The invention claimed is:1. A computer-implemented method performed by at least one processor, the method comprising:receiving, by the at least one processor, speech data provided by a user during a speech interaction with a conversational user interface (CUI) executing on a computing device;analyzing, by the at least one processor, the speech data to attempt a passive authentication of the user during the speech interaction, wherein the passive authentication is attempted based at least partly on the speech data and does not include explicitly prompting the user for a credential;storing, by the at least one processor, session information related to the speech interaction with the CUI, the session information including an indication of whether the passive authentication of the user has been successful during the speech interaction;in response to determining that the passive authentication of the user has been successful during the speech interaction, storing, with the session information, a time value that indicates a period of time following a cessation of speech interaction between the user and the CUI after which the successful passive authentication expires;receiving, by the at least one processor, a request to access sensitive information associated with the user, the request submitted by the user through the CUI during the speech interaction; andin response to the request: a) determining, from the session information, that the user has been successfully authenticated during the speech interaction, and b) based on the determination that the user has been successfully authenticated, providing, by the at least one processor, access to the sensitive information through the CUI.2. The method of claim 1, wherein the speech data is further analyzed to identify the user among a plurality of users who are registered as users of the computing device.3. The method of claim 1, further comprising:based on a determination that the passive authentication has expired when the request is received, attempting, by the at least one processor, to actively authenticate the user through the CUI.4. The method of claim 3, wherein attempting to actively authenticate the user includes prompting the user to provide, through the CUI, one or more of a personal identification number (PIN), a password, and a passphrase.5. The method of claim 1, further comprising:receiving, by the at least one processor, video data that is captured by at least one camera of the computing device;wherein the video data is analyzed with the speech data to attempt to passively authenticate the user.6. The method of claim 5, wherein analyzing the video data includes one or more of a facial recognition analysis, a posture recognition analysis, a gesture recognition analysis, and a gait recognition analysis.7. The method of claim 1, wherein analyzing the speech data to attempt to passively authenticate the user includes:providing the speech data as input to a model of a speech pattern of the user, the model having been previously developed based on collected speech data of the user;receiving, from the model, a confidence metric indicating a likelihood that the speech data is spoken by the user; anddetermining that the user is authenticated based on the confidence metric exceeding a threshold value.8. The method of claim 1, wherein the analyzed speech data is audio data that is recorded by at least one microphone of the computing device.9. The method of claim 1, wherein the analyzed speech data is text data that is generated by transcribing at least a portion of audio data that is recorded by at least one microphone of the computing device.10. The method of claim 1, wherein the request to access the sensitive information includes one or more of:a request to access financial account information describing at least one account of the user;a request to perform a financial transaction involving at least one account of the user; anda request to perform a funds transfer involving at least one account of the user.11. A system, comprising:at least one processor; anda memory communicatively coupled to the at least one processor, the memory storing instructions which, when executed by the at least one processor, cause the at least one processor to perform operations comprising:receiving speech data provided by a user during a speech interaction with a conversational user interface (CUI) executing on a computing device;analyzing the speech data to attempt a passive authentication of the user during the speech interaction, wherein the passive authentication is attempted based at least partly on the speech data and does not include explicitly prompting the user for a credential;storing session information related to the speech interaction with the CUI, the session information including an indication of whether the passive authentication of the user has been successful during the speech interaction;in response to determining that the passive authentication of the user has been successful during the speech interaction, storing, with the session information, a time value that indicates a period of time following a cessation of speech interaction between the user and the CUI after which the successful passive authentication expires;receiving a request to access sensitive information associated with the user, the request submitted by the user through the CUI during the speech interaction; andin response to the request: a) determining, from the session information, that the user has been successfully authenticated during the speech interaction, and b) based on the determination that the user has been successfully authenticated, providing, by the at least one processor, access to the sensitive information through the CUI.12. The system of claim 11, wherein the speech data is further analyzed to identify the user among a plurality of users who are registered as users of the computing device.13. The system of claim 11, the operations further comprising:based on a determination that the passive authentication has expired when the request is received, attempting, by the at least one processor, to actively authenticate the user through the CUI.14. The system of claim 13, wherein attempting to actively authenticate the user includes prompting the user to provide, through the CUI, one or more of a personal identification number (PIN), a password, and a passphrase.15. The system of claim 11, the operations further comprising:receiving video data that is captured by at least one camera of the computing device;wherein the video data is analyzed with the speech data to attempt to passively authenticate the user.16. The system of claim 15, wherein analyzing the video data includes one or more of a facial recognition analysis, a posture recognition analysis, a gesture recognition analysis, and a gait recognition analysis.17. The system of claim 11, wherein analyzing the speech data to attempt to passively authenticate the user includes:providing the speech data as input to a model of a speech pattern of the user, the model having been previously developed based on collected speech data of the user;receiving, from the model, a confidence metric indicating a likelihood that the speech data is spoken by the user; anddetermining that the user is authenticated based on the confidence metric exceeding a threshold value.18. The system of claim 11, wherein the analyzed speech data is audio data that is recorded by at least one microphone of the computing device.19. The system of claim 11, wherein the analyzed speech data is text data that is generated by transcribing at least a portion of audio data that is recorded by at least one microphone of the computing device.20. One or more non-transitory computer-readable media storing instructions which, when executed by at least one processor, cause the at least one processor to perform operations comprising:receiving speech data provided by a user during a speech interaction with a conversational user interface (CUI) executing on a computing device;analyzing the speech data to attempt a passive authentication of the user during the speech interaction, wherein the passive authentication is attempted based at least partly on the speech data and does not include explicitly prompting the user for a credential;storing session information related to the speech interaction with the CUI, the session information including an indication of whether the passive authentication of the user has been successful during the speech interaction;in response to determining that the passive authentication of the user has been successful during the speech interaction, storing, with the session information, a time value that indicates a period of time following a cessation of speech interaction between the user and the CUI after which the successful passive authentication expires;receiving a request to access sensitive information associated with the user, the request submitted by the user through the CUI during the speech interaction; andin response to the request: a) determining, from the session information, that the user has been successfully authenticated during the speech interaction, and b) based on the determination that the user has been successfully authenticated, providing, by the at least one processor, access to the sensitive information through the CUI.21. The method of claim 3, further comprising:determining that the active authentication of the user is successful; andin response, applying the speech data from the speech interaction with the CUI to refine a model of the user's speech.