GHSA-JVVF-X445-J334
Vulnerability from github – Published: 2026-09-29 23:51 – Updated: 2026-09-29 23:51Impact
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.
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "fast-uri"
},
"ranges": [
{
"events": [
{
"introduced": "4.1.3"
},
{
"fixed": "4.1.5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-86818"
],
"database_specific": {
"cwe_ids": [
"CWE-172",
"CWE-436"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-29T23:51:16Z",
"nvd_published_at": "2026-09-15T11:17:12Z",
"severity": "MODERATE"
},
"details": "### Impact\n\n`fast-uri`\u0027s `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`.\n\nAn 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.\n\n### Patches\n\nUpgrade to `fast-uri` 4.1.5.\n\n### Workarounds\n\nPercent-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.",
"id": "GHSA-jvvf-x445-j334",
"modified": "2026-09-29T23:51:16Z",
"published": "2026-09-29T23:51:16Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86818"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/commit/f40a88f33e684a46faec3f5b820bcbb1e85add64"
},
{
"type": "WEB",
"url": "https://cna.openjsf.org/security-advisories.html"
},
{
"type": "PACKAGE",
"url": "https://github.com/fastify/fast-uri"
},
{
"type": "WEB",
"url": "https://github.com/fastify/fast-uri/releases/tag/v4.1.5"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.