EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
eweReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Hex packages — download data is not available via public APIs for these ecosystems.
Description
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 tokenscookie/set-cookie— attacker can inject session cookiesproxy-authorization— attacker can inject proxy credentialsx-forwarded-for,x-forwarded-host,x-forwarded-proto— attacker can spoof proxy-trust headersx-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_trailerto includeauthorization,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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💧Hex | ewe | ≥ 0.6.0&&< 3.0.5 | 3.0.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ewe. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update ewe to 3.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-9w88-79f8-m3vp is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-9w88-79f8-m3vp is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-9w88-79f8-m3vp. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-9w88-79f8-m3vp in your dependencies?
O3 detects GHSA-9w88-79f8-m3vp across Hex dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.