Important note about SSL VPN compatibility for 20.0 MR1 with EoL SFOS versions and UTM9 OS. Learn more in the release notes.

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Sophos Firewall v21.0 GA - Kyber TLS (Edge/Chrome) connection reset error for transparent TLS decryption

AIFS IT Support

We recently upgraded our Sophos XGS 4300 to SFOS v21. Since then, we are finding that a number of our users were receieving connection reset messages in their browser (Edge and Chrome) when attempting to access some websites with transparent TLS decryption enabled.

The error is: Dropped due to TLS engine error: FLOW_TIMEOUT[5]

Example websites we receive the error in are: www.theage.com.au www.news.com.au

The issue occurs regardless of if the site is available over TLSv1.2 or TLSv1.3 and regardless of the cipher suite being used.

Upon further investigation, we've found that the issue appears to relate to the PostQuantumKeyAgreementEnabled policy in Edge and Chrome.

Acocrding to the Edge policy docs, the policy:

"Offering a post-quantum key agreement is backwards-compatible. Existing TLS servers and networking middleware are expected to ignore the new option and continue selecting previous options.

However, devices that don't implement TLS correctly may malfunction when offered the new option. For example, they might disconnect in response to unrecognized options or the resulting larger messages. These devices are not post-quantum-ready and will interfere with an enterprise's post-quantum transition. If this issue is encountered, administrators should contact the vendor for a fix."

We were wondering if this is a known issue with the GA release of SFOS 21 or if others are encountering the same issue?

We are using the current Edge (131.0.2903.70) and Chrome (131.0.6778.86) releases.



Edited TAGs
[edited by: Erick Jan at 3:59 AM (GMT -8) on 2 Dec 2024]
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    Hi  ,

    The much larger TLS Client Hello resulting from the newly supported Kyber Key Exchange algorithm by browsers, or ML-KEM in its recently standardized form, is indeed causing problems in networks worldwide.

    In the case of the Sophos firewall, it has long been supporting this larger TLS Client Hello in the TLS connection negotiation. But a problem arises in those networks where some network device along the packet route, on the TCP server side of the firewall, does not support (or have malfunctioning) IP fragmentation.

    See this KB Article for a possible workaround (bottom section: "Sophos Firewall"):
    https://support.sophos.com/support/s/article/KBA-000009276

    In the meantime, we are making changes to SFOS to deal with this situation on the firewall itself. This work is targeted to 21.0 MR1 and 21.5 GA.

    Hope this helps.

    Kind regards,

    Elardus

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