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(non-US) DIR-842/E :: HW Rev. R1 :: FW v3.0.2 :: Telnetd brute-force protection bypass


On February 13, 2021, a 3rd party security researcher submitted a report accusing the DIR-842/E Rev. R1 using firmware v3.0.2 of a LAN-side telnet authentication brute-force protection mechanism vulnerability   The Vulnerability was confirmed and a patch was issued too close the reported issue.  Please find the beta/hotfix release below.

3rd Party Report information


          - Report: Link to Public Disclosure from Author HERE

          - Reference 
CVE-2021-27342 DIR-842E Brute Force Protection Bypass


             The vulnerability stems from the fact that telnetd only sleeps 3 seconds on *invalid* login attempts (to prevent brute-force attacks), but responds with a "Welcome" immediately to valid login attempts. This opens the door to a timing-based side-channel attack:

An attacker can send a telnet login request and then only wait, for example, 0.05 seconds to figure out if the password was correct. If the login was correct, a "Welcome" would be immediately sent back. Otherwise, if the password wasn't correct, nothing will be sent back. 

So after 0.05 seconds the attacker knows if the password was correct, and if it wasn't, the attacker can create a new connection to try another password right away. This bypasses the intended 3 second cool-down period between failed login attempts.


 Affected Models


Model Hardware Revision Affected FW Fixed FW Recommendation  Last Updated
DIR-842E All R1 Hardware Revisions v3.0.2 & Below v3.0.2_Hotfix Please Download and Update Device 05/14//2021


Regarding Security patch for your D-Link Devices
Firmware updates address the security vulnerabilities in affected D-Link devices. D-Link will update this continually and we strongly recommend all users to install the relevant updates.
Please note that this is a device beta software, beta firmware, or hot-fix release which is still undergoing final testing before its official release. The beta software, beta firmware, or hot-fix is provided on an “as is” and “as available” basis and the user assumes all risk and liability for use thereof. D-Link does not provide any warranties, whether express or implied, as to the suitability or usability of the beta firmware. D-Link will not be liable for any loss, whether such loss is direct, indirect, special or consequential, suffered by any party as a result of their use of the beta firmware.
As there are different hardware revisions on our products, please check this on your device before downloading the correct corresponding firmware update. The hardware revision information can usually be found on the product label on the underside of the product next to the serial number. Alternatively, they can also be found on the device web configuration.