BREW-MAILCATCHER-CVE-202… (GHSA-WVG4-6P58-3GH3)
Vulnerability from osv_homebrew – Published: 2026-11-11 09:07 – Updated: 2026-11-11 09:07 – Source websiteSummary
Rack::Static normalizes the request path with Utils.clean_path_info when deciding which file to serve, but matches header_rules against a path that is only URL-decoded. A request containing dot segments, raw or percent-encoded, is served the normalized target file while the header rules for that file's location are not applied.
This allows an attacker to receive a protected static file without the security headers configured for it.
Details
Rack::Static#call computes the served path as Utils.clean_path_info(Utils.unescape_path(path)). Rack::Static#applicable_rules applies only Utils.unescape_path before testing string and regular expression rules. A request for /assets/../uploads/file.html or /assets/..%2fuploads/file.html is served as uploads/file.html, but the decoded path /assets/../uploads/file.html does not start with uploads, so a rule scoped to that directory never matches.
The fix for CVE-2026-34786 (GHSA-q4qf-9j86-f5mh) added URL decoding to rule matching but did not add dot segment normalization, so versions that include that fix remain affected.
Impact
Applications that use Rack::Static header_rules to attach security-relevant headers to particular paths or file types are affected. An unauthenticated attacker who knows the path of a protected file can request it through a dot segment path and receive it without those headers, for example without Content-Disposition: attachment or a restrictive Content-Security-Policy that prevents uploaded HTML from rendering inline. The file itself is served unchanged. Applications that do not use path-specific header_rules are not affected.
Mitigation
- Update to a patched version of Rack in which
header_rulesare matched against the same normalized path used to serve the file. - Reject requests containing dot segments at a reverse proxy or application boundary.
- Do not rely solely on
Rack::Staticheader_rulesto prevent inline rendering of untrusted uploads; serve them from a separate origin.
Credit
- Reporter and Remediation developer
{
"affected": [
{
"ecosystem_specific": {
"fix": null,
"range_state": "affected",
"resource": "rack",
"resource_purl": "pkg:gem/rack@3.2.7",
"upstream_fixed_in": "3.2.8"
},
"package": {
"ecosystem": "Homebrew",
"name": "mailcatcher",
"purl": "pkg:brew/mailcatcher"
},
"ranges": [
{
"events": [
{
"introduced": "0.8.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "RubyGems",
"key": "pkg:gem/rack@3.2.7",
"name": "rack",
"resource": "rack",
"strategy": "registry",
"subject_version": "3.2.7"
}
]
},
"details": "## Summary\n\n`Rack::Static` normalizes the request path with `Utils.clean_path_info` when deciding which file to serve, but matches `header_rules` against a path that is only URL-decoded. A request containing dot segments, raw or percent-encoded, is served the normalized target file while the header rules for that file\u0027s location are not applied.\n\nThis allows an attacker to receive a protected static file without the security headers configured for it.\n\n## Details\n\n`Rack::Static#call` computes the served path as `Utils.clean_path_info(Utils.unescape_path(path))`. `Rack::Static#applicable_rules` applies only `Utils.unescape_path` before testing string and regular expression rules. A request for `/assets/../uploads/file.html` or `/assets/..%2fuploads/file.html` is served as `uploads/file.html`, but the decoded path `/assets/../uploads/file.html` does not start with `uploads`, so a rule scoped to that directory never matches.\n\nThe fix for CVE-2026-34786 (GHSA-q4qf-9j86-f5mh) added URL decoding to rule matching but did not add dot segment normalization, so versions that include that fix remain affected.\n\n## Impact\n\nApplications that use `Rack::Static` `header_rules` to attach security-relevant headers to particular paths or file types are affected. An unauthenticated attacker who knows the path of a protected file can request it through a dot segment path and receive it without those headers, for example without `Content-Disposition: attachment` or a restrictive `Content-Security-Policy` that prevents uploaded HTML from rendering inline. The file itself is served unchanged. Applications that do not use path-specific `header_rules` are not affected.\n\n## Mitigation\n\n* Update to a patched version of Rack in which `header_rules` are matched against the same normalized path used to serve the file.\n* Reject requests containing dot segments at a reverse proxy or application boundary.\n* Do not rely solely on `Rack::Static` `header_rules` to prevent inline rendering of untrusted uploads; serve them from a separate origin.\n\n## Credit\n* Reporter and Remediation developer",
"id": "BREW-mailcatcher-CVE-2026-125110",
"modified": "2026-11-11T09:07:56Z",
"published": "2026-11-11T09:07:56Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/rack/rack/security/advisories/GHSA-wvg4-6p58-3gh3"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-125110"
},
{
"type": "WEB",
"url": "https://github.com/rack/rack/commit/18a07fe553afb4d8a678d6a948cd6ba1e11e28be"
},
{
"type": "WEB",
"url": "https://github.com/rack/rack/commit/9d7f19484ffd5ed5e032678c26ee6eff96d3a0a3"
},
{
"type": "WEB",
"url": "https://github.com/rack/rack/commit/e88097ccb88be44d06d47b42daffe04929516ee4"
},
{
"type": "PACKAGE",
"url": "https://github.com/rack/rack"
},
{
"type": "WEB",
"url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/GHSA-wvg4-6p58-3gh3.yml"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "Rack: Rack::Static header_rules bypass via dot-segment normalization mismatch",
"upstream": [
"GHSA-wvg4-6p58-3gh3",
"CVE-2026-125110"
]
}
Drag nodes to arrange them. Double-click a related vulnerability to open it, or a source to visit its link. The Physics button on the left tunes the layout and the simulation; the legend at the bottom left hides or shows nodes by kind and by sighting type.
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.
Vulnerability timeline
Sightings and recorded dates, grouped by day · UTC
How durations are calculated
Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.
Each result shows its source dates. A negative interval means the end event came first; when date-only inputs leave the order open, the result says so. Date-only inputs are approximate. The earliest evidence wins: KEV report dates and sightings are labelled as such. Records describing another vulnerability are listed but never calculated from, and a PoC alone does not establish exploitation. Missing patch evidence does not prove that no patch exists. Filters below change the event list only.
Loading timeline…
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.