeover (bsc#1284219).
  • CVE-2026-71891: forged ECPoint curves in BLS12-381 key validation can cause aggregate signature verification bypasses (bsc#1284220).
  • CVE-2026-71892: type mismatch in CMS key-transport validation can cause key-size enforcement bypasses for HKDF-derived content keys (bsc#1284221).
  • CVE-2026-85515: laundered EOF exceptions in OpenPGP message stream processing can cause unregistered truncation errors and integrity check bypasses (bsc#1284222).
  • CVE-2026-97873: unbounded iteration counts in legacy PBES1 and PKCS#12 PBE implementations can cause resource exhaustion (bsc#1284224).

Changes for bouncycastle:

  • Updated to version 1.86.
Record
SUSE-SU-2026:4797-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
bouncycastle
Debian:12
 All versions, plus listed versions
1.72-21.77-11.80-11.80-21.80-31.86-1
 None known
bouncycastle
Debian:13
 All versions, plus listed versions
1.80-31.86-1
 None known
bouncycastle
Debian:14
 before 1.86-1, plus listed versions
1.80-3
1.86-1
bouncycastle
SUSE:Linux Enterprise Server 16.0
From an advisory covering 14 CVEs
 before 1.86-160000.1.1
1.86-160000.1.1
bouncycastle
SUSE:Linux Enterprise Server for SAP applications 16.0
From an advisory covering 14 CVEs
 before 1.86-160000.1.1
1.86-160000.1.1
bouncycastle
SUSE:Linux Enterprise Module for Development Tools 15 SP7
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise High Performance Computing 15 SP4-LTSS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise High Performance Computing 15 SP5-ESPOS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise High Performance Computing 15 SP5-LTSS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
Show 15 moreShow less
Affected packages
PackageAffectedFixed
bouncycastle
SUSE:Linux Enterprise Server 15 SP4-LTSS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise Server 15 SP5-LTSS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise Server 15 SP6-LTSS
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise Server for SAP Applications 15 SP4
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise Server for SAP Applications 15 SP5
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
SUSE:Linux Enterprise Server for SAP Applications 15 SP6
From an advisory covering 14 CVEs
 before 1.86-150200.3.41.1
1.86-150200.3.41.1
bouncycastle
Ubuntu:Pro:16.04:LTS
 All versions, plus listed versions
1.49+dfsg-3ubuntu11.51-4ubuntu11.51-4ubuntu1+esm1
 None known
bouncycastle
Ubuntu:Pro:18.04:LTS
 All versions, plus listed versions
1.57-11.58-11.59-11.59-1ubuntu0.1~esm1
 None known
bouncycastle
Ubuntu:Pro:20.04:LTS
 All versions, plus listed versions
1.61-11.61-1ubuntu0.1~esm1
 None known
bouncycastle
Ubuntu:Pro:22.04:LTS
 All versions, plus listed versions
1.68-21.68-41.68-51.68-5ubuntu0.1~esm1
 None known
bouncycastle
Ubuntu:Pro:24.04:LTS
 All versions, plus listed versions
1.72-21.77-11.77-1ubuntu0.1~esm1
 None known
bouncycastle
Ubuntu:26.10
 All versions, plus listed versions
1.80-3
 None known
bouncycastle
Ubuntu:26.04:LTS
 All versions, plus listed versions
1.80-3
 None known
bouncycastle
openSUSE:Tumbleweed
From an advisory covering 14 CVEs
 before 1.86-1.1
1.86-1.1
bouncycastle
openSUSE:Leap 16.0
From an advisory covering 14 CVEs
 before 1.86-160000.1.1
1.86-160000.1.1

History

History
RecordedChangedSources
affected
affected, aliases, facts, referencesOSV
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-18040 is a medium vulnerability (CVSS 5.9) in Legion Of The Bouncy Castle Inc. Bc-java, published October 3, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.11%, the 1st percentile. Fixed in bouncycastle 1.86-1.

References

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