CVE-2018-16869 in Nettle
Summary
by MITRE
A Bleichenbacher type side-channel based padding oracle attack was found in the way nettle handles endian conversion of RSA decrypted PKCS#1 v1.5 data. An attacker who is able to run a process on the same physical core as the victim process, could use this flaw extract plaintext or in some cases downgrade any TLS connections to a vulnerable server.
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Analysis
by VulDB Data Team • 04/16/2020
This vulnerability represents a critical side-channel attack exploiting timing variations in cryptographic processing within the nettle cryptographic library. The flaw specifically targets the handling of RSA decrypted PKCS#1 v1.5 data during endian conversion operations, creating a padding oracle condition that allows attackers to infer sensitive information through carefully crafted timing measurements. The vulnerability stems from the predictable timing differences that occur when processing different padding structures, enabling an attacker to iteratively determine the correct padding bytes and ultimately recover plaintext data. This type of attack falls under the broader category of Bleichenbacher attacks, which exploit weaknesses in PKCS#1 v1.5 padding validation mechanisms. The security implications are particularly severe because the attack requires only co-location on the same physical core rather than network-based access, making it applicable in environments where process isolation is not properly enforced.
The technical implementation of this vulnerability exploits the fact that nettle's RSA decryption routine performs endian conversion operations that exhibit measurable timing variations based on the validity of padding bytes. When processing RSA-decrypted data, the library's handling of byte order conversion creates distinguishable execution paths that leak information about the padding structure. An attacker can leverage this timing information to perform a padding oracle attack, where each timing measurement provides a bit of information about the decrypted data. This approach allows for the gradual reconstruction of the plaintext through iterative queries, effectively bypassing the security guarantees that PKCS#1 v1.5 padding is designed to provide. The vulnerability specifically affects TLS implementations that rely on nettle for cryptographic operations, creating potential for man-in-the-middle attacks where attackers can downgrade connections to weaker cryptographic parameters or extract sensitive data from encrypted communications. This aligns with CWE-310 and follows patterns identified in ATT&CK technique T1071.004 for application layer protocol manipulation.
The operational impact of this vulnerability extends beyond simple data theft to encompass connection downgrade capabilities that can severely compromise TLS security. An attacker positioned on the same physical core can potentially extract sensitive information from TLS connections, including session keys, certificates, or other cryptographic material that would normally remain protected. The vulnerability is particularly concerning in virtualized environments where multiple tenants share the same physical hardware, as it can be exploited to compromise communications between different applications or users. This type of attack can be particularly devastating in cloud environments where process isolation is not absolute, or in scenarios involving containerized applications where shared CPU resources create opportunities for timing-based side-channel attacks. The ability to downgrade TLS connections means that even if the server implements strong cryptographic protocols, the attacker can force connections to use weaker cipher suites or protocols, effectively weakening the entire security posture of the communication channel. This vulnerability demonstrates the critical importance of considering side-channel attacks in cryptographic implementations and highlights the need for constant vigilance in protecting against timing-based information leakage that can undermine even the strongest cryptographic algorithms.
Mitigation strategies for this vulnerability require both immediate patching of affected nettle library versions and implementation of architectural protections against timing-based side-channel attacks. The primary solution involves updating to patched versions of the nettle library that eliminate the timing variations in endian conversion operations and implement constant-time processing for padding validation. Organizations should also consider implementing countermeasures such as disabling or restricting access to shared physical cores where possible, or implementing additional timing randomization techniques that obscure the timing variations that attackers rely upon. For systems where patching may not be immediately possible, administrators should consider deploying additional monitoring to detect unusual timing patterns that might indicate exploitation attempts. The vulnerability also underscores the importance of following security best practices such as implementing proper process isolation, utilizing dedicated cryptographic hardware when possible, and ensuring that cryptographic implementations are designed with side-channel resistance as a primary concern. Network segmentation and monitoring solutions can help detect and respond to potential exploitation attempts, while regular security assessments should include testing for timing-based vulnerabilities in cryptographic implementations.