CRITICAL ALERT: CVE-2025–55315 — HTTP Request Smuggling in ASP.NET Core (Kestrel)

Severity: CRITICAL (CVSS 9.9) Vulnerability: HTTP Request/Response Smuggling (CWE-444) in Kestrel web server (ASP.NET Core) Status: Patches available — urgent patching recommended POC : LINK

🔥 Summary

CVE-2025–55315 is a high-severity HTTP request smuggling vulnerability in ASP.NET Core's Kestrel web server. Due to inconsistent parsing between front-end proxies/load-balancers and Kestrel, an attacker can smuggle a malicious HTTP request inside another, bypassing security controls and causing severe impacts including authentication bypass, SSRF, data exposure, and potential full compromise.

🛠 Technical Breakdown

Request smuggling occurs when a front-end component (e.g., proxy, CDN, WAF) and a backend server interpret the boundaries of HTTP requests differently. An attacker crafts a single TCP-level payload that contains two logical HTTP requests. The front-end forwards one request to the backend while the backend processes the hidden request — giving the attacker a way to bypass filters, inject requests, or tamper with session state.

Potential outcomes include:

  • Authentication bypass or privilege escalation
  • Server-Side Request Forgery (SSRF) to internal resources (e.g., metadata endpoints)
  • Circumventing CSRF protections, caching logic, or logging
  • Data exfiltration or request/response tampering

⚠️ Affected Versions & Patches (apply immediately)

  • ASP.NET Core 8.0: <= 8.0.20 → update to 8.0.21
  • ASP.NET Core 9.0: <= 9.0.9 → update to 9.0.10
  • ASP.NET Core 10.0 RC1: update to RC2
  • Kestrel.Core package: <= 2.3.0 → update to 2.3.6
  • Note: .NET 6 is affected but is end-of-life and will not receive official patches — consider upgrade/migration and mitigations.

🛡 Immediate Mitigation & Hardening

If you cannot patch immediately, apply risk-reduction steps:

  1. Patch first — prioritize applying vendor fixes.
  2. Restart applications after runtime updates; recompile & redeploy self-contained apps.
  3. Harden front-end proxies/WAFs — configure them to reject ambiguous or non-conformant HTTP requests and to normalize headers (Content-Length / Transfer-Encoding discrepancies).
  4. Deny ambiguous transfer encodings — block requests with both Content-Length and Transfer-Encoding discrepancies.
  5. Restrict internal endpoint access — isolate management and metadata services from the application network.
  6. Network monitoring — monitor for anomalous request patterns, unexpected backend connections, and doubled or pipelined requests.
  7. Temporary WAF rules — create signatures to block known exploitation patterns until patches are applied.

🔍 Detection & Hunting

  • Search logs for suspicious patterns: multiple HTTP requests in a single TCP stream, unexpected HTTP verbs or header anomalies, or backend processing of unexpected requests.
  • Monitor for unusual session hijacks, unexplained cached response tampering, or internal-request bursts.
  • Use IDS signatures that detect smuggling patterns (e.g., conflicting Content-Length vs Transfer-Encoding headers).

📚 References & Resources

  • Microsoft Security Advisory / Patch: check official Microsoft .NET/ASP.NET security pages for vendor guidance and KB links.
  • OWASP Request Smuggling: https://owasp.org/www-community/attacks/HTTP_Request_Smuggling
  • Project/Community write-ups and detection templates (search community repos and Nuclei templates for CVE-2025–55315)

🧾 Incident Response Notes

  • If compromise is suspected: isolate affected hosts, preserve network logs (PCAPs), collect full request logs, and coordinate with your vendor or incident response provider.
  • Rotate secrets and credentials that may have been exposed.
  • Conduct a full audit of ingress points (proxies, gateways, load balancers) to ensure consistent HTTP parsing and normalization.

Share widely — patch immediately. Prepared by @cybersecplayground — follow for real-time CVE alerts and remediation tips.

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