Linux Distros Unpatched Vulnerability : CVE-2026-71887

high Nessus Plugin ID 362871

Synopsis

The Linux/Unix host has one or more packages installed with a vulnerability that the vendor indicates will not be patched.

Description

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

- In Bouncy Castle for Java before 1.86, the high-level OpenPGP API accepted a data signature made by a signing subkey whose Subkey Binding signature carried no embedded Primary Key Binding (cross- certification) signature, in the case where that binding omits a Key Flags subpacket. RFC 9580 sec.
5.2.1.8 and sec. 10.1.3 require the embedded Primary Key Binding signature on any subkey that can issue signatures; it is the subkey's own statement that it belongs to the primary key it is bound under.
OpenPGPCertificate resolved the subkey's key flags two different ways. isSigningKey() goes through getKeyFlags() and getApplyingSubpacket(), which falls back to the primary key's direct-key or primary User ID self-signature when the binding signature omits the subpacket, so the subkey inherited the primary's SIGN_DATA and counted as signing-capable; verifyEmbeddedPrimaryKeyBinding(), which enforces the requirement, reads the binding signature's own hashed subpackets, found no SIGN_DATA there, and returned early as a non-signing key without ever demanding the back signature. The same subkey was therefore signing-capable - so its signatures were attributed to the certificate and OpenPGPSignature.OpenPGPDocumentSignature.isValid() returned true - while being exempt from cross- certification, where GnuPG refuses the identical certificate and message. An attacker needs only the victim's public signing subkey, which is public material: they bind it to their own primary key with a Subkey Binding signature they are able to make, carrying no Key Flags and no embedded Primary Key Binding signature, which they cannot make without the subkey's private key, and a relying party verifying one of the victim's genuinely signed messages against that certificate is told the signature is valid and given the attacker's certificate as its issuer. Because a certificate's User IDs are self-asserted, a verifier that pins on the subkey's fingerprint or key ID while taking the identity from the enclosing certificate reports a real signature under an attacker-chosen identity. This is misattribution of a genuine signature rather than forgery of a new one: no private key is recovered, and the signature must be one the grafted subkey actually made. The low-level PGPSignature / PGPPublicKeyRing API performs no binding checks by design and is unaffected. Key Flags are a statement about the key the carrying signature refers to (RFC 9580 sec. 5.2.3.29), so a subkey no longer inherits them from the certificate-wide signatures of the primary key: a Subkey Binding signature that omits the subpacket now leaves the subkey with no capabilities rather than the primary's, which makes the flags the cross-certification check consults the same flags every other decision consults. Preferences and the other subpackets a direct-key signature carries are inherited as before, and the primary key itself, whose flags legitimately come from its own direct-key or User ID self-signature, is unaffected. (CVE-2026-71887)

Note that Nessus relies on the presence of the package as reported by the vendor.

Solution

There is no known solution at this time.

Plugin Details

Severity: High

ID: 362871

File Name: unpatched_CVE_2026_71887.nasl

Version: 1.1

Type: Local

Agent: unix

Family: Misc.

Published: 10/3/2026

Updated: 10/3/2026

Supported Sensors: Frictionless Assessment Agent, Nessus Agent, Agentless Assessment, Tenable Cloud Security, Tenable Self-Hosted Container Security, Nessus

Risk Information

VPR

Risk Factor: Low

Score: 3

Percentile: 23.67

CVSS v2

Risk Factor: High

Base Score: 7.5

Temporal Score: 6.4

Vector: CVSS2#AV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS Score Source: CVE-2026-71887

CVSS v3

Risk Factor: Critical

Base Score: 9.8

Temporal Score: 9

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Temporal Vector: CVSS:3.0/E:U/RL:U/RC:C

CVSS v4

Risk Factor: High

Base Score: 8.2

Threat Score: 4.6

Threat Vector: CVSS:4.0/E:U

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

Vulnerability Information

CPE: cpe:/o:debian:debian_linux:12.0, cpe:/o:debian:debian_linux:13.0, p-cpe:/a:debian:debian_linux:bouncycastle

Required KB Items: Host/local_checks_enabled, Host/cpu, global_settings/vendor_unpatched, Host/OS/identifier

Exploit Ease: No known exploits are available

Vulnerability Publication Date: 10/3/2026

Reference Information

CVE: CVE-2026-71887

Detections

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