Summary
The pattern '/\domain.com' is not disallowed when redirecting, allowing for open redirect
Workarounds
At this stage there is no work around.
Impact
An open redirect vulnerability has been found in oauth2_proxy. Anyone who uses oauth2_proxy may potentially be impacted.
For a context [detectify] have an in depth blog post about the potential impact of an open redirect. Particularly see the OAuth section.
tl;dr: People's authentication tokens could be silently harvested by an attacker. e.g:facebook.com/oauth.php?clientid=123&state=abc&redirect_url=https://yourdomain.com/red.php?url%3dhttps://attacker.com/
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
CVE-2020-5233 has a CVSS score of 5.9 (Medium). The vector is network-reachable, low privileges required, and user interaction required. 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 (5.0.0); 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.
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@sauyon found the issue, and has submitted a patch.
diff --git a/oauthproxy.go b/oauthproxy.go
index 72ab580..f420df6 100644
--- a/oauthproxy.go
+++ b/oauthproxy.go
@@ -517,7 +517,7 @@ func (p *OAuthProxy) GetRedirect(req *http.Request) (redirect string, err error)
// IsValidRedirect checks whether the redirect URL is whitelisted
func (p *OAuthProxy) IsValidRedirect(redirect string) bool {
switch {
- case strings.HasPrefix(redirect, "/") && !strings.HasPrefix(redirect, "//"):
+ case strings.HasPrefix(redirect, "/") && !strings.HasPrefix(redirect, "//") && !strings.HasPrefix(redirect, "/\\"):
return true
case strings.HasPrefix(redirect, "http://") || strings.HasPrefix(redirect, "https://"):
redirectURL, err := url.Parse(redirect)
This patch will be applied to the next release, which is scheduled for when this is publicly disclosed.
Frequently Asked Questions
- What is CVE-2020-5233? CVE-2020-5233 is a medium-severity open redirect vulnerability in github.com/oauth2-proxy/oauth2-proxy (go), affecting versions < 5.0.0. It is fixed in 5.0.0. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- How severe is CVE-2020-5233? CVE-2020-5233 has a CVSS score of 5.9 (Medium). 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/oauth2-proxy/oauth2-proxy are affected by CVE-2020-5233? github.com/oauth2-proxy/oauth2-proxy (go) versions < 5.0.0 is affected.
- Is there a fix for CVE-2020-5233? Yes. CVE-2020-5233 is fixed in 5.0.0. Upgrade to this version or later.
- Is CVE-2020-5233 exploitable, and should I be worried? Whether CVE-2020-5233 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-2020-5233 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-2020-5233? Upgrade
github.com/oauth2-proxy/oauth2-proxyto 5.0.0 or later.