GHSA-ghc4-35x6-crw5 is a high-severity (CVSS 7.5) CWE-863 vulnerability in github.com/envoyproxy/envoy. A fix is available for github.com/envoyproxy/envoy — see the affected versions and patch details below.
Envoy has RBAC Header Validation Bypass via Multi-Value Header Concatenation
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-ghc4-35x6-crw5.
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.
How urgent is this, really
GHSA-ghc4-35x6-crw5 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
1. Summary
The Envoy RBAC (Role-Based Access Control) filter contains a logic vulnerability in how it validates HTTP headers when multiple values are present for the same header name. Instead of validating each header value individually, Envoy concatenates all values into a single comma-separated string. This behavior allows attackers to bypass RBAC policies—specifically "Deny" rules—by sending duplicate headers, effectively obscuring the malicious value from exact-match mechanisms.
2. Attack Scenario
Consider an environment where an administrator wants to block external access to internal resources using a specific header flag.
Configuration
The Envoy proxy is configured with a Deny rule to reject requests containing the header internal: true.
- Rule Type: Exact Match
- Target:
internalheader must not equaltrue.
The Bypass Logic
-
Standard Request (Blocked):
- Input:
internal: true - Envoy Processing: Sees string
"true". - Result: Match found. Request Denied.
- Input:
-
Exploit Request (Bypassed):
- Input:
internal: true internal: true - Envoy Processing: Concatenates values into
"true,true". - Matcher Evaluation: Does
"true,true"equal"true"? No. - Result: The Deny rule fails to trigger. Request Allowed.
- Input:
3. Implications
- RBAC Bypass: Remote attackers can bypass configured access controls.
- Unauthorized Access: Sensitive internal resources or administrative endpoints protected by header-based Deny rules become accessible.
- Risk: High, particularly for deployments relying on "Exact Match" strategies for security blocking.
4. Reproduction Steps
To verify this vulnerability:
- Deploy Envoy: Configure an instance with an RBAC Deny rule that performs an exact match on a specific header (e.g.,
internal: true). - Baseline Test: Send a request containing the header
internal: true.- Observation: Envoy blocks this request (HTTP 403).
- Exploit Test: Send a second request containing the same header twice:
GET /restricted-resource HTTP/1.1 Host: example.com internal: true internal: true- Observation: Envoy allows the request, granting access to the resource.
6. Recommendations
Fix Header Validation Logic:
Modify the RBAC filter to validate each header value instance individually. Avoid relying on the concatenated string output of getAllOfHeaderAsString() for security-critical matching unless the matcher is explicitly designed to parse comma-separated lists.
** Examine the DENY role to use a Regex style fix.
Credit: Dor Konis
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/envoyproxy/envoy | ≥ 1.37.0&&< 1.37.1 | 1.37.1go get github.com/envoyproxy/envoy@v1.37.1 |
| 🐹Go | github.com/envoyproxy/envoy | ≥ 1.36.0&&< 1.36.5 | 1.36.5go get github.com/envoyproxy/envoy@v1.36.5 |
| 🐹Go | github.com/envoyproxy/envoy | ≥ 1.35.0&&< 1.35.9 | 1.35.9go get github.com/envoyproxy/envoy@v1.35.9 |
| 🐹Go | github.com/envoyproxy/envoy | all versions | 1.34.13go get github.com/envoyproxy/envoy@v1.34.13 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/envoyproxy/envoy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/envoyproxy/envoy to 1.37.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-ghc4-35x6-crw5 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-ghc4-35x6-crw5 can be triaged on real exposure rather than presence alone.
Tailored to GHSA-ghc4-35x6-crw5. 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-ghc4-35x6-crw5 in your dependencies?
O3 Security finds GHSA-ghc4-35x6-crw5 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.