TL;DR

CVE-2025-12543 is a critical vulnerability in the Undertow HTTP server core that allows attackers to send malicious or unexpected Host headers which are incorrectly accepted as valid. Because the Host header is widely used across HTTP infrastructure (proxies, routing, absolute URL generation, and cache keying), accepting malformed or unexpected Host values can lead to issues like cache poisoning, unsafe redirects, cross-tenant mix-ups, or trust-boundary bypasses. The issue affects enterprise Java environments where Undertow is used directly or indirectly (e.g., WildFly / JBoss EAP). Organisations should upgrade as soon as patches are available and enforce strict Host header validation at the application or edge layer.

Affected Versions

Technical Analysis

What Is the Vulnerability?

Affected versions of Undertow do not strictly validate Host header values during request parsing. As a result, malformed or unexpected Host headers are accepted instead of being rejected with an error.

Examples of attacker-controlled Host headers include:

• Host: evil.attacker.com
• Host: app.company.com@evil.attacker.com
• Host: internal-admin.company.local

Once accepted, these values may be trusted by:

• application logic
• framework utilities,
• reverse proxies or caches,
• tenant or domain resolution code.
  
  

This constitutes a server-layer input validation failure at a critical trust boundary.

How the Attack Works

1. Attacker sends a crafted HTTP request with a manipulated Host header.
   
   
2. Undertow accepts the request due to insufficient validation.
   
   
3. Downstream code trusts the Host value for security-relevant decisions.
   
   
4. Impact occurs, depending on how the Host header is used.
   
   

An example would be absolute URL generation:

String baseUrl = "https://" + request.getHeader("Host");

If the Host header is attacker-controlled, the application may generate links pointing to malicious domains, enabling token theft or phishing.

Impacted Scenarios

Undertow is a high-performance, embeddable Java HTTP server designed for scalable, non-blocking workloads. While not as universally deployed as Tomcat, Undertow’s role as the default web server in major enterprise Java platforms gives it a large and concentrated footprint in corporate environments.

Organizations are particularly exposed in the following cases:

1. Password Reset and Link Generation : Applications that construct absolute URLs using the Host header may leak password reset tokens or session identifiers to attacker-controlled domains.
2. Cache Poisoning: In deployments behind CDNs or reverse proxies, poisoned Host headers can create persistent malicious cache entries, affecting multiple users.
3. Multi-Tenant Environments: If tenant identity is inferred from the Host header, attackers may access or influence other tenants’ data or behaviour.‍
4. Internal Trust Bypasses: Applications that treat certain hostnames as “internal” may inadvertently expose administrative or internal-only functionality.

Mitigation

Primary Remediation

• Upgrade Undertow as soon as a patched release or vendor update is available (see below).
• Apply security updates from platform vendors (e.g., WildFly / JBoss EAP).

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The fix commit is in progress (https://github.com/undertow-io/undertow/pull/1857) as a classic upstream/downstream model:

• What PR #1857 does: Adds a new HostHeaderHandler to validate the Host header consistently and early in request processing.
• Behavior change: Invalid or ambiguous Host headers are rejected with 400 Bad Request (and the connection is closed).
• Coverage: Handles common tricky cases (missing/multiple Host, malformed IP-literals/ports, bad characters/percent-encoding, and Host mismatch in absolute-form requests).
• Supporting change: Tightens NetworkUtils IP matching used by the validation logic.
• Safety net: Adds a dedicated test suite to prevent regressions.
• Upstream/downstream note: This is upstream hardening work that can be shipped downstream in products like JBoss EAP while the PR is still being finalized upstream.

Hardening Controls

The following points outline generic measures for protecting your applications against header-based attacks such as this one.

1. Enforce strict Host header allow-lists at the application level. Reverse proxies and load balancers should only forward requests for known, expected hostnames.
   ‍
   ‍Example:
   
   Allow: app.company.com
   Allow: api.company.com
   Deny: everything else
   
   Reject unexpected or malformed Host headers at the reverse proxy or WAF‍‍

2. Application-Level Host Validation: If an upgrade is not immediately possible, applications should explicitly validate the Host header as early in the request processing as possible.
   
   ‍Example:
   
   if (!allowedHosts.contains(request.getHeader("Host"))) {
      return 400;
   }
   ‍
   This ensures that downstream logic never consumes an untrusted Host value.

3. Applications that map tenants to hostnames should validate hosts against a trusted registry rather than relying on raw headers.
4. Logging and alerting on rejected Host headers helps detect exploitation attempts.

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Conclusion

CVE-2025-12543 underscores how small validation gaps in foundational components can have outsized impact in enterprise systems. The root issue is not application-specific logic but a failure at the HTTP server layer, meaning:

• Multiple applications can be affected simultaneously.
  
• Even well-written application code may be exposed.
  
• Exploitation requires no authentication or special privileges.

Because Host headers influence routing and trust decisions, exploitation can result in account compromise, data exposure, and systemic security failures.

Given Undertow’s role in Java application servers and micro-services, organizations should prioritize remediation and review how Host headers are used across their stacks.

References

• https://test.osv.dev/vulnerability/CVE-2025-12543
• https://bugzilla.redhat.com/show_bug.cgi?id=2408784
• https://github.com/undertow-io/undertow

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