VulnerabilityPublished 3 Aug 2026

CVE-2026-59639: Legion of the Bouncy Castle BC-JAVA · Improper Verification of Cryptographic Signature

Affects Legion of the Bouncy Castle BC-JAVASource CVE Database V5

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Not known to be exploited · CVSS 8.7 High : upgrade to 1.85 soon.

Data as of 31 Oct 2026, 08:20 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 15% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.85, 2.73.12, 2.1.12 +2

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

Base metrics

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Supplemental metrics

Provider Urgency: Amber

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
Provider Urgency: Amber: Provider has assessed the impact of this vulnerability as having a moderate urgency.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade BC-JAVA and bcpkix to 1.85; BC-LTS-JAVA and bcpkix-lts8on to 2.73.12; BC-FJA to 2.1.12, 2.0.12 or 1.0.12.

Affected software

Product	Affected	Fixed in
Legion of the Bouncy Castle Inc.BC-JAVA	Affected <1.85	Fixed in 1.85
Legion of the Bouncy Castle Inc.BC-LTS-JAVA	Affected >=2.73.0 <2.73.12	Fixed in 2.73.12
Legion of the Bouncy Castle Inc.BC-FJA	Affected >=2.1.0 <2.1.12	Fixed in 2.1.12
Legion of the Bouncy Castle Inc.BC-FJA	Affected >=2.0.0 <2.0.12	Fixed in 2.0.12
Legion of the Bouncy Castle Inc.BC-FJA	Affected >=1.0.0 <1.0.12	Fixed in 1.0.12
GitHubbcpkix`pkg:github/bcpkix`Other threats	Affected <1.85	Fixed in 1.85

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Until then, avoid relying on the affected verifySignatures method for critical signature validation or implement additional verification controls.

Description

CVE-2026-59639 is a high severity vulnerability in Legion of the Bouncy Castle Inc.'s BC-JAVA library. The issue occurs in versions before 1.85 and certain 2.73.x versions, where the CMS verifySignatures method incorrectly returns true for SignedData objects that have zero signers. This improper verification of cryptographic signatures can undermine the trustworthiness of signature validation in affected versions.

Impact

Successful exploitation of this vulnerability allows an attacker to bypass signature verification checks by presenting SignedData with no signers, potentially causing applications relying on BC-JAVA to accept unsigned or improperly signed data as valid. This undermines data integrity and authenticity guarantees provided by cryptographic signatures.

Summary

The vulnerability identified as CVE-2026-59639 affects Bouncy Castle for Java (BC-JAVA) in versions prior to 1.85 and versions in the range 2.73.0 up to but not including 2.73.12. The CMS verifySignatures function returns a successful verification result even when the SignedData structure contains zero signers, violating proper cryptographic signature verification principles (CWE-347). This flaw also impacts Bouncy Castle for Java LTS before 2.73.12 and Bouncy Castle for Java FIPS in specified versions. The CVSS 4.0 base score is 8.7, indicating a high severity due to network attack vector, low attack complexity, no privileges or user interaction required, and high impact on integrity.

CWE-347Improper Verification of Cryptographic Signature (opens in a new tab)
Base weaknessOWASP Top 10 A04:2025: Cryptographic Failures (opens in a new tab)
The product does not verify, or incorrectly verifies, the cryptographic signature for data.
OWASP Cheat Sheets
XML Security (opens in a new tab)
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
REST Assessment (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.2 (opens in a new tab)
V3.5.3 (opens in a new tab)
V13.2.6 (opens in a new tab)
Attack patterns
Padding Oracle Crypto Attack (opens in a new tab)
Signature Spoofing by Improper Validation (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Aug 2026, 15:25 UTC

WeaknessesCWE-347

Technical details

References

Community reviews

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