HighCVE-2026-71888

    CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent (CVE-2026-71888)

    Published Last modified Sources: Red Hat, BSI, CVE.org, SUSE

    What is affected, what to do

    A flaw was found in Bouncy Castle. In its Cryptographic Message Syntax (CMS) streaming parser, messages with mismatched attribute and algorithm fields are improperly handled, allowing attributes to be returned as valid even though they were not protected by the Message Authentication Code (MAC).

    Update Legion of the Bouncy Castle BC-JAVA to version 1.86, Legion of the Bouncy Castle BC-LTS-JAVA to version 2.73.13, Legion of the Bouncy Castle BC-FJA to version 1.0.13.

    Affected
    • Legion of the Bouncy Castle BC-JAVA lt 1.70..1.86
    • Legion of the Bouncy Castle BC-LTS-JAVA lt 2.73.0..2.73.13
    • Legion of the Bouncy Castle BC-FJA lt 1.0.0..1.0.13
    • Legion of the Bouncy Castle BC-FJA lt 2.0.0..2.0.13
    • Legion of the Bouncy Castle BC-FJA lt 2.1.0..2.1.13
    • Legion of the Bouncy Castle BC-FJA lt 2.0.0..2.0.8
    • Legion of the Bouncy Castle BC-FJA lt 2.1.0..2.1.8
    Fixed in
    • Legion of the Bouncy Castle BC-JAVA 1.86
    • Legion of the Bouncy Castle BC-LTS-JAVA 2.73.13
    • Legion of the Bouncy Castle BC-FJA 1.0.13
    • Legion of the Bouncy Castle BC-FJA 2.0.13
    • Legion of the Bouncy Castle BC-FJA 2.1.13
    • Legion of the Bouncy Castle BC-FJA 2.0.8
    • Legion of the Bouncy Castle BC-FJA 2.1.8

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    Metrics

    8.7

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v4

    8.7

    CVSS v4

    Severity High on the scale from 0 to 10.

    Source: Red Hat, BSI, CVE.org, SUSE

    0.1 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 1.4

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    73.51 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.7 Methodology
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.7
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.1 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

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    1.4
    Low — this CVE sits in the lower 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 5 October 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-354

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A flaw was found in Bouncy Castle. In its Cryptographic Message Syntax (CMS) streaming parser, messages with mismatched attribute and algorithm fields are improperly handled, allowing attributes to be returned as valid even though they were not protected by the Message Authentication Code (MAC). A remote attacker capable of intercepting and modifying network traffic can exploit this flaw by inserting unauthorized attributes into an otherwise valid message. This can allow an attacker to manipulate application behavior and bypass security checks if the application relies on those attributes to make authorization or routing decisions.

    Source: redhat_csaf, BSI WID Portal, BSI CSAF, NVD (NIST), cvelistv5, suse_csaf

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    Legion of the Bouncy Castle Inc.BC-FJA
    • 2.0.0 – < 2.0.13
    • 2.1.0 – < 2.1.13
    • 1.0.0 – < 1.0.13
    • 2.0.0 – < 2.0.8
    • 2.1.0 – < 2.1.8
    • 2.0.13
    • 2.1.13
    • 1.0.13
    • 2.0.8
    • 2.1.8
    Legion of the Bouncy Castle Inc.BC-JAVA
    • 1.70 – < 1.86
    • 1.86
    Legion of the Bouncy Castle Inc.BC-LTS-JAVA
    • 2.73.0 – < 2.73.13
    • 2.73.13
    Open SourceBouncy Castle Java
    • < 1.86
    • 1.86
    Open SourceBouncy Castle Java LTS
    • < 2.73.13
    • 2.73.13

    Vendors and products

    Weakness classes (CWE)

    • CWE-354Improper Validation of Integrity Check Value

      Base

      The product does not validate or incorrectly validates the integrity check values or "checksums" of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission.

      Entry at MITRE CWE-354

    References & sources

    • github.combcgit/bc-java/wiki/CVE‐2026‐71888Vendor advisory
    • github.combcgit/bc-java/commit/dcb683b32b440f1beb1d2276a34930164281fb95Patch

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: yes

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2026-71888","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-05T16:51:49.725826Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    2. Receivedbcorg

      CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/U:Amber

      Weakness type: CWE-354

      Description added or changed

      Show 6 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      In Bouncy Castle for Java before 1.86, the streaming CMS AuthenticatedData parser accepted a message whose digestAlgorithm and authAttrs fields disagreed about whether authenticated attributes were present. RFC 5652 sec. 9.1 pairs the two, requiring that authAttrs be present whenever digestAlgorithm is, and sec. 9.2 makes the MAC cover the DER encoding of authAttrs when they are present and the eContent OCTET STRING directly when they are not. CMSAuthenticatedDataParser has to choose between those two in its constructor, before it can reach authAttrs, which comes later in the SEQUENCE, so it chose on digestAlgorithm alone: for a message with digestAlgorithm absent but authAttrs present it verified the content MAC and then returned the attributes through getAuthAttrs() as though they had been authenticated, when the MAC had never covered them. An attacker able to modify a message in transit could insert an authenticated attribute, such as an RFC 2634 ESSSecurityLabel, into an otherwise valid message while holding neither the key-encryption key nor the content-MAC key, and an application taking an authorization, routing or labelling decision from those attributes would act on attacker-chosen values. The content itself remained MAC-bound. asn1.cms.AuthenticatedData now rejects the mismatched pairing when parsing and CMSAuthenticatedDataParser cross-checks the two fields once authAttrs is read. This is a variant of CVE-2026-59642, which bound the content to the MAC for messages that legitimately carry authAttrs, and which does not address this case. This issue also affects Bouncy Castle for Java LTS before 2.73.13, and Bouncy Castle for Java FIPS (BC-FJA) before bcpkix-fips 1.0.13 (1.0.X series), 2.0.13 (2.0.X series) and 2.1.13 (2.1.X series), and bcutil-fips 2.0.8 (2.0.X series) and 2.1.8 (2.1.X series).
      CVSSadded
      —
      AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/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 type (CWE)added
      —
      CWE-354
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/71xxx/CVE-2026-71888.json">CVE-2026-71888</a>
      Referenceadded
      —
      https://github.com/bcgit/bc-java/commit/dcb683b32b440f1beb1d2276a34930164281fb95
      Referenceadded
      —
      https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071888

      Source identifier: 91579145-5d7b-4cc5-b925-a0262ff19630

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