CVE-2026-37977
LOWCVE-2026-37977 is a low-severity (CVSS 3.7) CWE-346 vulnerability in redhat build of keycloak. O3 Security detects and blocks CVE-2026-37977 exploitation at runtime via eBPF exploit-chain detection and L7 egress monitoring, even on unpatched systems.
A flaw was found in Keycloak. A remote attacker can exploit a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint.…
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-37977.
Description
A flaw was found in Keycloak. A remote attacker can exploit a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint. This flaw occurs because the azp claim from a client-supplied JSON Web Token (JWT) is used to set the Access-Control-Allow-Origin header before the JWT signature is validated. When a specially crafted JWT with an attacker-controlled azp value is processed, this value is reflected as the CORS origin, even if the grant is later rejected. This can lead to the exposure of low-sensitivity information from authorization server error responses, weakening origin isolation, but only when a target client is misconfigured with webOrigins: ["*"].
Affected Products
build of keycloakredhatDetection & mitigation playbook
Vendor / applianceDetect
Inventory every redhat build of keycloak deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for exploitation at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.
Remediation status
No patch has shipped for CVE-2026-37977 yet — track the redhat build of keycloak advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.
How O3 protects you
O3 detects and blocks CVE-2026-37977 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.
Tailored to CVE-2026-37977. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Low impact: This vulnerability in Keycloak's UMA token endpoint allows for CORS header injection when a client is misconfigured with `webOrigins: ["*"]`. This can lead to the exposure of low-sensitivity information from authorization server error responses. Exploitation requires a specific client misconfiguration and…
Frequently Asked Questions
Is CVE-2026-37977 being exploited in your environment?
O3's eBPF runtime sensors and L7 egress monitoring detect and block the CVE-2026-37977 exploit chain at execution — protecting unpatched and end-of-life systems until the vendor patch is applied.