EUVD-2026-51645
GHSA-8mxp-hf23-8hp4
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Network-facing web server reachable unauthenticated via a single crafted request with low complexity; scored to the described RCE impact, though a smuggling-only outcome could instead warrant S:C with lower A.
Primary rating from Vendor (mitre).
CVSS VectorVendor: mitre
Lifecycle Timeline
4DescriptionCVE.org
An issue in Hugo Leisink Hiawatha v.12.1 and before allows a remote attacker to execute arbitrary code via a crafted request
AnalysisAI
Remote code execution is claimed in Hugo Leisink's Hiawatha web server version 12.1 and earlier, reachable by an unauthenticated attacker sending a single crafted HTTP request (CVSS 9.8, CWE-94). Notably, the primary referenced research (fenrisk.com/hiawatha-http-smuggling) and the cited RFCs on chunked/message parsing indicate the underlying defect is HTTP request smuggling rather than classic code injection, so the RCE framing should be treated cautiously. Publicly available exploit code exists (SSVC 'poc'); this is not listed in CISA KEV, and EPSS is low at 0.36% (29th percentile).
Technical ContextAI
Hiawatha is a lightweight, security-focused open-source HTTP/HTTPS web server written in C. The CWE-94 (Code Injection) classification and the 'RCE/Code Injection' tags describe a crafted-request path to arbitrary code execution, but the reference set - RFC 9112 §6.3 (message body length), RFC 2616 and RFC 2616 §4.4 (chunked transfer-encoding / message length rules), plus the Fenrisk 'hiawatha-http-smuggling' writeup - points to inconsistent parsing of HTTP message framing (Content-Length vs. Transfer-Encoding: chunked). That class of desync/HTTP request smuggling bug arises when the server and a fronting proxy disagree on where one request ends and the next begins. The CPE is unhelpfully generic ('cpe:2.3:a:n/a:n/a:*'), so exact affected product enumeration relies on the description ('Hiawatha v12.1 and before') rather than structured NVD CPE data.
RemediationAI
Upgrade Hiawatha to a release newer than 12.1 once the vendor publishes a fixed build; no exact fixed version number is present in the provided data, so consult Hugo Leisink's official Hiawatha changelog/release notes before deploying (do not assume a specific version). Because the referenced defect is HTTP message-framing/request smuggling, effective compensating controls until patched include placing Hiawatha behind a strict, RFC-9112-compliant reverse proxy or WAF that normalizes requests and rejects messages containing both Content-Length and Transfer-Encoding headers or malformed chunked encoding (trade-off: adds a hop and may break clients relying on lax framing); disabling or blocking chunked transfer-encoding on untrusted inbound paths where feasible; and restricting network exposure of the server to trusted sources. Review the technical analysis at https://fenrisk.com/hiawatha-http-smuggling and the advisories at https://nvd.nist.gov/vuln/detail/CVE-2026-51785 and https://vuldb.com/vuln/385142 to align mitigations with the actual parsing flaw.
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External POC / Exploit Code
Leaving vuln.today