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Why is it necessary to limit server access to internal network addresses in the place where external links can be filled in the background?

📅 2026-11-01 👁️ 0

Why does the server need to be restricted from accessing the internal network address when it can fill in the external link? Because these input boxes are not for users to see, the server will replace the user to retrieve the address. Once the link verification is bypassed, the request will be initiated from within, targeting internal services in the link-local address segment, and the external input thus obtains an internal network perspective. This is the cause of server-side request forgery: the vulnerability is not in the page, but in the fact that 'the server initiates the request for you.'

Who does the server replace to access this address?

There are three common entry points: fetching images when filling in remote image addresses, bulk extraction when importing external content, and checking the reachability when configuring callback addresses or friendship links in the background. The common point of the three is that the program sends requests directly to the strings submitted by the user as addresses, rather than confirming whether they point to externally reachable targets on the public network first.

WordPress released the 7.0.3 security version on August 6, 2026, which listed a server-side request forgery issue in the link verification, indicating that requests are allowed to be sent to the link-local address space, reported by an independent reporter. This was included in the same fix list as the multi-site registration permission escalation, several stored cross-site scripting and information leakage items.

Why is link-local address sensitive

Services on the link-local segment are typically assumed to be accessible only by the local machine or the same network segment: the management panel, metric ports, caching services, and task queues are all located here. They do not perform authentication, relying on the premise that 'external cannot reach'. Once the on-site program is allowed to make requests to these addresses, the premise is gone, and the attacker does not need to directly connect to the internal network, but can read back the content with a single input box on the back-end.

Risk stratification makes it more intuitive: read-back status is information leakage; being able to carry a request body is internal network write operation; the rendered content is data leakage. The three capabilities have a big difference in value to the attacker, but for the defender, all three exist as long as the request is allowed to be initiated.

Check should be done at the time of writing or at the time of request

Both are required, neither can be omitted. The verification during writing is for format and address segment judgment: parse out the actual IP, reject loopback, private and link-local segments, and avoid using string prefix judgment in this way. The pre-verification before the request is for the second check after redirect following, because once redirected to a public domain name and the next hop back to the intranet, it is the most common way to bypass this kind of thing.

The defense line that only performs write-time verification will be redirected around; the dirty addresses that only perform pre-request verification have already been stored, and other call points may not check again. In engineering, an additional layer of backup is needed: limit the network target of service accounts to only allow access to necessary public network address segments, and hand over the issue of 'whether it can go out' to the network layer rather than the code.

Which functions within the station will hit this path

Compare with the implementation of the self-built program, importing and fetching images remotely is the most prone to errors. The document content import interface of Anqi CMS uses the import and archive actions, and when importing external compressed packages and tables in batches, it will also handle the remote resource addresses contained in them; the interface documentation can find the frequently used API calling capabilities, and fields such as uploading images and site contact information also accept external addresses. The orientation of these entry points is 'receive first and then process', which is exactly the position where double verification and network restriction are needed.

Version upgrade should also be checked. Anqi CMS fixed the injection issue of list sorting parameters and the link where the login credentials of site switching can be forged in v3.6.6. The upgrade recommendation requires the rotation of site signature keys and the modification of the administrator password. After the change of the verification logic, adjacent aspects such as credentials and permissions should also be reviewed, not just focus on a single path.

Common questions

Q: Is it enough to only verify the address format? A: No. You should first parse out the actual IP and then judge the address segment, and re-check after following the redirect.

What about limiting outgoing network traffic compared to modifying the code?\nAs a fallback, it is. However, if the business indeed needs to access external addresses, the functionality still requires two layers of verification when writing and when making requests.

How can you confirm if your website has such entry points?\nList all the fields that may initiate external requests from the server, focusing on the configuration of import, remote image retrieval, and callback address.

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