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GHSA-jvvf-x445-j334
fast-uriImpact
fast-uri's mailto scheme parser compares each query field name to the reserved names (to, subject, body) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as %74o (percent-encoded to) is not recognized as a recipient at parse time (parse().to shows only the legitimate recipient) but materializes as a literal to= field after serialize(), and reparsing then treats it as a recipient. The same technique smuggles subject and body through %73ubject and %62ody.
An application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on parse().to, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra to= sees nothing, because the injected field appears only after fast-uri serializes.
Patches
Upgrade to fast-uri 4.1.5.
Workarounds
Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.
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The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. 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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