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How CVE-2026-12606 Works
Eclipse Grizzly is an open-source NIO framework maintained by the Eclipse Foundation, widely used as the HTTP protocol processor in Java EE application servers such as GlassFish. Its `grizzly-http` module is responsible for parsing HTTP/1.1 requests, including trailer headers under chunked transfer encoding.
CVE-2026-12606 resides in the Trailer Parser component of Grizzly versions prior to 5.0.2. The vulnerability stems from the library’s inability to correctly interpret the trailer section when presented with malformed trailer header lines. In HTTP/1.1, chunked transfer encoding allows a trailer section to be sent after the message body, containing additional header fields. The trailer section is demarcated by a zero-sized chunk followed by the trailer headers.
The parsing flaw occurs when Grizzly encounters a specially crafted, malformed trailer header line within this section. Instead of rejecting the malformed input or parsing it consistently with downstream agents, Grizzly mishandles the boundary between the trailer and the subsequent HTTP request. This inconsistent interpretation is classified under CWE-444: Inconsistent Interpretation of HTTP Requests (‘HTTP Request/Response Smuggling’).
An attacker can exploit this by sending a sequence of HTTP requests where the first request contains a malformed trailer header. Grizzly, acting as an intermediary (proxy, load balancer, or application server front-end), will parse the request in one way, while the ultimate backend server parses it differently. This desynchronization allows the attacker to “smuggle” a second, malicious HTTP request within the trailer section of the first. The smuggled request is then interpreted by the backend as a separate, legitimate request, effectively bypassing security controls, manipulating request routing, or poisoning caches.
The attack requires no authentication and can be launched remotely over the network. The CVSS v3.1 base score is 5.3 (Medium) with the vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N, while the CVSS v4.0 base score is 6.3 (Medium) with the vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N. The lower v4 score reflects the requirement of an attack presence (AT:P) due to the need for a malformed trailer.
DailyCVE Form:
Platform: Eclipse Grizzly
Version: < 5.0.2
Vulnerability: HTTP Request Smuggling
Severity: Medium (CVSS 5.3)
date: 2026-07-14
Prediction: 2026-07-14
What Undercode Say:
Analytics:
- CWE: CWE-444 (Inconsistent Interpretation of HTTP Requests)
- CAPEC: CAPEC-33 (HTTP Request Smuggling)
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Integrity Impact: Low
- Exploit Maturity: Not Defined (no public exploit available as of disclosure)
- Affected Products: Eclipse GlassFish 4.0.0 through 4.0.2, and 5.0.0 through 5.0.1
Detection & Verification Commands:
Check Grizzly version in a Maven project mvn dependency:tree | grep grizzly Check version in a Gradle project gradle dependencies | grep grizzly For standalone Grizzly, check the JAR manifest unzip -p grizzly-http-server-.jar META-INF/MANIFEST.MF | grep "Implementation-Version" Verify if the vulnerability is present (version < 5.0.2) Example using grep to filter versions java -cp grizzly-http-server-.jar org.glassfish.grizzly.http.server.HttpServer 2>&1 | grep -i version
Smuggling Test Payload (Conceptual):
POST /target HTTP/1.1 Host: vulnerable-server.com Transfer-Encoding: chunked Trailer: X-Smuggled 0 X-Smuggled: GET /admin HTTP/1.1 Host: internal-backend.com
Note: The exact malformed trailer header that triggers the bug is not publicly disclosed. The above illustrates the smuggling technique.
Exploit:
Currently, there is no public exploit available for CVE-2026-12606. However, the vulnerability is considered easy to exploit due to its low attack complexity and the fact that no authentication is required. An attacker with network access to an affected Grizzly instance could craft a sequence of HTTP requests with a malformed trailer section to desynchronize the front-end and back-end parsing states. This could lead to:
– Bypassing security controls (e.g., WAF, authentication gates)
– Cache poisoning (if a caching proxy is involved)
– Request hijacking (smuggled requests can access resources the attacker shouldn’t have)
– Session fixation or credential theft in specific deployment scenarios
The exploitation relies on the presence of an intermediary (Grizzly) that forwards requests to a backend server with a different parsing logic. The success of the attack depends on the discrepancy between how Grizzly and the backend interpret the malformed trailer.
Protection:
- Upgrade to Grizzly 5.0.2 or later – This is the primary and most effective mitigation. The fix was released on the same day as the CVE publication (2026-07-14).
2. If immediate upgrade is not possible:
- Disable chunked transfer encoding support if your application does not require it (though this may break legitimate clients).
- Deploy a reverse proxy (e.g., Nginx, HAProxy) in front of Grizzly that normalizes or rejects malformed trailer headers before they reach the vulnerable parser.
- Implement a Web Application Firewall (WAF) rule to block requests containing suspicious `Trailer` headers or malformed chunked encoding sequences.
- Regularly apply security patches to all software components, especially those that handle HTTP parsing.
- Monitor logs for unusual request patterns, particularly requests with malformed `Transfer-Encoding` or `Trailer` headers.
Impact:
- Confidentiality: None – the vulnerability does not directly leak information.
- Integrity: Low – an attacker can manipulate request processing, potentially leading to unauthorized actions or data alteration.
- Availability: None – the vulnerability does not cause denial of service.
- Overall Risk: Medium. While the direct impact is limited to integrity, the ability to smuggle requests can be chained with other vulnerabilities or misconfigurations to achieve significant compromise (e.g., privilege escalation, data exfiltration, or cache poisoning).
- Affected Environments: Any deployment using Eclipse Grizzly as an HTTP intermediary, including GlassFish application servers, custom Java web services, and reverse-proxy setups that rely on Grizzly’s HTTP parser.
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Sources:
Reported By: nvd.nist.gov
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