Software Engineering Institute

CWE-295: Improper Certificate Validation

RSA signatures are used to authenticate the source of a message. To prevent RSA signatures from being forged, messages are padded with data to ensure message hashes are adequately sized. The Public-Key Cryptography Standard #1 version 1.5 (PKCS#1 v1.5), which is defined in RFC 2313, specifies "the mathematical properties and format of RSA public and private keys (ASN.1-encoded in clear-text), and the basic algorithms and encoding/padding schemes for performing RSA encryption, decryption, and producing and verifying signatures" (Wikipedia). The Mozilla Network Security Services (NSS) library incorrectly parses PKCS#1 v1.5 padded signatures due to the use of a vulnerable implementation of ASN.1 encoding of DigestInfo. Vulnerable implementations parse the DigestInfo field using the BER encoding, which allows multiple ways of encoding the same ASN.1 object. The parser implementation allows for bytes to skip validation, allowing an attacker to forge a signature when a RSA key with a low public exponent (e.g., three) is used.

This vulnerability is a variant of the Bleichenbacher vulnerability, where unvalidated bytes are allowed in the least significant bytes of the signature.

Mozilla NSS is used by other software products including a number of Linux distributions and packages, Google Chrome, and Google Chrome OS. It is also possible that other cryptographic libraries may have similar vulnerabilities.

This vulnerability may allow an attacker to forge a RSA signature, such as a SSL certificate.

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Mozilla has released patch for this vulnerability (MSFA 2014-73). Mozilla NSS is used by other software products including a number of Linux distributions and packages, Google Chrome, and Google Chrome OS.