Headline
GHSA-98px-6486-j7qc: Synapse has URL deny list bypass via oEmbed and image URLs when generating previews
Impact
A discovered oEmbed or image URL can bypass the url_preview_url_blacklist setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the url_preview_ip_range_blacklist setting (by default this only allows public IPs) and by the limited information returned to the client:
- For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded.
- For discovered image URLs, any non-image response is discarded.
Systems which have URL preview disabled (via the url_preview_enabled setting) or have not configured a url_preview_url_blacklist are not affected.
Because of the uncommon configuration required, the limited information a malicious user, and the amount of guesses/time the attack would need; the severity is rated as low.
Patches
The issue is fixed by #15601.
Workarounds
The default configuration of the url_preview_ip_range_blacklist should protect against requests being made to internal infrastructure, URL previews of public URLs is expected.
Alternately URL previews could be disabled using the url_preview_enabled setting.
Impact
A discovered oEmbed or image URL can bypass the url_preview_url_blacklist setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the url_preview_ip_range_blacklist setting (by default this only allows public IPs) and by the limited information returned to the client:
- For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded.
- For discovered image URLs, any non-image response is discarded.
Systems which have URL preview disabled (via the url_preview_enabled setting) or have not configured a url_preview_url_blacklist are not affected.
Because of the uncommon configuration required, the limited information a malicious user, and the amount of guesses/time the attack would need; the severity is rated as low.
Patches
The issue is fixed by #15601.
Workarounds
The default configuration of the url_preview_ip_range_blacklist should protect against requests being made to internal infrastructure, URL previews of public URLs is expected.
Alternately URL previews could be disabled using the url_preview_enabled setting.
References
- GHSA-98px-6486-j7qc
- matrix-org/synapse#15601
- https://github.com/matrix-org/synapse/releases/tag/v1.85.0
Related news
Synapse is a Matrix protocol homeserver written in Python with the Twisted framework. A discovered oEmbed or image URL can bypass the `url_preview_url_blacklist` setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the `url_preview_ip_range_blacklist` setting (by default this only allows public IPs) and by the limited information returned to the client: 1. For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded. 2. For discovered image URLs, any non-image response is discarded. Systems which have URL preview disabled (via the `url_preview_enabled` setting) or have not configured a `url_preview_url_blacklist` are not affected. This issue has been addressed in version 1.85.0. Users are advised to upgrade. User unable to upgrade may also disable URL previews.