Detection of undesired computer files in archives

1. A computer-implemented method comprising:

receiving, at a network gateway computer system logically interposed between a client workstation and an email server, an archive file that is in transit across a network as an attachment to an email message destined for the client workstation; and

without decrypting and without decompressing contents of the archive file:

identifying, by an anti-virus detection module running on the network gateway computer system, a type and associated structure of the archive file by examining (i) one or more primary identification bytes stored within the archive file that contain information intended to identify the type of the archive file or (ii) one or more secondary identification bytes stored within the archive file that contain identifiable data intended for a purpose other than identifying the type of the archive file;

based on the identified type and the associated structure, obtaining, by the anti-virus detection module, descriptive information from the archive file describing a contained file of a plurality of contained files within the archive file, the descriptive information including at least a hash value of the contained file in uncompressed format;

evaluating, by the anti-virus detection module, the descriptive information by comparing the descriptive information to signatures of known malicious or known undesired computer files;

if the evaluating results in a determination that the contained file is a known malicious or a known undesired computer file, then producing, by the anti-virus detection module, a clean version of the archive file by removing the contained file from the archive file and regenerating the archive file without the contained file; and

delivering, by the network gateway computer system, the clean version of the archive file to the client workstation.

2. The method of claim 1 , wherein the descriptive information includes a size of the contained file in uncompressed format.

3. The method of claim 1 , wherein the descriptive information includes a hash value of the contained file in compressed format and a size of the contained file in compressed format.

4. The method of claim 1 , wherein the descriptive information includes a size of the contained file in uncompressed format, a size of the contained file in compressed format and a file name of the contained file.

5. The method of claim 1 , wherein said receiving, at a network gateway computer system logically interposed between a client workstation and an email server, an archive file that is in transit across a network as an attachment to an email message destined for the client workstation comprises intercepting the email message by a transparent proxy running on the network gateway computer, the transparent proxy implementing simple mail transfer protocol (SMTP) handlers for a subset of SMTP commands and replies.

6. The method of claim 5 , wherein prior to said identifying, by an anti-virus detection module running on the network gateway computer system, a type and associated structure of the archive file, determining, by the transparent proxy, whether there exist one or more attachments associated with the email message and if so, then causing the one or more attachments to be scanned by the anti-virus detection module.

7. The method of claim 1 , further comprising responsive to the determination that the contained file is a known malicious or a known undesired computer file, issuing an alert to a user or administrator indicating detection of the malicious or undesired computer file and describing any action taken.

8. The method of claim 7 , where information about the malicious or undesired computer file and any action taken is stored in a log file.

9. A computer-implemented method comprising:

receiving, at a network gateway computer system logically interposed between a client workstation and an email server, an archive file that is in transit across a network as an attachment to an email message destined for the client workstation; and

without decrypting and without decompressing contents of the archive file:

identifying, by an anti-virus detection module running on the network gateway computer system, a type and associated structure of the archive file by examining (i) one or more primary identification bytes stored within the archive file that contain information intended to identify the type of the archive file or (ii) one or more secondary identification bytes stored within the archive file that contain identifiable data intended for a purpose other than identifying the type of the archive file;

based on the identified type and the associated structure, obtaining, by the anti-virus detection module, descriptive information from the archive file describing a contained file of a plurality of contained files within the archive file, the descriptive information including at least a digital signature and a size of the contained file in compressed format;

evaluating, by the anti-virus detection module, the descriptive information by comparing the descriptive information to signatures of known malicious or known undesired computer files;

if the evaluating results in a determination that the contained file is a known malicious or a known undesired computer file, then producing, by the anti-virus detection module, a clean version of the archive file by removing the contained file from the archive file and regenerating the archive file without the contained file; and

delivering, by the network gateway computer system, the clean version of the archive file to the client workstation.

10. The method of claim 9 , wherein said receiving, at a network gateway computer system logically interposed between a client workstation and an email server, an archive file that is in transit across a network as an attachment to an email message destined for the client workstation comprises intercepting the email message by a transparent proxy running on the network gateway computer, the transparent proxy implementing simple mail transfer protocol (SMTP) handlers for a subset of SMTP commands and replies.

11. The method of claim 10 , wherein prior to said identifying, by an anti-virus detection module running on the network gateway computer system, a type and associated structure of the archive file, determining, by the transparent proxy, whether there exist one or more attachments associated with the email message and if so, then causing the one or more attachments to be scanned by the anti-virus detection module.

12. The method of claim 9 , further comprising responsive to the determination that the contained file is a known malicious or a known undesired computer file, issuing an alert to a user or administrator indicating detection of the malicious or undesired computer file and describing any action taken.

13. The method of claim 12 , where information about the malicious or undesired computer file and any action taken is stored in a log file.

14. A computer-implemented method comprising:

receiving, at a network gateway computer system logically interposed between a client workstation and an email server, an archive file that is in transit across a network as an attachment to an email message destined for the client workstation; and

without decrypting and without decompressing contents of the archive file:

identifying, by an anti-virus detection module running on the network gateway computer system, a type and associated structure of the archive file by examining (i) one or more primary identification bytes stored within the archive file that contain information intended to identify the type of the archive file or (ii) one or more secondary identification bytes stored within the archive file that contain identifiable data intended for a purpose other than identifying the type of the archive file;

based on the identified type and the associated structure, obtaining, by the anti-virus detection module, descriptive information from the archive file describing a contained file of a plurality of contained files within the archive file, wherein a type, form or amount of the descriptive information varies for a plurality of different types of archive file formats;

evaluating, by the anti-virus detection module, the descriptive information by comparing the descriptive information to signatures of known malicious or known undesired computer files;

if the evaluating results in a determination that the contained file is a known malicious or a known undesired computer file, then producing, by the anti-virus detection module, a clean version of the archive file by removing the contained file from the archive file and regenerating the archive file without the contained file; and

delivering, by the network gateway computer system, the clean version of the archive file to the client workstation.

15. The method of claim 14 , wherein the descriptive information is comprised essentially of a hash value of the contained file in uncompressed format.

16. The method of claim 14 , wherein, for a first archive file type of the plurality of different types of archive files, the descriptive information includes a hash value of the contained file in uncompressed format and a size of the contained file in uncompressed format.

17. The method of claim 16 , wherein, for a second archive file type of the plurality of different types of archive files, the descriptive information includes a hash value of the contained file in compressed format and a size of the contained file in compressed format.

18. The method of claim 16 , wherein, for a second archive file type of the plurality of different types of archive files, the descriptive information includes the hash value of the contained file in uncompressed format, the size of the contained file in uncompressed format, a size of the contained file in compressed format and a size of the contained file in compressed format.

19. The method of claim 16 , wherein, for a second archive file type of the plurality of different types of archive files, the descriptive information includes a size of the contained file in compressed format, the size of the contained file in uncompressed format and a compression type.

20. The method of claim 16 , wherein, for a second archive file type of the plurality of different types of archive files, the descriptive information includes a digital signature and a size of the contained file in compressed format.

21. The method of claim 16 , wherein, for a second archive file type of the plurality of different types of archive files, the descriptive information includes the hash value of the contained file in uncompressed format, the size of the contained file in uncompressed format, a size of the contained file in compressed format and a file name of the contained file.