CVE-2019-1543 in Enterprise Manager Base Platform
Summary
by MITRE
ChaCha20-Poly1305 is an AEAD cipher, and requires a unique nonce input for every encryption operation. RFC 7539 specifies that the nonce value (IV) should be 96 bits (12 bytes). OpenSSL allows a variable nonce length and front pads the nonce with 0 bytes if it is less than 12 bytes. However it also incorrectly allows a nonce to be set of up to 16 bytes. In this case only the last 12 bytes are significant and any additional leading bytes are ignored. It is a requirement of using this cipher that nonce values are unique. Messages encrypted using a reused nonce value are susceptible to serious confidentiality and integrity attacks. If an application changes the default nonce length to be longer than 12 bytes and then makes a change to the leading bytes of the nonce expecting the new value to be a new unique nonce then such an application could inadvertently encrypt messages with a reused nonce. Additionally the ignored bytes in a long nonce are not covered by the integrity guarantee of this cipher. Any application that relies on the integrity of these ignored leading bytes of a long nonce may be further affected. Any OpenSSL internal use of this cipher, including in SSL/TLS, is safe because no such use sets such a long nonce value. However user applications that use this cipher directly and set a non-default nonce length to be longer than 12 bytes may be vulnerable. OpenSSL versions 1.1.1 and 1.1.0 are affected by this issue. Due to the limited scope of affected deployments this has been assessed as low severity and therefore we are not creating new releases at this time. Fixed in OpenSSL 1.1.1c (Affected 1.1.1-1.1.1b). Fixed in OpenSSL 1.1.0k (Affected 1.1.0-1.1.0j).
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Analysis
by VulDB Data Team • 11/26/2024
The ChaCha20-Poly1305 cipher implemented in OpenSSL contains a critical vulnerability related to nonce handling that compromises cryptographic security guarantees. This vulnerability stems from OpenSSL's deviation from RFC 7539 standards, which explicitly mandates a 96-bit (12-byte) nonce for ChaCha20-Poly1305 operations. The cipher's security model fundamentally relies on nonce uniqueness to maintain confidentiality and integrity properties, making proper nonce management essential for cryptographic strength.
The technical flaw manifests in OpenSSL's permissive nonce length handling where the implementation accepts nonces up to 16 bytes in length, despite RFC 7539's specification requiring exactly 12 bytes. When users configure nonces longer than 12 bytes, OpenSSL processes only the final 12 bytes while silently ignoring the leading bytes. This behavior creates a dangerous scenario where applications might believe they are using unique nonces when in fact they are reusing the last 12 bytes, leading to nonce reuse vulnerabilities. The CWE-327 weakness classification applies here as this vulnerability involves the use of weak cryptographic primitives through improper implementation of nonce handling mechanisms.
The operational impact of this vulnerability extends beyond simple cryptographic failure to encompass potential data compromise and integrity breaches. Applications that modify default nonce lengths to exceed 12 bytes and subsequently change leading bytes to create new nonces are particularly at risk, as the ignored bytes do not contribute to the cipher's integrity protection. This means that any application relying on the integrity of these ignored leading bytes may experience further security degradation, as the Poly1305 authenticator does not validate these portions of the nonce. The attack surface is further expanded when considering that applications using the cipher directly, rather than through SSL/TLS protocols, are vulnerable since OpenSSL's internal usage remains safe due to its default nonce handling practices.
Mitigation strategies must address both immediate and long-term security concerns for affected systems. Organizations should immediately review all applications utilizing OpenSSL's ChaCha20-Poly1305 cipher with custom nonce lengths to ensure proper nonce generation and uniqueness enforcement. The recommended approach involves either reverting to the standard 12-byte nonce length or implementing robust nonce management that explicitly accounts for the ignored leading bytes. System administrators should upgrade to OpenSSL versions 1.1.1c or 1.1.0k, which contain the necessary patches to address the vulnerability. The ATT&CK framework's T1552.001 technique for "Unsecured Credentials" and T1074.001 for "Data Staged" could be relevant in scenarios where compromised confidentiality leads to credential exposure or data staging attacks. Additionally, the vulnerability's classification as low severity by OpenSSL's security team reflects the limited scope of deployments affected, though security practitioners should not dismiss its potential impact on specific high-value applications. The patch implementation addresses the core issue by ensuring that nonces exceeding 12 bytes are properly validated or rejected, thereby restoring compliance with RFC 7539 standards and maintaining cryptographic security guarantees.