Concourse CI CVE-2026-49826
LOW
GHSA-8w27-c4vc-88q9
Severity by source
Network-delivered, low-complexity redirect requiring user interaction; scope changes as victim is sent to external attacker domain with phishing potential.
Lifecycle Timeline
1DescriptionCVE.org
Impact
An attacker is able to craft and send a user a URL that will redirect the user from the Concourse web server to any other site. This could be used in a phishing attack to steal user's credentials.
Patches
This has been fixed in 8.2.3
Workarounds
None.
Exploit
Vulnerable code was in: https://github.com/concourse/concourse/blob/ea7b812e3a88fdd070f0faece874e8a2d4fbb31c/skymarshal/skyserver/skyserver.go#L162-L170
The issue stems from the underlying processing logic of Go's url package. Normally, ParseRequestURI() will eventually reach an internal url.setPath() function, where the URL will be decoded. However, if RawPath is not empty and validEncoded(RawPath) is true, and the decoded result equals Path, then return RawPath as is; otherwise, escape Path again, i.e., decode it again.
In other words, if the URL contains dangerous characters that should be escaped, such as backslashes (\), then an extra decoding step will be performed. Therefore, /%2Fexample.com will be parsed as //example.com.
On vulnerable versions of Concourse, add /sky/login?redirect_uri=/%252Fexample.com/\ to your Concourse external URL, login as usual, and you should be redirected to example.com instead of your Concourse web server. The redirect happens after the login flow completes. No credentials are leaked.
AnalysisAI
Open redirect in Concourse's web login flow allows a remote unauthenticated attacker to craft a URL that silently redirects an authenticated user to an arbitrary external site upon completing the login process. Concourse versions prior to 8.2.3 are affected via the /sky/login endpoint's redirect_uri parameter. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | The victim must be a user of the affected Concourse instance and must click an attacker-crafted URL targeting the `/sky/login` endpoint with a maliciously encoded `redirect_uri` parameter (e.g., `/%252Fexample.com/\`). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | No official CVSS vector was assigned to this CVE, so risk signals are limited. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker identifies a Concourse instance and crafts a phishing link such as `https://concourse.example.com/sky/login?redirect_uri=/%252Fattacker.com/\` disguised as a routine CI/CD login link. A developer clicks the link, authenticates normally via Concourse's SSO/OAuth flow, and is silently redirected to an attacker-controlled site mimicking a Concourse dashboard or credential prompt. … |
| Remediation | Upgrade Concourse to v8.2.3, which contains the fix at commit ac60be5f0435b6592f5a4fcc089050d72ad2452c addressing the double-decoding issue in the `redirect_uri` validation logic. … Detailed patch versions, workarounds, and compensating controls in full report. |
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