Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-18040. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-18040 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

5 public PoC repositories reported; 0 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with Not reported where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-18040 using CVSS 5.9, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

mediumpublic exploit

CVE-2026-18040

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

Q: What is CVE-2026-18040?

CVE-2026-18040 is HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler. The supplied enrichment data classifies it as Unclassified mapped to CWE-208 CWE-208.

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.

Published Oct 3, 2026Updated Oct 3, 2026Sources: NVD, Legion of the Bouncy Castle Inc. (CNA), GitHub, GitHub Security Advisory, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Exploit code

public exploit, none observed

EPSS

ahead of 0% of scored CVEs

Affects

Legion of the Bouncy Castle Inc.

BC-JAVA

CVSS base

5.9

medium

Remediation

The vendor's own words where we have them.

Upgrade BC-JAVA to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Legion of the Bouncy Castle Inc.	BC-JAVA	1.73 to < 1.86	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Scope

Attack Complexity

High

Attack Vector

Local

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
5.9	CVSS 4.0	medium	—	—	Legion of the Bouncy Castle Inc. (CNA)

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-208 · Observable Timing Discrepancy

Read Application Data
Bypass Protection Mechanism::

Weakness & attack patterns

CWE-208

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-462 · Cross-Domain Search Timing

T1592.002Gather Victim Host Information: Software
T1082System Information Discovery

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Monitor for scanner or exploit-pattern traffic after 5 public PoC repositories were reported.

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 5.9 (AV:L)
Oct 3, 2026 · NVD
Initial · CVE published
Oct 3, 2026 · NVD
Added · GHSA-rxw8-22c7-wgq6 published (medium)
Oct 3, 2026 · GitHub Security Advisory
CVE published by NIST.
Oct 3, 2026 · SOCRadar CTI

References

4 on the record

github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03
Exploit, Third Party Advisory
github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040
Exploit, Third Party Advisory, NIST
nvd.nist.gov/vuln/detail/CVE-2026-18040
NVD
github.com/advisories/GHSA-rxw8-22c7-wgq6
Exploit, Third Party Advisory

Elsewhere on this site

Legion of the Bouncy Castle Inc.every CVE for this vendor
Unclassifiedsame class
CWE-208other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-18040 is HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler, a medium vulnerability affecting BC-JAVA from Legion of the Bouncy Castle Inc.. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-18040. Public exploit evidence is: 5 public PoC repositories reported; 0 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 5.9 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.