CVE-2021-23351 is a medium-severity (CVSS 4.9) vulnerability in github.com/pires/go-proxyproto. O3 Security confirms whether CVE-2021-23351 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
github.com/pires/go-proxyproto denial of service vulnerability
Real-World Exposure
github.com/pires/go-proxyprotoReal-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
The package github.com/pires/go-proxyproto before 0.5.0 are vulnerable to Denial of Service (DoS) via the parseVersion1() function. The reader in this package is a default bufio.Reader wrapping a net.Conn. It will read from the connection until it finds a newline. Since no limits are implemented in the code, a deliberately malformed V1 header could be used to exhaust memory in a server process using this code - and create a DoS. This can be exploited by sending a stream starting with PROXY and continuing to send data (which does not contain a newline) until the target stops acknowledging. The risk here is small, because only trusted sources should be allowed to send proxy protocol headers.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/pires/go-proxyproto | all versions | 0.5.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/pires/go-proxyproto. 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 github.com/pires/go-proxyproto to 0.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23351 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 CVE-2021-23351 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 CVE-2021-23351. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-23351 in your dependencies?
O3 detects CVE-2021-23351 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.