Re: CVE-2024-40761: Apache Answer: Avatar URL leaked user email addresses

[Demi Marie Obenour <demi () invisiblethingslab com>]

On Wed, Sep 25, 2024 at 06:28:16AM +0000, Enxin Xie wrote:
> Severity: low
>
> Affected versions:
>
> - Apache Answer through 1.3.5
>
> Description:
>
> Inadequate Encryption Strength vulnerability in Apache Answer.
>
> This issue affects Apache Answer: through 1.3.5.
>
> Using the MD5 value of a user's email to access Gravatar is insecure and can lead to the leakage of user email. The 
> official recommendation is to use SHA256 instead.
> Users are recommended to upgrade to version 1.4.0, which fixes the issue.
>
> Credit:
>
> 张岳熙 (reporter)
>
> References:
>
> https://answer.incubator.apache.org
> https://www.cve.org/CVERecord?id=CVE-2024-40761

What is the specific property of SHA256 required here?  Email addresses
have low entropy and I suspect they can be easily brute-forced, so
leaking the SHA256 has is still bad.  Instead, I would use a seeded PRF
with a seed only known to the server, ensuring that the resulting value
does not leak any information about the email.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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