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https://issues.apache.org/jira/browse/NIFI-16056?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18092660#comment-18092660
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Yuanhao Zhu commented on NIFI-16056:
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[~exceptionfactory] Hey David, sure, then probably it's a good idea to mention
it in the admin guide, because figuring this out is quite cumbersome and it's
because of the behavior of the proxy
> X-Forwarded-Host could contain multiple comma-separated values, which leads
> to Invalid Proxy Hosts
> --------------------------------------------------------------------------------------------------
>
> Key: NIFI-16056
> URL: https://issues.apache.org/jira/browse/NIFI-16056
> Project: Apache NiFi
> Issue Type: Bug
> Affects Versions: 2.10.0
> Reporter: Yuanhao Zhu
> Priority: Major
>
> When NiFi sits behind a chain of reverse proxies that each append to
> {{X-Forwarded-Host}} (standard {{apr_table_mergen}} behavior in Apache httpd
> {{mod_proxy}} with {{{}ProxyAddHeaders On{}}}), the header arrives as a
> single field containing a comma-separated list, e.g.:
> {{X-Forwarded-Host: host.example.com, host.example.com}}
> {{ProxyHeaderValidatorCustomizer.processProxyHostHeaders}} reads the first
> field value and, after stripping a single trailing {{{}:port{}}}, compares
> the *entire string* against the request {{Host}} and the
> {{nifi.web.proxy.host}} allow-list. Because it does not split on comma, the
> value {{"host.example.com, host.example.com"}} is never found in the
> allow-list and the request is rejected with {*}HTTP 421 – Invalid Proxy Host
> Requested{*}, even though every individual value is a legitimate,
> allow-listed host.
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