CVE-2026-34182: One-Byte Auth Tag Vulnerability in CMS Parsing for OpenSSL/wolfSSL/Bouncy Castle/GnuPG

Vulnerability Overview Vulnerability Name: CVE-2026-34182 Description: This vulnerability involves a weakness in a one-byte tag affecting multiple cryptographic libraries (OpenSSL, wolfSSL, Bouncy Castle, GnuPG). Core Issue: During PKCS#7/CMS parsing, the length of the authentication tag generated by AES-GCM is incorrectly set to 1 byte, allowing attackers to forge valid messages via brute force. Affected Scope Affected Libraries: - OpenSSL - wolfSSL - Bouncy Castle - GnuPG (gpgsm) Specific Impacts: - OpenSSL: Accepts 1-byte authentication tags, leading to reduced security. - wolfSSL: Also accepts 1-byte authentication tags. - Bouncy Castle: Although it enforces a minimum limit of 4 bytes, it skips length checks during decryption. - GnuPG: Uses the wolfSSL code path; while it retains an extra layer of protection, it ultimately accepts 1-byte authentication tags. Remediation Root Cause: Authentication tag length should not be embedded as a variable parameter within the ciphertext; it should be determined by the key and algorithm. Recommendations: - Best Practices: Ciphertext formats should be as simple as possible, avoiding any metadata. - Key Management: Use local key IDs to look up key material instead of global key IDs. - Multi-tenant Environments: Create named key buckets for each tenant to avoid the use of global key IDs. Code Example Summary Key Takeaway: Ciphertext format is an attack surface, and any parameters should be selectable by the attacker. Recommendation: When constructing keys, carry your own parameters. Keep the ciphertext as close to a "opaque blob plus local key handle" as possible, and exercise caution with every field.

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CVE-2026-34182: One-Byte Auth Tag Vulnerability in CMS Parsing for OpenSSL/wolfSSL/Bouncy Castle/GnuPG