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CVE-2026-54735 – Server-Side Request Forgery (SSRF) – Prebid Server before v4.4.0

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Summary

CVE-2026-54735 is a critical server-side request forgery (SSRF) vulnerability in Prebid Server, the open-source header-bidding server maintained by the Prebid project. Certain bidder adapters interpolate user-supplied parameters into outbound request URLs without properly validating host and subdomain values, allowing an unauthenticated attacker to redirect server-initiated requests to unintended destinations. The issue carries a CVSS v3.1 base score of 10.0 (Critical).

Technical details

  • Root cause: Certain bidder adapters build outbound request URLs by interpolating user-supplied parameters without properly validating the host and subdomain values (CWE-918: Server-Side Request Forgery).
  • Trigger conditions: An attacker submits a crafted bid request containing malicious parameter values that reach a vulnerable adapter’s URL-construction logic.
  • Attack vector: Network (AV:N). Exploitation requires no authentication and no user interaction, and can be performed remotely against an exposed Prebid Server instance.
  • Impact: The server can be coerced into issuing HTTP requests to attacker-chosen destinations, enabling probing and access to internal network services and potential extraction of host-environment data. The scope is changed (S:C), with high confidentiality, integrity, and availability impact.

Affected software

  • github.com/prebid/prebid-server (v1): versions ≤ 0.275.0
  • github.com/prebid/prebid-server/v2: versions ≤ 2.32.0
  • github.com/prebid/prebid-server/v3: versions ≤ 3.30.0
  • github.com/prebid/prebid-server/v4: versions < 4.4.0

Severity

  • CVSS v3.1 base score: 10.0 (Critical)
  • Vector string: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to Prebid Server v4.4.0 or later, which contains the security patch. No fixes were issued for earlier major version lines (v1, v2, v3), so affected deployments should migrate to the patched v4 release.
  • If no patch can be applied immediately: Disable the vulnerable bidder adapters until the upgrade can be completed. Additionally, restrict the server’s outbound network egress so it can only reach legitimate, expected bidder endpoints, reducing the reach of any forged request.

References

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