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xmlseclibs

xmlseclibs is a library written in PHP for working with XML Encryption and Signatures.

The author of xmlseclibs is Rob Richards.

Branches

Master is currently the only actively maintained branch.

• master: requires PHP version 8.0+
• 3.1: Added AES-GCM support requiring 7.1+
• 3.0: Removes mcrypt usage requiring 5.4+ (5.6.24+ recommended for security reasons)
• 2.0: Contains namespace support requiring 5.3+
• 1.4: Contains auto-loader support while also maintaining backwards compatiblity with the older 1.3 version using the xmlseclibs.php file. Supports PHP 5.2+

Requirements

xmlseclibs requires PHP version 8.0 or greater. OpenSSL is optional (phpseclib is used for crypto).

Security notes

• Prefer the safe-by-default verifier verifyDocument() (see below). It requires a caller-supplied (pinned) key, never derives the key from the document's KeyInfo, enforces an algorithm allowlist for both the SignatureMethod and every DigestMethod, and only reports success when every reference validated. It returns the validated nodes for you to operate on.
• If you use the low-level primitives directly, always check verification with a strict comparison: $objDSig->verify($key) === 1. A return of -1 is an error and is truthy in boolean context.
• After validateReference(), use getValidatedNodes() and operate only on those nodes (especially for SAML / WS-Security). Do not re-select assertions by Id from the whole document.
• Do not trust a signing certificate from KeyInfo alone. Load and pin trusted keys yourself.
• By default verifyDocument() accepts only SHA-256/384/512 digests and RSA-SHA-256/384/512 (and RSA-PSS) signatures. To interoperate with legacy peers, widen the sets explicitly, e.g. $objDSig->allowedSignatureAlgorithms[] = XMLSecurityKey::RSA_SHA1;.
• Prefer RSA-OAEP and AES-GCM for encryption. RSA-1.5 key transport is denied by default on decryption (Bleichenbacher risk); opt in with $objenc->allowRSA15KeyTransport = true; only for legacy interop. You can additionally pin exact algorithms via $objenc->allowedKeyAlgorithms / $objenc->allowedDataAlgorithms (presets: XMLSecEnc::DEFAULT_KEY_ALGORITHMS and XMLSecEnc::DEFAULT_DATA_ALGORITHMS, both authenticated/OAEP-only). Unauthenticated CBC modes remain available for interop but are malleable — prefer AES-GCM.

• XPath (REC-xpath-19991116) transforms are rejected during verification by default. They evaluate a document-supplied XPath expression in validateReference() before any signature crypto runs, so a crafted expression is a pre-authentication CPU denial-of-service — and the expression is arbitrary XPath by design, so it cannot be sanitized. SAML and WS-Security do not use them. Set $objDSig->allowXPathTransforms = true only if you must verify signatures that legitimately rely on XPath transforms and you trust the source. Signing is unaffected. When enabled, the count/namespace caps below still apply.

• XPath transforms (once enabled) are capped by default (maxXPathTransforms / maxXPathNamespaces, defaults 5 and 20). Raise or lower these on the XMLSecurityDSig instance if your use case needs different limits.
• add509Cert(..., $isURL = true) fetches over http/https only and rejects hosts that resolve to loopback/private/link-local/reserved/CGNAT addresses, with redirects disabled (SSRF hardening). file:// is disabled unless you pass array('allow_file_scheme' => true) in $options. Only fetch certificates from trusted URLs — a small DNS-rebinding window remains.

• Decrypted XML containing a DOCTYPE is rejected to guard against entity-expansion / XXE. Always load untrusted input documents yourself with DTD/entity processing disabled.

• Documents carrying a DOCTYPE are rejected during signature verification (locateSignature() / verifyDocument()). This closes the entity-reference bypass in which an Id="&e;" attribute is resolved by getAttribute() but is invisible to the XPath reference lookup (a libxml2 hashing bug, the same root cause as CVE-2025-23369), causing verify() to validate a different node than the one your application reads. Set $objDSig->forbidDoctype = false only if you fully trust the document source and require DTD support.

• Legacy interoperability (temporary): if you must accept documents/peers that rely on pre-4.0 behaviour, call $objDSig->enableLegacyMode() and/or $objenc->enableLegacyMode() once after construction. That restores the interoperability settings in one place (DOCTYPE allowed on verify, XPath transforms allowed without count caps, RSA-1.5 key transport allowed). It does not undo always-on hardening such as SignatureMethod/key binding, uniform decryption errors, or DOCTYPE rejection in decrypted XML. Prefer migrating peers and removing the call.

Breaking changes (3.1 → 4.0)

Platform

• PHP 8.0+ required (was 5.4+)
• phpseclib/phpseclib ~3.0 required; ext-openssl is now optional

Signature verification

• DOCTYPE rejected by default during verification (locateSignature / verifyDocument). Opt out: $dsig->forbidDoctype = false or enableLegacyMode()
• XPath Filtering Transforms rejected by default on verify. Opt in: $dsig->allowXPathTransforms = true or enableLegacyMode(). Signing unchanged
• XPath caps when enabled: max 5 transforms / 20 namespaces per transform ($maxXPathTransforms / $maxXPathNamespaces; raised by enableLegacyMode())
• verify() always requires SignatureMethod === key algorithm; mismatch throws (not restored by legacy mode)
• HMAC keys cannot be loaded from certs/PEM
• References fail closed: unresolved, external, or duplicate-Id URIs throw; same-document only
• Unknown CanonicalizationMethod rejected
• HMAC verify returns 1/0 — always check === 1

Encryption / decryption

• RSA-1.5 key transport denied by default. Opt in: $enc->allowRSA15KeyTransport = true or enableLegacyMode()
• Single decrypt error message: Failure decrypting Data (do not branch on exception text)
• DOCTYPE rejected in decrypted XML (not restored by legacy mode)
• EncryptedKey/RetrievalMethod depth capped
• ISO 10126 pad length validated on CBC decrypt

Certificate URL fetch (add509Cert)

• Only http/https by default; file:// needs ['allow_file_scheme' => true]
• Private/loopback/link-local/reserved/CGNAT targets rejected; redirects disabled

API signature changes (defaults preserve most callers)

• processTransforms(..., $signing = false)
• staticLocateKeyInfo(..., $depth = 0, $allowRSA15 = false)
• fromEncryptedKeyElement(..., $depth = 0, $allowRSA15 = false)

Legacy mode

$dsig->enableLegacyMode() / $enc->enableLegacyMode() restores DOCTYPE (verify), XPath transforms + uncapped limits, and RSA-1.5 only. It does not undo algorithm/key binding, uniform decrypt errors, decrypted-XML DOCTYPE rejection, Reference fail-closed behavior, SSRF rules, or the PHP/phpseclib requirements.

Migration tip: Prefer verifyDocument() with a pinned key. Use legacy mode only while updating peers.

Verifying a signature (recommended)

Decrypting XML (recommended recipe)

xmlseclibs does not derive the decryption key from the document: you supply your own private key, and the document's session key is unwrapped with it. The recipe below covers the common EncryptedKey (key transport) + EncryptedData (data) layout used by SAML and WS-Security, with the algorithm policy pinned.

If the recipient key is selected by KeyName (rather than an embedded EncryptedKey), read $objKeyInfo->name and load the matching local private key yourself before decrypting. Never treat key material from the document as trusted.

How to Install

Install with composer.phar.

Use cases

xmlseclibs is being used in many different software.

• SimpleSAMLPHP
• LightSAML
• OneLogin

Basic usage

The example below shows basic usage of xmlseclibs, with a SHA-256 signature.

How to Contribute

• Open Issues
• Open Pull Requests

Mailing List: https://groups.google.com/forum/#!forum/xmlseclibs