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GHSA-gp7f-rwcx-9369
No affected components available
jsoup may incorrectly sanitize HTML including javascript: URL expressions, which could allow cross-site scripting (XSS) attacks when a reader subsequently clicks that link. If the non-default SafeList.preserveRelativeLinks option is enabled, HTML including javascript: URLs that have been crafted with control characters will not be sanitized. If the site that this HTML is published on does not set a Content Security Policy, an XSS attack is then possible.
Impact
Sites that accept input HTML from users and use jsoup to sanitize that HTML, may be vulnerable to cross-site scripting (XSS) attacks, if they have enabled SafeList.preserveRelativeLinks and do not set an appropriate Content Security Policy.
Patches
This issue is patched in jsoup 1.15.3.
Users should upgrade to this version. Additionally, as the unsanitized input may have been persisted, old content should be cleaned again using the updated version.
Workarounds
To remediate this issue without immediately upgrading:
- disable
SafeList.preserveRelativeLinks, which will rewrite input URLs as absolute URLs - ensure an appropriate Content Security Policy is defined. (This should be used regardless of upgrading, as a defence-in-depth best practice.)
Background and root cause
jsoup includes a Cleaner component, which is designed to sanitize input HTML against configurable safe-lists of acceptable tags, attributes, and attribute values.
This includes removing potentially malicious attributes such as <a href="javascript:...">, which may enable XSS attacks. It does this by validating URL attributes against allowed URL protocols (e.g. http, https).
However, an attacker may be able to bypass this check by embedding control characters into the href attribute value. This causes the Java URL class, which is used to resolve relative URLs to absolute URLs before checking the URL's protocol, to treat the URL as a relative URL. It is then resolved into an absolute URL with the configured base URI.
For example, java\tscript:... would resolve to https://example.com/java\tscript:....
By default, when using a safe-list that allows a tags, jsoup will rewrite any relative URLs (e.g. /foo/) to an absolute URL (e.g. https://example.com/foo/). Therefore, this attack attempt would be successfully mitigated. However, if the option SafeList.preserveRelativeLinks is enabled (which does not rewrite relative links to absolute), the input is left as-is.
While Java will treat a path like java\tscript: as a relative path, as it does not match the allowed characters of a URL spec, browsers may normalize out the control characters, and subsequently evaluate it as a javascript: spec inline expression. That disparity then leads to an XSS opportunity.
Sites defining a Content Security Policy that does not allow javascript expressions in link URLs will not be impacted, as the policy will prevent the script's execution.
For more information
If you have any questions or comments about this advisory:
- Open an issue in jsoup
- Email the author of jsoup at jonathan@hedley.net
Credits
Thanks to Jens Häderer, who reported this issue, and contributed to its resolution.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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