It was found that the GnuTLS implementation of HMAC-SHA-256 was vulnerable to a Lucky thirteen style attack. Remote attackers could use this flaw to conduct distinguishing attacks and plaintext-recovery attacks via statistical analysis of timing data using crafted packets.
Covert timing channels convey information by modulating some aspect of system behavior over time, so that the program receiving the information can observe system behavior and infer protected information.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Gnutls | Gnu | * | 3.6.12 (excluding) |
| Gnutls26 | Ubuntu | esm-infra-legacy/trusty | * |
| Gnutls26 | Ubuntu | precise/esm | * |
| Gnutls26 | Ubuntu | trusty | * |
| Gnutls26 | Ubuntu | trusty/esm | * |
| Gnutls28 | Ubuntu | bionic | * |
| Gnutls28 | Ubuntu | trusty | * |
| Gnutls28 | Ubuntu | upstream | * |
| Gnutls28 | Ubuntu | xenial | * |
| Red Hat Enterprise Linux 7 | RedHat | gnutls-0:3.3.29-8.el7 | * |
In some instances, knowing when data is transmitted between parties can provide a malicious user with privileged information. Also, externally monitoring the timing of operations can potentially reveal sensitive data. For example, a cryptographic operation can expose its internal state if the time it takes to perform the operation varies, based on the state. Covert channels are frequently classified as either storage or timing channels. Some examples of covert timing channels are the system’s paging rate, the time a certain transaction requires to execute, and the time it takes to gain access to a shared bus.