Mailpit: WebSocket origin check bypass via percent-encoded path (regression of CVE-2026-22689)

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Mailpit is an email testing tool and API for developers. From 1.29.0 until 1.30.6, Mailpit's server/server.go origin middleware checks the raw RequestURI for the /api/ prefix while Go's ServeMux routes using the percent-decoded URL path, and server/websockets/client.go configures websocket.Upgrader.CheckOrigin to return true. A malicious website can request /%61pi/events, skip corsOriginAccessControl(), reach the /api/events WebSocket handler, and receive live message IDs, Message-Id values, sender and recipient fields, subjects, tags, and body snippets from an unauthenticated default Mailpit instance after the user visits the site. This is a regression of the earlier WebSocket origin protection and does not affect deployments protected by --ui-auth-file. This issue is fixed in version 1.30.6.

Source published

20 Aug 2026 21:29:06 UTC

Source modified

21 Aug 2026 11:31:27 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

Mailpit is an email testing tool and API for developers. From 1.29.0 until 1.30.6, Mailpit's server/server.go origin middleware checks the raw RequestURI for the /api/ prefix while Go's ServeMux routes using the percent-decoded URL path, and server/websockets/client.go configures websocket.Upgrader.CheckOrigin to return true. A malicious website can request /%61pi/events, skip corsOriginAccessControl(), reach the /api/events WebSocket handler, and receive live message IDs, Message-Id values, sender and recipient fields, subjects, tags, and body snippets from an unauthenticated default Mailpit instance after the user visits the site. This is a regression of the earlier WebSocket origin protection and does not affect deployments protected by --ui-auth-file. This issue is fixed in version 1.30.6.

Source published

Aug 20, 2026, 9:29:06 PM (timezone not stated)

Source modified

Aug 21, 2026, 11:31:27 AM (timezone not stated)

Observed here

7 Nov 2026 14:20:42 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

Mailpit is an email testing tool and API for developers. From 1.29.0 until 1.30.6, Mailpit's server/server.go origin middleware checks the raw RequestURI for the /api/ prefix while Go's ServeMux routes using the percent-decoded URL path, and server/websockets/client.go configures websocket.Upgrader.CheckOrigin to return true. A malicious website can request /%61pi/events, skip corsOriginAccessControl(), reach the /api/events WebSocket handler, and receive live message IDs, Message-Id values, sender and recipient fields, subjects, tags, and body snippets from an unauthenticated default Mailpit instance after the user visits the site. This is a regression of the earlier WebSocket origin protection and does not affect deployments protected by --ui-auth-file. This issue is fixed in version 1.30.6.

Source published

20 Aug 2026 22:17:56 UTC

Source modified

18 Oct 2026 20:10:00 UTC

Observed here

25 Oct 2026 15:18:51 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

## Summary The cross-site WebSocket hijacking fix was reimplemented as an origin check gated on a raw-URI prefix test, but Go's ServeMux routes on the percent-decoded path, so requesting /%61pi/events reaches the WebSocket handler while skipping the only origin control, and the upgrader itself accepts every origin. Confirmed at HEAD 408b30d. Affects 1.29.0 through 1.30.5. ## The defect Two halves that were each correct in isolation. server/websockets/client.go accepts any origin and delegates the check elsewhere: ```go var upgrader = websocket.Upgrader{ // line 33 ... CheckOrigin: func(_ *http.Request) bool { // line 37 // origin is checked via server.go's CORS settings return true // line 39 }, } ``` server/server.go performs that check but keys it on the RAW request target: ```go if strings.HasPrefix(r.RequestURI, config.Webroot+"api/") || htmlPreviewRouteRe.MatchString(r.RequestURI) { // line 320 if allowed := corsOriginAccessControl(r); !allowed { http.Error(w, "Blocked due to CORS violation", http.StatusForbidden) return } ``` r.RequestURI is the untouched wire target; Go's ServeMux routes on the percent-DECODED path. So for /%61pi/events: `strings.HasPrefix("/%61pi/events", "/api/")` is FALSE (origin check skipped), ServeMux decodes %61 to "a" and routes to /api/events, and the upgrader's CheckOrigin returns true. Measured, default config, no auth: /api/events with `Origin: https://evil.example` returns 403; /%61pi/events with the same Origin returns 101 Switching Protocols and begins streaming. With a message delivered over SMTP while the cross-origin socket was open, the attacker origin received the ID, Message-Id, From, To, Cc, Bcc, Subject ("SECRET password reset token abc123"), size, tags, and body Snippet, live. WebSockets are not subject to CORS response-header enforcement, so the absent Access-Control-Allow-Origin header provides no protection once the upgrade succeeds. Regression provenance: commit 6f1f4f3 (2026-01-10, v1.28.2) fixed CVE-2026-22689 by DELETING CheckOrigin; commit a63bcd9 (2026-01-31, first in v1.29.0) reintroduced CheckOrigin returning true and replaced the protection with the bypassable raw-prefix test. ## Attacker model and verification Any website the developer visits while Mailpit is running. No credentials, no ability to send mail, no interaction beyond visiting a page. Requires Mailpit without --ui-auth-file (the default, and the same precondition as the original CVE). The bypass was measured live against a real Mailpit instance on loopback, including the 403-versus-101 control pair; authentication still holds (the encoded path returns 401 when --ui-auth-file is set); browser reachability was confirmed against the WHATWG URL parser, which preserves %61. ## Suggested fix Do not make security decisions on r.RequestURI. Key the check on r.URL.Path, the decoded value the router uses, so the gate and the route agree. Better, restore a real CheckOrigin on the upgrader so the WebSocket carries its own origin enforcement rather than depending on a middleware prefix match. Secondary (Low, not claimed as XSS): server/apiv1/message.go lines 154-155 echo an attacker-chosen Content-Type with Content-Disposition: inline; this is blocked today by the nonce CSP. ## Tooling I used AI assistance while investigating. The bypass was measured live against a real Mailpit instance on loopback, including the 403-versus-101 control pair and the authenticated 401 case, and I separately confirmed at HEAD the CheckOrigin returning true with its delegating comment and the RequestURI-keyed prefix gate.

Source published

20 Aug 2026 21:34:58 UTC

Source modified

2 Sep 2026 23:45:04 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

Publisher rating · MODERATE

Mailpit: WebSocket origin check bypass via percent-encoded path (regression of CVE-2026-22689) in github.com/axllent/mailpit

Source published

25 Aug 2026 19:43:10 UTC

Source modified

26 Aug 2026 01:19:26 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.