CVE-2026-32881

ewe is a Gleam web server. ewe is a Gleam web server. Versions 0.6.0 through 3.0.4 are vulnerable to authentication bypass or spoofed proxy-trust headers. Chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. A malicious client can exploit this by declaring these headers in the Trailer field and appending them after the final chunk, causing request.set_header to overwrite legitimate values (e.g., those set by a reverse proxy). This enables attackers to forge authentication credentials, hijack sessions, bypass IP-based rate limiting, or spoof proxy-trust headers in any downstream middleware that reads headers after ewe.read_body is called. This issue has been fixed in version 3.0.5.

ewe is a Gleam web server. ewe is a Gleam web server. Versions 0.6.0 through 3.0.4 are vulnerable to authentication bypass or spoofed proxy-trust headers. Chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. A malicious client can exploit this by declaring these headers in the Trailer field and appending them after the final chunk, causing request.set_header to overwrite legitimate values (e.g., those set by a reverse proxy). This enables attackers to forge authentication credentials, hijack sessions, bypass IP-based rate limiting, or spoof proxy-trust headers in any downstream middleware that reads headers after ewe.read_body is called. This issue has been fixed in version 3.0.5.

Summary

ewe's chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. Security-sensitive headers like authorization, cookie, and x-forwarded-for can be injected or overwritten by a malicious client via trailers, potentially bypassing authentication or spoofing proxy-trust headers.

Impact

When ewe.read_body processes a chunked request with a Trailer header, it calls handle_trailers (ewe/internal/http1.gleam:493), which merges declared trailer fields into req.headers via request.set_header (line 517). The is_forbidden_trailer denylist (line 534) only blocks 9 header names: transfer-encoding, content-length, host, cache-control, expect, max-forwards, pragma, range, and te.

Security-sensitive headers are not blocked, including:

• authorization — attacker can inject or overwrite Bearer tokens
• cookie / set-cookie — attacker can inject session cookies
• proxy-authorization — attacker can inject proxy credentials
• x-forwarded-for, x-forwarded-host, x-forwarded-proto — attacker can spoof proxy-trust headers
• x-real-ip — attacker can spoof client IP

A malicious client can inject these headers by declaring them in the Trailer request header and including them after the final 0\r\n chunk. If the header already exists (e.g., set by a reverse proxy), request.set_header overwrites it. Any application logic that reads these headers after calling ewe.read_body — such as authentication middleware, IP-based rate limiting, or session validation — will see the attacker-controlled values.

Proof of Concept

Inject an authorization header that didn't exist:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: authorization\r\n\r\n4\r\ntest\r\n0\r\nauthorization: Bearer injected-token\r\n\r\n' | nc -w 2 localhost 8080

Overwrite a legitimate authorization header set by a proxy:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nAuthorization: Bearer legitimate-token\r\nTransfer-Encoding: chunked\r\nTrailer: authorization\r\n\r\n4\r\ntest\r\n0\r\nauthorization: Bearer evil-token\r\n\r\n' | nc -w 2 localhost 8080

Inject x-forwarded-for to spoof client IP:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: x-forwarded-for\r\n\r\n4\r\ntest\r\n0\r\nx-forwarded-for: 10.0.0.1\r\n\r\n' | nc -w 2 localhost 8080

Patches

• Expand the denylist in is_forbidden_trailer to include authorization, cookie, set-cookie, proxy-authorization, x-forwarded-for, x-forwarded-host, x-forwarded-proto, x-real-ip, and other security-sensitive headers.
• Alternatively, switch to an allowlist model that only permits explicitly safe trailer field names.

ewe is a Gleam web server. ewe is a Gleam web server. Versions 0.6.0 through 3.0.4 are vulnerable to authentication bypass or spoofed proxy-trust headers. Chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. A malicious client can exploit this by declaring these headers in the Trailer field and appending them after the final chunk, causing request.set_header to overwrite legitimate values (e.g., those set by a reverse proxy). This enables attackers to forge authentication credentials, hijack sessions, bypass IP-based rate limiting, or spoof proxy-trust headers in any downstream middleware that reads headers after ewe.read_body is called. This issue has been fixed in version 3.0.5.

ewe is a Gleam web server. ewe is a Gleam web server. Versions 0.6.0 through 3.0.4 are vulnerable to authentication bypass or spoofed proxy-trust headers. Chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. A malicious client can exploit this by declaring these headers in the Trailer field and appending them after the final chunk, causing request.set_header to overwrite legitimate values (e.g., those set by a reverse proxy). This enables attackers to forge authentication credentials, hijack sessions, bypass IP-based rate limiting, or spoof proxy-trust headers in any downstream middleware that reads headers after ewe.read_body is called. This issue has been fixed in version 3.0.5.

Summary

ewe's chunked transfer encoding trailer handling merges declared trailer fields into req.headers after body parsing, but the denylist only blocks 9 header names. Security-sensitive headers like authorization, cookie, and x-forwarded-for can be injected or overwritten by a malicious client via trailers, potentially bypassing authentication or spoofing proxy-trust headers.

Impact

When ewe.read_body processes a chunked request with a Trailer header, it calls handle_trailers (ewe/internal/http1.gleam:493), which merges declared trailer fields into req.headers via request.set_header (line 517). The is_forbidden_trailer denylist (line 534) only blocks 9 header names: transfer-encoding, content-length, host, cache-control, expect, max-forwards, pragma, range, and te.

Security-sensitive headers are not blocked, including:

• authorization — attacker can inject or overwrite Bearer tokens
• cookie / set-cookie — attacker can inject session cookies
• proxy-authorization — attacker can inject proxy credentials
• x-forwarded-for, x-forwarded-host, x-forwarded-proto — attacker can spoof proxy-trust headers
• x-real-ip — attacker can spoof client IP

A malicious client can inject these headers by declaring them in the Trailer request header and including them after the final 0\r\n chunk. If the header already exists (e.g., set by a reverse proxy), request.set_header overwrites it. Any application logic that reads these headers after calling ewe.read_body — such as authentication middleware, IP-based rate limiting, or session validation — will see the attacker-controlled values.

Proof of Concept

Inject an authorization header that didn't exist:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: authorization\r\n\r\n4\r\ntest\r\n0\r\nauthorization: Bearer injected-token\r\n\r\n' | nc -w 2 localhost 8080

Overwrite a legitimate authorization header set by a proxy:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nAuthorization: Bearer legitimate-token\r\nTransfer-Encoding: chunked\r\nTrailer: authorization\r\n\r\n4\r\ntest\r\n0\r\nauthorization: Bearer evil-token\r\n\r\n' | nc -w 2 localhost 8080

Inject x-forwarded-for to spoof client IP:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: x-forwarded-for\r\n\r\n4\r\ntest\r\n0\r\nx-forwarded-for: 10.0.0.1\r\n\r\n' | nc -w 2 localhost 8080

Patches

• Expand the denylist in is_forbidden_trailer to include authorization, cookie, set-cookie, proxy-authorization, x-forwarded-for, x-forwarded-host, x-forwarded-proto, x-real-ip, and other security-sensitive headers.
• Alternatively, switch to an allowlist model that only permits explicitly safe trailer field names.