Summary
Authelia vulnerable to an authentication bypassed with malformed request URI on nginx
Workarounds
The most relevant workaround is upgrading. If you need assistance with an upgrade please contact us on Matrix or Discord. Please just let us know you're needing help upgrading to above 4.29.2.
You can add an block which fails requests that contains a malformed URI in the internal location block. We have crafted one that should work in most instances, it basically checks no chars that are required to be URL-encoded for either the path or the query are in the URI. Basically this regex checks that the characters between the square braces are the only characters in the $request_uri header, if they exist, it returns a HTTP 401 status code. The characters in the regex match are tested to not cause a parsing error that would result in a failure, however they are not exhaustive since query strings seem to not always conform to the RFC.
authelia.conf:location /authelia {
internal;
# **IMPORTANT**
# This block rejects requests with a 401 which contain characters that are unable to be parsed.
# It is necessary for security prior to v4.29.3 due to the fact we returned an invalid code in the event of a parser error.
# You may comment this section if you're using Authelia v4.29.3 or above. We strongly recommend upgrading.
# RFC3986: http://tools.ietf.org/html/rfc3986
# Commentary on RFC regarding Query Strings: https://www.456bereastreet.com/archive/201008/what_characters_are_allowed_unencoded_in_query_strings/
if ($request_uri ~ [^a-zA-Z0-9_+-=\!@$%&*?~.:#'\;\(\)\[\]]) {
return 401;
}
# Include the remainder of the block here.
}
Discovery
This issue was discovered by:
Siemens Energy
Cybersecurity Red Team
- Silas Francisco
- Ricardo Pesqueira
Identifying active exploitation of the vulnerability
The following regex should match log entries that are an indication of the vulnerability being exploited:
level=error msg="Unable to parse target URL: Unable to parse URL (extracted from X-Original-URL header)?.*?: parse.*?net/url:.*github\.com/authelia/authelia/internal/handlers/handler_verify\.go
Example log entry with X-Original-URL configured:
time="2021-05-21T16:31:15+10:00" level=error msg="Unable to parse target URL: Unable to parse URL extracted from X-Original-URL header: parse \"https://example.com/": net/url: invalid control character in URL" method=GET path=/api/verify remote_ip=192.168.1.10 stack="github.com/authelia/authelia/internal/middlewares/authelia_context.go:65 (*AutheliaCtx).Error\ngithub.com/authelia/authelia/internal/handlers/handler_verify.go:431 VerifyGet.func1\ngithub.com/authelia/authelia/internal/middlewares/authelia_context.go:50 AutheliaMiddleware.func1.1\ngithub.com/fasthttp/[email protected]/router.go:414 (*Router).Handler\ngithub.com/authelia/authelia/internal/middlewares/log_request.go:14 LogRequestMiddleware.func1\ngithub.com/valyala/[email protected]/server.go:2219 (*Server).serveConn\ngithub.com/valyala/[email protected]/workerpool.go:223 (*workerPool).workerFunc\ngithub.com/valyala/[email protected]/workerpool.go:195 (*workerPool).getCh.func1\nruntime/asm_amd64.s:1371 goexit"
Example log entry without X-Original-URL configured:
time="2021-05-21T16:30:17+10:00" level=error msg="Unable to parse target URL: Unable to parse URL https://example.com/: parse \"https://example.com/": net/url: invalid control character in URL" method=GET path=/api/verify remote_ip=192.168.1.10 stack="github.com/authelia/authelia/internal/middlewares/authelia_context.go:65 (*AutheliaCtx).Error\ngithub.com/authelia/authelia/internal/handlers/handler_verify.go:431 VerifyGet.func1\ngithub.com/authelia/authelia/internal/middlewares/authelia_context.go:50 AutheliaMiddleware.func1.1\ngithub.com/fasthttp/[email protected]/router.go:414 (*Router).Handler\ngithub.com/authelia/authelia/internal/middlewares/log_request.go:14 LogRequestMiddleware.func1\ngithub.com/valyala/[email protected]/server.go:2219 (*Server).serveConn\ngithub.com/valyala/[email protected]/workerpool.go:223 (*workerPool).workerFunc\ngithub.com/valyala/[email protected]/workerpool.go:195 (*workerPool).getCh.func1\nruntime/asm_amd64.s:1371 goexit"
For more information
If you have any questions or comments about this advisory:
- Open a Discussion
- Email us at [email protected]
Edit / Adjustment
This CVE has been edited adjusting the score to more accurately reflect the guidance in the official CVSS 3.1 guide. Due to misunderstandings about the CVSS indicators this was incorrectly assigned but this has been corrected. Under close evaluation the score we originally assigned to this CVE is inappropriate in two clearly identifiable criteria:
- Complexity (Low -> High): This attack requires the administrator be using NGINX's auth_request module. This means the attack cannot be exploited at will but rather requires a pre-condition separate to the vulnerable system outside of the attackers control (a vulnerable version of NGINX - at the time of this writing NGINX's security team has refused to fix the clear bug on their end but that's effectively irrelevant since we operate with more than just a NGINX proxy and no other proxy has this vulnerability), and requires the attacker have gathered knowledge about the system for this likely to be exploited.
- Availability (High -> None): This attack does not alter availability directly.
Impact
This affects uses who are using nginx ngx_http_auth_request_module with Authelia, it allows a malicious individual who crafts a malformed HTTP request to bypass the authentication mechanism. It additionally could theoretically affect other proxy servers, but all of the ones we officially support except nginx do not allow malformed URI paths.
The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.
CVE-2021-32637 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.29.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
The problem is rectified entirely in v4.29.3. As this patch is relatively straightforward we can back port this to any version upon request. Alternatively we are supplying a git patch to 4.25.1 which should be relatively straightforward to apply to any version, the git patches for specific versions can be found below.
Patch for 4.25.1:From ca22f3d2c44ca7bef043ffbeeb06d6659c1d550f Mon Sep 17 00:00:00 2001
From: James Elliott <[email protected]>
Date: Wed, 19 May 2021 12:10:13 +1000
Subject: [PATCH] fix(handlers): verify returns 200 on malformed request
This is a git patch for commit at tag v4.25.1 to address a potential method to bypass authentication in proxies that forward malformed information to Authelia in the forward auth process. Instead of returning a 200 this ensures that Authelia returns a 401 when this occurs.
---
internal/handlers/handler_verify.go | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/internal/handlers/handler_verify.go b/internal/handlers/handler_verify.go
index 65c064ce..4dd9702d 100644
--- a/internal/handlers/handler_verify.go
+++ b/internal/handlers/handler_verify.go
@@ -396,7 +396,9 @@ func VerifyGet(cfg schema.AuthenticationBackendConfiguration) middlewares.Reques
targetURL, err := getOriginalURL(ctx)
if err != nil {
- ctx.Error(fmt.Errorf("Unable to parse target URL: %s", err), operationFailedMessage)
+ ctx.Logger.Error(fmt.Errorf("Unable to parse target URL: %s", err))
+ ctx.ReplyUnauthorized()
+
return
}
--
2.31.1
Frequently Asked Questions
- What is CVE-2021-32637? CVE-2021-32637 is a critical-severity improper authentication vulnerability in github.com/authelia/authelia/v4 (go), affecting versions >= 4.0.0-alpha1, <= 4.29.2. It is fixed in 4.29.3. The application does not adequately verify the identity of a user, device, or process before granting access.
- How severe is CVE-2021-32637? CVE-2021-32637 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/authelia/authelia/v4 are affected by CVE-2021-32637? github.com/authelia/authelia/v4 (go) versions >= 4.0.0-alpha1, <= 4.29.2 is affected.
- Is there a fix for CVE-2021-32637? Yes. CVE-2021-32637 is fixed in 4.29.3. Upgrade to this version or later.
- Is CVE-2021-32637 exploitable, and should I be worried? Whether CVE-2021-32637 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-32637 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-32637? Upgrade
github.com/authelia/authelia/v4to 4.29.3 or later.