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CVE-2026-37977 is a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint. The flaw occurs because the endpoint uses the azp claim from a client-supplied JSON Web Token to populate the Access-Control-Allow-Origin response header before the JWT signature has been validated. An attacker can submit a specially crafted JWT containing an attacker-controlled azp value, causing that value to be reflected as the allowed origin even when the grant request is ultimately rejected. This breaks the expected trust boundary between token validation and CORS policy generation and can expose low-sensitivity information contained in authorization server error responses. The issue is only exploitable in practice when the affected target client is misconfigured to allow wildcard origins through webOrigins: ["*"].
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What an attacker gets, and what they’ve been doing with it.
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webOrigins: ["*"]. Restrict allowed origins to the minimum required trusted origins and limit cross-origin access to the UMA token endpoint until patched versions are deployed.Patch, then assume compromise.
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
2 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.
No news coverage yet. Advisories and community discussion only.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
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Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.