hunt-aspnet – Coding skill for AI agents

Crown Jewel Targets

ASP.NET deserialization bugs pay among the highest amounts in bug bounty when they reach RCE. Even when patched, the disclosure-tier findings (signed-only ViewState, dual-parser differential, request-validator quirks) reliably pay Low-Medium.

Highest-value targets:

• SharePoint farms (any version — 2013/2016/2019/SE) — sign-only ViewState + permissive ToolPane.aspx + anonymous FormDigest creates the CVE-2025-53770 ToolShell precondition chain
• Telerik UI for ASP.NET AJAX — Telerik.Web.UI.WebResource.axd is a documented RCE sink when keys leak (CVE-2017-11317, CVE-2017-11357, CVE-2019-18935)
• Classic ASP.NET Webforms enterprise apps — banking portals, dealer portals, HR systems left on .NET Framework 4.x
• WCF services (*.svc?WSDL) — often forgotten admin endpoints with looser auth than the main app
• Sitecore CMS — ViewState + Sitecore-specific deserialization chains (CVE-2021-42237)
• DotNetNuke (DNN) — historic ViewState RCE chains
• Umbraco CMS — ViewState + custom deserialization sinks

Asset types that pay most: internet-reachable ASP.NET Webforms apps > WCF admin services > Telerik-integrated sites > Classic ASP.NET MVC with VSF (very rare)

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Attack Surface Signals

Response headers indicating ASP.NET:

X-AspNet-Version: 4.0.30319          (classic — disclosure on its own)
X-Powered-By: ASP.NET
X-AspNetMvc-Version: 5.2
Server: Microsoft-IIS/10.0
Set-Cookie: ASP.NET_SessionId=...
Set-Cookie: .ASPXAUTH=...            (Forms auth cookie)
Set-Cookie: .ASPXFORMSAUTH=...
Set-Cookie: ASP.NET_SessionId=...; SameSite=None  (suggests cross-origin embedding)

Body signals (in form HTML):

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="..." />
<input type="hidden" name="__VIEWSTATEGENERATOR" id="__VIEWSTATEGENERATOR" value="..." />
<input type="hidden" name="__VIEWSTATEENCRYPTED" id="__VIEWSTATEENCRYPTED" value="" />
                                        ↑ EMPTY = signed-only, not encrypted = exploitable if key leaks
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="..." />
<input type="hidden" name="__REQUESTDIGEST" id="__REQUESTDIGEST" value="0x...,...">
                                        ↑ SharePoint CSRF token; if anon-issued, see hunt-sharepoint

URL patterns to probe:

/trace.axd                            (per-app trace viewer; sometimes anon-accessible)
/elmah.axd                            (ELMAH error log viewer)
/elmah.axd/?id=...                    (ELMAH RCE / stack-trace leak)
/*.svc                                (WCF services)
/*.svc?wsdl                           (WCF WSDL)
/*.svc/mex                            (Metadata Exchange)
/*.asmx                               (legacy SOAP)
/*.asmx?WSDL                          (legacy SOAP description)
/*.asmx?disco                         (legacy discovery)
/Telerik.Web.UI.WebResource.axd       (Telerik AJAX components)
/ChartImg.axd                         (DataVisualization controls; historic deserialization)
/ScriptResource.axd                   (script resource handler; sometimes leaks paths)
/WebResource.axd                      (web resource handler)
/_vti_bin/*                           (SharePoint Web Service Forwarder)
/api/                                 (Web API 2.x is ASP.NET on classic framework)
/signin                               (often FedAuth / WS-Federation)

Tech-stack signals:

• Server: Microsoft-IIS/10.0 (or /8.5, /7.5) — confirmed Windows + IIS
• X-AspNet-Version header — classic .NET Framework (4.x); .NET Core/5+ does NOT emit this
• Cookies with ASP.NET_SessionId, .ASPXAUTH, FedAuth — Forms or claims auth
• __VIEWSTATE in form bodies — Webforms (NOT MVC, NOT Razor Pages, NOT Blazor)
• MicrosoftSharePointTeamServices header (sometimes stripped by ELB but leaks in start.aspx body) — SharePoint

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Step-by-Step Hunting Methodology

1. Fingerprint the framework version. Trigger any 500 error (stale ViewState POST is a reliable way) and look for Version Information: Microsoft .NET Framework Version:X.X.XXXXX; ASP.NET Version:X.X.XXXX.X in the error body. This banner discloses both the runtime and ASP.NET-version-specific patch level. .NET 4.0.30319 + ASP.NET 4.8.x is the most common modern combination.

2. Locate every form with __VIEWSTATE. Spider the target and grep for name="__VIEWSTATE". Each is a candidate sink for deserialization attacks if MAC / encryption is bypassable.

3. Check __VIEWSTATEENCRYPTED value. Empty (value="") means ViewState is signed-only via <machineKey> but NOT encrypted. Recovery of the validation key → arbitrary deserialization. Non-empty (value="something") means ViewState is BOTH signed and encrypted; both keys needed to forge.

4. Test the ViewState parser-error differential (the dual-parser anti-pattern). Send 7+ ViewState shapes and classify responses:

• Trivial garbage (AAAA) → "Validation of viewstate MAC failed"
• Real prefix from current page → "Validation of viewstate MAC failed"
• Flipped-bit real ViewState → "Validation of viewstate MAC failed"
• Oversize (A * 100000) → "Validation of viewstate MAC failed"
• XML-shaped (<xss/>) → "The state information is invalid for this page and might be corrupted" ← different parser path
• LosFormatter-style prefix (/wEPDwUKMTcxNzgyOTQwMmRkkz9p4lzA...) → "The state information is invalid for this page and might be corrupted"

The differential proves there are two distinct deserialization entry points, one of which dispatches BEFORE the MAC check on some payload shapes. Historically this enables MAC-before-parse-bypass exploits.

5. Look for load-balanced cross-node ViewState MAC failures. If POST gets a 500 with "Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey...", the farm has multiple WFEs WITHOUT machineKey sync, or without sticky-session affinity. Operationally this breaks legit users; security-wise it confirms farm topology.

6. Probe trace.axd and elmah.axd. If either returns 200 anonymously, it's a Critical finding (trace leaks every request + headers + form data; ELMAH leaks every server error including stack traces).

7. Enumerate WCF services (.svc). For each, fetch ?wsdl and ?mex (metadata exchange). MEX endpoints sometimes return full service contracts including admin operations.

8. Test request-validator bypass. ASP.NET's request validator blocks < in query strings by default. Bypass categories that may still get through:

• HTML-entity-encoded payloads (<script> — but these don't execute)
• Encoded inside JSON / XML POST bodies (different content-type ≠ same validator)
• In path segments (not query) — validator scope depends on framework version
• In Cookie / Referer headers (varies)
• Inside <%@ ... %> ASP directives if reached via WebDAV PUT (rare)

9. Check customErrors mode. If 500s expose full stack traces, framework versions, file paths, internal method names → customErrors mode="Off" is set. Should be RemoteOnly for production.

10. Look for Telerik components. Telerik.Web.UI.WebResource.axd?type=rau is the historic upload-to-RCE chain (CVE-2017-11317). The dialogParametersHolder parameter chain (CVE-2019-18935) requires the encryption key but is otherwise RCE.

11. SharePoint-specific deserialization paths — see hunt-sharepoint skill for the ToolPane.aspx + anonymous FormDigest + unencrypted ViewState chain.

12. SafeControl enumeration via reflection. SharePoint's Picker.aspx?PickerDialogType=<TypeName> (and DNN-equivalent endpoints) accept class names and return DIFFERENT error messages for "type exists but not whitelisted" vs "type does not exist." Feed a wordlist of Microsoft.SharePoint.*.WebControls.* types to enumerate the SafeControl list — useful for CVE-2019-0604-family hunting.

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Payload & Detection Patterns

Stack-trace fingerprint (trigger via stale ViewState POST):

curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.example/page.aspx" \
  --data "__VIEWSTATE=AAAA&__VIEWSTATEGENERATOR=AAAA"
# Inspect body for:
#  - "Validation of viewstate MAC failed" → confirms signed ViewState
#  - "The state information is invalid for this page" → confirms ALTERNATE parser path
#  - "Version Information: Microsoft .NET Framework Version:X.X.XXXXX" → exact patch level
#  - "Microsoft.SharePoint.Client.ServerStub..." → SharePoint farm

ViewState parser-error differential probe (Python):

import requests, re, json
S = requests.Session(); S.verify = False
# Get fresh form
r = S.get("https://target.example/path/page.aspx")
real_vs = re.search(r'__VIEWSTATE" id="__VIEWSTATE" value="([^"]+)', r.text).group(1)
real_vsg = re.search(r'__VIEWSTATEGENERATOR" 

… (full content is in the download package)