CVE-2026-23603

Gitea: Blind SSRF in OAuth2 avatar synchronization via unvalidated OIDC picture claim

func oauth2UpdateAvatarIfNeed(ctx *context.Context, url string, u *user_model.User) {
    if setting.OAuth2Client.UpdateAvatar && len(url) > 0 {
        resp, err := http.Get(url)
        if err == nil {
            defer func() { _ = resp.Body.Close() }()
        }
        if err == nil && resp.StatusCode == http.StatusOK {
            data, err := io.ReadAll(io.LimitReader(resp.Body, setting.Avatar.MaxFileSize+1))
            if err == nil && int64(len(data)) <= setting.Avatar.MaxFileSize {
                _ = user_service.UploadAvatar(ctx, u, data)
            }
        }
    }
}

func oauth2SignInSync(ctx *context.Context, authSourceID int64, u *user_model.User, gothUser goth.User) {
    oauth2UpdateAvatarIfNeed(ctx, gothUser.AvatarURL, u)
    ...
}

python3 -m pip install requests pyjwt cryptography

[oauth2_client]
UPDATE_AVATAR = true
ENABLE_AUTO_REGISTRATION = true
USERNAME = userid

python3 fake_oidc.py http://127.0.0.1:8888/ 9999

nc -lvnp 8888

GET / HTTP/1.1
Host: 127.0.0.1:8888
User-Agent: Go-http-client/1.1
Accept-Encoding: gzip

When [oauth2_client] UPDATE_AVATAR = true is enabled, a low-privileged OAuth2/OIDC user who can influence their own picture claim can force the Gitea server to make outbound HTTP GET requests to attacker-selected URLs. This allows blind SSRF from the Gitea server’s network position, including requests to loopback addresses, RFC1918 private addresses, link-local addresses such as 169.254.169.254, and other internal services that may not be reachable from the public internet. In practical terms, this can enable internal service probing and interaction with localhost-only or private-network services depending on the deployment’s network access controls.

The vulnerability is blind in the common case because non-image responses such as HTML, JSON, or plaintext are rejected during avatar processing and are not directly returned to the attacker. However, impact can increase in cloud or internal-network deployments where metadata services, internal admin panels, monitoring endpoints, or image-generating internal services are reachable from the Gitea host. If an internal endpoint returns a valid supported image format within the configured avatar size limit, the response may be stored as the attacker’s avatar, creating a limited response retrieval primitive.

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-23603 has a CVSS score of 3.1 (Low). The vector is network-reachable, low 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 (1.27.0); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-23603? CVE-2026-23603 is a low-severity server-side request forgery (SSRF) vulnerability in code.gitea.io/gitea (go), affecting versions < 1.27.0. It is fixed in 1.27.0. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-23603? CVE-2026-23603 has a CVSS score of 3.1 (Low). 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.
3. Which versions of code.gitea.io/gitea are affected by CVE-2026-23603? code.gitea.io/gitea (go) versions < 1.27.0 is affected.
4. Is there a fix for CVE-2026-23603? Yes. CVE-2026-23603 is fixed in 1.27.0. Upgrade to this version or later.
5. Is CVE-2026-23603 exploitable, and should I be worried? Whether CVE-2026-23603 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
6. What actually determines whether CVE-2026-23603 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.
7. How do I fix CVE-2026-23603? Upgrade code.gitea.io/gitea to 1.27.0 or later.

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