hunt-aspnet skill

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)

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

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.

1. 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.

1. Check VIEWSTATEENCRYPTED value.** Empty (value="") means ViewState is signed-only via 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.

1. 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 () → "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.

1. 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 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.

1. 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).

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

1. 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 (&lt;script&gt; — 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)

1. 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.

1. 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.

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

1. SafeControl enumeration via reflection. SharePoint's Picker.aspx?PickerDialogType= (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.

Payload & Detection Patterns

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

curl -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" id="__VIEWSTATEGENERATOR" value="([^"]+)', r.text).group(1)

# Test 7 payload shapes
for label, vs in [
    ("trivial",      "AAAA"),
    ("real",         real_vs),
    ("flipped-bit",  real_vs[:50] + "X" + real_vs[51:]),
    ("oversize",     "A" * 100000),
    ("base64",       "VGVzdE1hcmtlcjY3OFhZWg=="),
    ("xml-shaped",   "<xss/>"),
    ("losformatter", "/wEPDwUKMTcxNzgyOTQwMmRkkz9p4lzA" + "A"*50),
]:
    r = S.post("https://target.example/path/page.aspx",
               data={"__VIEWSTATE": vs, "__VIEWSTATEGENERATOR": real_vsg})
    title = re.search(r'<title>([^<]+)</title>', r.text)
    title = title.group(1)[:100] if title else "—"
    print(f"  [{label:14s}] {r.status_code}  {title}")

trace.axd anonymous check:

curl -sk -o /dev/null -w "%{http_code}\n" "https://target.example/trace.axd"
# 200 = full trace dump exposed → Critical
# 403 = mod set to localhost-only → check via X-Forwarded-For: 127.0.0.1

WCF service enumeration:

# Find all .svc files
curl -sk "https://target.example/" -o body.html
grep -oE '/[a-zA-Z0-9/_-]+\.svc' body.html | sort -u
# For each found:
curl -sk "https://target.example/Service.svc?wsdl" | xmllint --format - | head -60

Request-validator bypass categories:

# Default: <script>alert(1)</script> in ?q= → "Potentially dangerous Request.QueryString value detected"
# Bypasses that sometimes work:
?q=%3cscript%3e            (URL-encoded — depends on validator config)
?q=<svg/onload=alert(1)>  (depends on validator version)
?q=<%00script>             (NUL-byte; older validators)
?q=javascript:alert(1)     (no < at all — passes validator)
Cookie: foo=<script>       (cookie body not validated by default)
Referer: http://x.com/<script>  (referer not validated in classic ASP.NET)

Telerik exploit gate (CVE-2019-18935 — requires encryption keys):

# Fingerprint Telerik
curl -sk "https://target.example/Telerik.Web.UI.WebResource.axd?type=rau" -X POST
# If response is RadAsyncUploadHandler-style → Telerik present; try keys
# Public exploits require leaked machineKey AND telerikEncryptionKey

Common Root Causes

1. viewStateEncryption="Auto" defaults to signed-only on pages without sensitive ViewState data. Many SharePoint pages are configured this way. When __VIEWSTATEENCRYPTED is empty, ViewState is signed-only — recovery of validationKey alone enables forgery.

1. AutoGenerate in a Web Farm. Each WFE generates a different key on first boot; ViewState issued by one WFE fails MAC validation on another. Operationally produces 500s; security-wise broadcasts the topology (the error message names the cluster).

1. left from development. Stack traces with full method names, file paths, version banners exposed to anonymous internet users.

1. trace.axd / elmah.axd left enabled in production. Often forgotten in blocks.

1. Forgotten WCF .svc admin endpoints. Built for internal admin tooling, never disabled when the main app went to internet exposure.

1. Dual-parser anti-pattern: ObjectStateFormatter (legacy) vs LosFormatter (modern) deserialize in different orders relative to MAC validation. Some payload shapes hit the legacy parser BEFORE MAC check.

1. Request validator only applies to URL-encoded body and querystring. Headers, cookies, XML/JSON bodies, and multipart fields are NOT validated by default. Developers assume validator is universal; it is not.

1. checked into source repos. Configuration check-ins to GitHub frequently leak validation/decryption keys. Combine with hunt-misc source-recon for Telerik / SharePoint / DNN keys.

1. SafeControls web.config entries trusted to gate deserialization. SharePoint's list determines which classes Picker.aspx can instantiate. Bypasses exist when the inheritance check is the only gate (CVE-2019-0604 family).

Bypass Techniques

Gate 0 Validation

Before writing the report, confirm:

1. What can the attacker DO right now with the disclosed information?

• trace.axd 200 with full request dump → Critical (PII / session cookies / Authorization headers exposed)
• elmah.axd 200 with error log → High (stack traces + internal paths + sometimes credentials)
• VIEWSTATEENCRYPTED empty + recoverable machineKey via separate finding → Critical chain to RCE**
• VIEWSTATEENCRYPTED empty without key recovery → Low-Medium** (primitive present, not exploitable on its own)
• Stack traces in 500s → Low unless they include credentials / connection strings

1. Have you reproduced the full chain to attacker-attainable impact, or only the primitive?

• Cross-reference triage-validation Pre-Severity Gate. "Primitive confirmed" is not Critical until the chain ends in impact.