Access-control bypass via HTTP request smuggling
Published Jul 31, 2026 · Updated Jul 31, 2026
HTTP request smuggling in Hugo Leisink Hiawatha 12.1 and earlier allows remote attackers to bypass access controls via crafted HTTP requests. The parser in src/http.c prioritizes Content-Length over Transfer-Encoding, while send_request_to_webserver forwards the conflicting Content-Length unchanged, desynchronizing HTTP message boundaries. Exploitation requires a proxy chain that accepts ambiguous framing; demonstrated outcomes include ACL bypass and cache poisoning, while cross-user disclosure depends on shared upstream connections.
Summary
What happened
HTTP request smuggling in Hugo Leisink Hiawatha 12.1 and earlier allows remote attackers to bypass access controls via crafted HTTP requests. The parser in src/http.c prioritizes Content-Length over Transfer-Encoding, while send_request_to_webserver forwards the conflicting Content-Length unchanged, desynchronizing HTTP message boundaries. Exploitation requires a proxy chain that accepts ambiguous framing; demonstrated outcomes include ACL bypass and cache poisoning, while cross-user disclosure depends on shared upstream connections.
The record
- CVE
- CVE-2026-51785
- Published
- Jul 31, 2026
- Updated
- Jul 31, 2026
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-94, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 31, 2026CVE publishedPublication date reported by the CVE source.
- Jul 31, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
What conditions does exploitation require?
What is affected?
Affected products and versions are unavailable in this record.
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Hiawatha HTTP request-smuggling proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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