CVE-2026-42784: Sequoia-OpenPGP Auth Bypass Vulnerability

CVE-2026-42784 Overview

CVE-2026-42784 is a cryptographic verification flaw in the sequoia-openpgp library. The library incorrectly infers key flags for older certificates when a key flags subpacket is missing. This creates a discrepancy in how key capabilities are interpreted across code paths. Attackers exploit the inconsistency to bypass the back-signature check on subkey binding signatures. Successful exploitation allows an attacker to bind an arbitrary subkey to their own certificate and forge signatures that appear valid. The flaw undermines the trust model of OpenPGP signatures processed by affected sequoia-openpgp versions and is tracked under [CWE-347: Improper Verification of Cryptographic Signature].

Critical Impact

Attackers can forge OpenPGP signatures and illegitimately bind arbitrary subkeys, fully compromising cryptographic integrity for downstream verifiers.

Affected Products

• sequoia-openpgp library (Rust OpenPGP implementation)
• Applications and distributions bundling vulnerable versions of sequoia-openpgp
• Red Hat ecosystem components consuming sequoia-openpgp (see Red Hat advisory)

Discovery Timeline

• 2026-09-16 - CVE-2026-42784 published to the National Vulnerability Database (NVD)
• 2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-42784

Vulnerability Analysis

The vulnerability resides in how sequoia-openpgp handles legacy OpenPGP certificates that lack an explicit key flags subpacket. When the subpacket is absent, the library falls back to inferring key capabilities from surrounding metadata. That inference is not consistent with the checks performed during subkey binding validation. Specifically, the back-signature verification path treats a subkey as non-signing while another path treats it as signing-capable. This mismatch is the pivot that enables forgery. An attacker crafts a certificate that exploits the inconsistency to attach an arbitrary subkey, then produces signatures that verifiers accept as authentic.

Root Cause

The root cause is inconsistent enforcement of key capability semantics when a key flags subpacket is missing on older certificates. The library relies on implicit assumptions rather than deterministic policy, weakening the guarantees of the back-signature check that is designed to prove a subkey consents to being bound to a primary key.

Attack Vector

Exploitation is remote and requires user interaction, such as importing a crafted certificate or verifying an attacker-supplied signed artifact. No privileges are required on the target system. The scope is changed because the trust decision affects the calling application and any downstream consumers of the verification result.

No public proof-of-concept code is available. Technical details are described in the Red Hat CVE record and Red Hat Bug Report #2464122.

Detection Methods for CVE-2026-42784

Indicators of Compromise

• OpenPGP certificates lacking a key flags subpacket on the primary key while presenting subkeys with unexpected capability usage.
• Signature verification events where a subkey appears bound without a valid, capability-consistent back-signature.
• Unexpected new subkeys appearing on trusted certificates after certificate refresh from untrusted sources.

Detection Strategies

• Inventory applications and packages that link against sequoia-openpgp and compare installed versions against the fixed release referenced in the vendor advisory.
• Audit certificate stores and keyrings for older certificates missing key flags subpackets and validate them with an unaffected OpenPGP implementation.
• Log and review OpenPGP verification outcomes for anomalies such as new signing subkeys not previously observed on known certificates.

Monitoring Recommendations

• Monitor package management events for installation or update of components that embed sequoia-openpgp.
• Alert on ingestion of externally sourced OpenPGP certificates into signing or release pipelines.
• Track signature verification failures and successes in build systems, mail gateways, and software update mechanisms that use OpenPGP.

How to Mitigate CVE-2026-42784

Immediate Actions Required

• Identify all systems and applications that depend on sequoia-openpgp and prioritize upgrade to a patched release.
• Re-verify any recently imported OpenPGP certificates and signatures with a known-good implementation until patched versions are deployed.
• Restrict certificate imports in production signing and verification workflows to trusted sources only.

Patch Information

Refer to the Red Hat CVE record for CVE-2026-42784 and Red Hat Bug Report #2464122 for fixed package versions and distribution-specific updates. Rebuild or repackage downstream software that statically links vulnerable versions of sequoia-openpgp once fixed upstream releases are available.

Workarounds

• Reject OpenPGP certificates that omit a key flags subpacket on the primary key when policy allows stricter validation.
• Cross-verify high-value signatures with a second OpenPGP implementation until the patched library is deployed.
• Constrain trust roots so that only vetted certificates can produce signatures accepted by internal verification services.

# Configuration example
# Identify Rust crates or system packages using sequoia-openpgp
cargo tree | grep sequoia-openpgp
rpm -qa | grep -i sequoia
dpkg -l | grep -i sequoia