Internet Engineering Task Force (IETF) D. Benjamin
Request for Comments: 9618 Google LLC
Updates: 5280 August 2024
Category: Standards Track
ISSN: 2070-1721
Updates to X.509 Policy Validation
Abstract
This document updates RFC 5280 to replace the algorithm for X.509
policy validation with an equivalent, more efficient algorithm. The
original algorithm built a structure that scaled exponentially in the
worst case, leaving implementations vulnerable to denial-of-service
attacks.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9618.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Summary of Changes from RFC 5280
2. Conventions and Definitions
3. Denial-of-Service Vulnerability
3.1. Policy Trees
3.2. Exponential Growth
3.3. Attack Vector
4. Avoiding Exponential Growth
4.1. Policy Graphs
4.2. Verification Outputs
5. Updates to RFC 5280
5.1. Updates to Section 6.1
5.2. Updates to Section 6.1.2
5.3. Updates to Section 6.1.3
5.4. Updates to Section 6.1.4
5.5. Updates to Section 6.1.5
5.6. Updates to Section 6.1.6
6. Other Mitigations
6.1. Verify Signatures First
6.2. Limit Certificate Depth
6.3. Limit Policy Tree Size
6.4. Inhibit Policy Mapping
6.5. Disable Policy Checking
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Author's Address
1. Introduction
[RFC5280] defines a suite of extensions for determining the policies
that apply to a certification path. A policy is described by an
object identifier (OID) and a set of optional qualifiers.
Policy validation in [RFC5280] is complex. As an overview, the
certificate policies extension (Section 4.2.1.4 of [RFC5280])
describes the policies, with optional qualifiers, under which an
individual certificate was issued. The policy mappings extension
(Section 4.2.1.5 of [RFC5280]) allows a CA certificate to map its
policy OIDs to other policy OIDs in certificates that it issues.
Subject to these mappings and other extensions, the certification
path's overall policy set is the intersection of policies asserted by
each certificate in the path.
The procedure in Section 6.1 of [RFC5280] determines this set in the
course of certification path validation. It does so by building a
policy tree containing policies asserted by each certificate and the
mappings between them. This tree can grow exponentially in the depth
of the certification path, which means an attacker, with a small
input, can cause a path validator to consume excessive memory and
computational resources. This cost asymmetry can lead to a denial-
of-service vulnerability in X.509-based applications, such as
[CVE-2023-0464] and [CVE-2023-23524].
Section 3 describes this vulnerability. Section 4.1 describes the
primary mitigation for this vulnerability, a replacement for the
policy tree structure. Section 5 provides updates to [RFC5280] that
implement this change. Finally, Section 6 discusses alternative
mitigation strategies for X.509 applications.
1.1. Summary of Changes from RFC 5280
The algorithm for processing certificate policies and policy mappings
is replaced with one that builds an equivalent but much more
efficient structure. This new algorithm does not change the validity
status of any certification path or which certificate policies are
valid for it.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Denial-of-Service Vulnerability
This section discusses how the path validation algorithm defined in
Section 6.1.2 of [RFC5280] can lead to a denial-of-service
vulnerability in X.509-based applications.
3.1. Policy Trees
Section 6.1.2 of [RFC5280] constructs the valid_policy_tree, a tree
of certificate policies, during certification path validation. The
nodes at any given depth in the tree correspond to policies asserted
by a certificate in the certification path. A node's parent policy
is the policy in the issuer certificate that was mapped to this
policy, and a node's children are the policies the node was mapped to
in the subject certificate.
For example, suppose a certification path contains:
* An intermediate certificate that asserts the following policy
OIDs: OID1, OID2, and OID5. It contains mappings from OID1 to
OID3 and from OID1 to OID4.
* An end-entity certificate that asserts the following policy OIDs:
OID2, OID3, and OID6.
This would result in the tree shown below. Note that OID5 and OID6
are not included or mapped across the whole path, so they do not
appear in the final structure.
+-----------+
Root: | anyPolicy |
+-----------+
|{anyPolicy}|
+-----------+
/ \
/ \
v v
+------------+ +------------+
Intermediate: | OID1 | | OID2 |
(OID5 discarded) +------------+ +------------+
|{OID3, OID4}| | {OID2} |
+------------+ +------------+
| |
| |
v v
+------------+ +------------+
End-entity: | OID3 | | OID2 |
(OID6 discarded) +------------+ +------------+
The complete algorithm for building this structure is described in
steps (d), (e), and (f) in Section 6.1.3 of [RFC5280]; steps (h),
(i), and (j) in Section 6.1.4 of [RFC5280]; and steps (a), (b), and
(g) in Section 6.1.5 of [RFC5280].
3.2. Exponential Growth
The valid_policy_tree grows exponentially in the worst case. In step
(d.1) in Section 6.1.3 of [RFC5280], a single policy P can produce
multiple child nodes if multiple issuer policies map to P. This can
cause the tree size to increase in size multiplicatively at each
level.
In particular, consider a certificate chain where every intermediate
certificate asserts policies OID1 and OID2 and then contains the full
Cartesian product of mappings:
* OID1 maps to OID1
* OID1 maps to OID2
* OID2 maps to OID1
* OID2 maps to OID2
At each depth, the tree would double in size. For example, if there
are two intermediate certificates and one end-entity certificate, the
resulting tree would be as depicted in Figure 1.
+-----------------------+
| anyPolicy |
+-----------------------+
| {anyPolicy} |
+-----------------------+
/ \
/ \
v v
+------------+ +------------+
| OID1 | | OID2 |
+------------+ +------------+
|{OID1, OID2}| |{OID1, OID2}|
+------------+ +------------+
/ \ / \
/ \ / \
v v v v
+------------+ +------------+ +------------+ +------------+
| OID1 | | OID2 | | OID1 | | OID2 |
+------------+ +------------+ +------------+ +------------+
|{OID1, OID2}| |{OID1, OID2}| |{OID1, OID2}| |{OID1, OID2}|
+------------+ +------------+ +------------+ +------------+
| | | | | | | |
v v v v v v v v
+------+ +------+ +------+ +------+ +------+ +------+ +------+ +------+
| OID1 | | OID2 | | OID1 | | OID2 | | OID1 | | OID2 | | OID1 | | OID2 |
+------+ +------+ +------+ +------+ +------+ +------+ +------+ +------+
Figure 1: An Example X.509 Policy Tree with Exponential Growth
3.3. Attack Vector
An attacker can use the exponential growth to mount a denial-of-
service attack against an X.509-based application. The attacker
sends a certificate chain as described in Section 3.2 and triggers
the target application's certificate validation process. For
example, the target application may be a TLS server [RFC8446] that
performs client certificate validation. The target application will
consume far more resources processing the input than the attacker
consumed to send it, which prevents the target application from
servicing other clients.
4. Avoiding Exponential Growth
This document mitigates the denial-of-service vulnerability described
in Section 3 by replacing the policy tree with a policy graph
structure, which is described in this section. The policy graph
grows linearly instead of exponentially. This removes the asymmetric
cost in policy validation.
X.509 implementations SHOULD perform policy validation by building a
policy graph, following the procedure described in Section 5. This
replacement procedure computes the same policies as in [RFC5280], but
one of the outputs is in a different form. See Section 4.2 for
details. Section 6 describes alternative mitigations for
implementations that depend on the original, exponential-sized
output.
4.1. Policy Graphs
The tree structure in [RFC5280] is an unnecessarily inefficient
representation of a certification path's policy mappings. When
multiple issuer policies map to a single subject policy, the subject
policy will correspond to multiple duplicate nodes in the policy
tree. Children of the subject policy are then duplicated
recursively. This duplication is the source of the exponential
growth described in Section 3.2.
A policy graph represents the same information with a directed
acyclic graph of policy nodes. It eliminates this duplication by
using a single node with multiple parents. See Section 5 for the
procedure for building this structure. Figure 2 shows the updated
representation of the example in Figure 1.
+-----------+
| anyPolicy |
+-----------+
|{anyPolicy}|
+-----------+
/ \
/ \
v v
+------------+ +------------+
| OID1 | | OID2 |
+------------+ +------------+
|{OID1, OID2}| |{OID1, OID2}|
+------------+ +------------+
| \ / |
| \ / |
| \/ |
| /\ |
| / \ |
v v v v
+------------+ +------------+
| OID1 | | OID2 |
+------------+ +------------+
|{OID1, OID2}| |{OID1, OID2}|
+------------+ +------------+
| \ / |
| \ / |
| \/ |
| /\ |
| / \ |
v v v v
+------------+ +------------+
| OID1 | | OID2 |
+------------+ +------------+
Figure 2: A More Efficient Representation of an X.509 Policy Tree
This graph's size is bounded linearly by the total number of
certificate policies (Section 4.2.1.4 of [RFC5280]) and policy
mappings (Section 4.2.1.5 of [RFC5280]). The policy tree in
[RFC5280] is the tree of all paths from the root to a leaf in the
policy graph, so no information is lost in the graph representation.
4.2. Verification Outputs
Section 6.1.6 of [RFC5280] describes the entire valid_policy_tree
structure as an output of the verification process. However,
Section 12.2 of [X.509] only describes the following as outputs: the
authorities-constrained policies, the user-constrained policies, and
their associated qualifiers.
As the valid_policy_tree is the exponential structure, computing it
reintroduces the denial-of-service vulnerability. X.509
implementations SHOULD NOT output the entire valid_policy_tree
structure; instead, they SHOULD limit output to just the set of
authorities-constrained and/or user-constrained policies, as
described in [X.509]. Sections 5.6 and 6 discuss other mitigations
for applications where this option is not available.
X.509 implementations MAY omit policy qualifiers from the output to
simplify processing. Note that Section 4.2.1.4 of [RFC5280] already
recommends that certification authorities omit policy qualifiers from
policy information terms.
5. Updates to RFC 5280
This section provides updates to [RFC5280]. These updates implement
the changes described in Section 4.
5.1. Updates to Section 6.1
Section 6.1 of [RFC5280] is updated as follows:
OLD:
| A particular certification path may not, however, be appropriate
| for all applications. Therefore, an application MAY augment this
| algorithm to further limit the set of valid paths. The path
| validation process also determines the set of certificate policies
| that are valid for this path, based on the certificate policies
| extension, policy mappings extension, policy constraints
| extension, and inhibit anyPolicy extension. To achieve this, the
| path validation algorithm constructs a valid policy tree. If the
| set of certificate policies that are valid for this path is not
| empty, then the result will be a valid policy tree of depth n,
| otherwise the result will be a null valid policy tree.
NEW:
| A particular certification path may not, however, be appropriate
| for all applications. Therefore, an application MAY augment this
| algorithm to further limit the set of valid paths. The path
| validation process also determines the set of certificate policies
| that are valid for this path, based on the certificate policies
| extension, policy mappings extension, policy constraints
| extension, and inhibit anyPolicy extension. To achieve this, the
| path validation algorithm constructs a valid policy set, which may
| be empty if no certificate policies are valid for this path.
5.2. Updates to Section 6.1.2
The following replaces entry (a) in Section 6.1.2 of [RFC5280]:
| (a) valid_policy_graph: A directed acyclic graph of certificate
| policies with their optional qualifiers; each of the leaves
| of the graph represents a valid policy at this stage in the
| certification path validation. If valid policies exist at
| this stage in the certification path validation, the depth of
| the graph is equal to the number of certificates in the chain
| that have been processed. If valid policies do not exist at
| this stage in the certification path validation, the graph is
| set to NULL. Once the graph is set to NULL, policy
| processing ceases. Implementations MAY omit qualifiers if
| not returned in the output.
|
| Each node in the valid_policy_graph includes three data
| objects: the valid policy, a set of associated policy
| qualifiers, and a set of one or more expected policy values.
|
| Nodes in the graph can be divided into depths, numbered
| starting from zero. A node at depth x can have zero or more
| children at depth x+1 and, with the exception of depth zero,
| one or more parents at depth x-1. No other edges between
| nodes may exist.
|
| If the node is at depth x, the components of the node have
| the following semantics:
|
| (1) The valid_policy is a single policy OID representing a valid
| valid policy for the path of length x.
|
| (2) The qualifier_set is a set of policy qualifiers associated
| associated with the valid policy in certificate x. It
| is only necessary
| to maintain this field if policy
| qualifiers are returned to
| the application. See
| Section 6.1.5, step (g).
|
| (3) The expected_policy_set contains one or more policy OIDs that
| that would satisfy this policy in the certificate x+1.
|
| The initial value of the valid_policy_graph is a single node
| with valid_policy anyPolicy, an empty qualifier_set, and an
| expected_policy_set with the single value anyPolicy. This
| node is considered to be at depth zero.
|
| The graph additionally satisfies the following invariants:
|
| * For any depth x and policy OID P-OID, there is at most one node
| node at depth x whose valid_policy is P-OID.
|
| * The expected_policy_set of a node whose valid_policy is
| anyPolicy is always {anyPolicy}.
|
| * A node at depth x whose valid_policy is anyPolicy, except for
| for the one at depth zero, always has exactly one parent:
| a node at
| depth x-1 whose valid_policy is also anyPolicy.
|
| * Each node at depth greater than 0 has either one or more parent
| parent nodes whose valid_policy is not anyPolicy or a
| single parent
| node whose valid_policy is anyPolicy. That
| is, a node cannot
| simultaneously be a child of both
| anyPolicy and some non-
| anyPolicy non-anyPolicy OID.
|
| Figure 3 is a graphic representation of the initial state of
| the valid_policy_graph. Additional figures will use this
| format to describe changes in the valid_policy_graph during
| path processing.
|
| +----------------+
| | anyPolicy | <---- valid_policy
| +----------------+
| | {} | <---- qualifier_set
| +----------------+
| | {anyPolicy} | <---- expected_policy_set
| +----------------+
|
| Figure 3: Initial Value of the valid_policy_graph State
| Variable
5.3. Updates to Section 6.1.3
The following replaces steps (d), (e), and (f) in Section 6.1.3 of
[RFC5280]:
| (d) If the certificate policies extension is present in the
| certificate and the valid_policy_graph is not NULL, process
| the policy information by performing the following steps in
| order:
|
| (1) For each policy P not equal to anyPolicy in the certificate
| certificate policies extension, let P-OID denote the OID
| for policy P and
| P-Q denote the qualifier set for policy
| P. Perform the
| following steps in order:
|
| (i) Let parent_nodes be the nodes at depth i-1 in the
| valid_policy_graph where P-OID is in the
| expected_policy_set. If parent_nodes is not
| empty, create a
| child node as follows: set the
| valid_policy to P-OID, set
| the qualifier_set to
| P-Q, set the expected_policy_set to
| {P-OID}, and
| set the parent nodes to parent_nodes.
|
| For example, consider a valid_policy_graph with a
| node of
| depth i-1 where the expected_policy_set is
| {Gold, White} and
| a second node where the
| expected_policy_set is {Gold,
| Yellow}. Assume the
| certificate policies Gold and Silver
| appear in the
| certificate policies extension of certificate
| i.
| The Gold policy is matched, but the Silver policy is
| is not. This rule will generate a child node of
| depth i for
| the Gold policy. The result is shown
| as Figure 4.
|
| +-----------------+ +-----------------+
| | Red | | Blue |
| +-----------------+ +-----------------+
| | {} | | {} | depth i-1
| +-----------------+ +-----------------+
| | {Gold, White} | | {Gold, Yellow} |
| +-----------------+ +-----------------+
| \ /
| \ /
| \ /
| v v
| +-----------------+
| | Gold |
| +-----------------+
| | {} | depth i
| +-----------------+
| | {Gold} |
| +-----------------+
|
| Figure 4: Processing an Exact Match
|
| (ii) If there was no match in step (i) and the valid_policy_graph
| valid_policy_graph includes a node of depth i-1
| with the valid_policy
| anyPolicy, generate a child
| node with the following values:
| set the
| valid_policy to P-OID, set the qualifier_set to P-Q,
| P-Q, set the expected_policy_set to {P-OID}, and
| set the parent
| node to the anyPolicy node at depth
| i-1.
|
| For example, consider a valid_policy_graph with a
| node of
| depth i-1 where the valid_policy is
| anyPolicy. Assume the
| certificate policies Gold
| and Silver appear in the
| certificate policies
| extension of certificate i. The Gold
| policy does
| not have a qualifier, but the Silver policy has
| the qualifier Q-Silver. If Gold and Silver were
| not matched
| in (i) above, this rule will generate
| two child nodes of
| depth i, one for each policy.
| The result is shown as
| Figure 5.
|
| +-----------------+
| | anyPolicy |
| +-----------------+
| | {} |
| +-----------------+ depth i-1
| | {anyPolicy} |
| +-----------------+
| / \
| / \
| / \
| v v
| +-----------------+ +-----------------+
| | Gold | | Silver |
| +-----------------+ +-----------------+
| | {} | | {Q-Silver} | depth i
| +-----------------+ +-----------------+
| | {Gold} | | {Silver} |
| +-----------------+ +-----------------+
|
| Figure 5: Processing Unmatched Policies When a
| Leaf Node
| Specifies anyPolicy
|
| (2) If the certificate policies extension includes the policy
| policy anyPolicy with the qualifier set AP-Q and either (a)
| (a) inhibit_anyPolicy is greater than 0 or (b) i<n and the
| the certificate is self-issued, then:
|
| For each policy OID P-OID (including anyPolicy) that appears
| appears in the expected_policy_set of some node in the
| valid_policy_graph for depth i-1, if P-OID does not
| appear as
| the valid_policy of some node at depth i,
| create a single
| child node with the following values:
| set the valid_policy to
| P-OID, set the qualifier_set to
| AP-Q, set the
| expected_policy_set to {P-OID}, and set
| the parents to the
| nodes at depth i-1 where P-OID
| appears in
| expected_policy_set.
|
| This is equivalent to running step (1) above as if the
| certificate policies extension contained a policy with OID
| OID P-OID and qualifier set AP-Q.
|
| For example, consider a valid_policy_graph with a node of
| of depth i-1 where the expected_policy_set is {Gold,
| Silver} and
| a second node of depth i-1 where the
| expected_policy_set is
| {Gold}. Assume anyPolicy appears
| in the certificate policies
| extension of certificate i
| with policy qualifiers AP-Q, but
| Gold and Silver do not
| appear. This rule will generate two
| child nodes of
| depth i, one for each policy. The result is
| shown below
| as Figure 6.
|
| +-----------------+ +-----------------+
| | Red | | Blue |
| +-----------------+ +-----------------+
| | {} | | {} | depth i-1
| +-----------------+ +-----------------+
| | {Gold, Silver} | | {Gold} |
| +-----------------+ +-----------------+
| | \ |
| | \ |
| | \ |
| | \ |
| | \ |
| v v v
| +-----------------+ +-----------------+
| | Silver | | Gold |
| +-----------------+ +-----------------+
| | {AP-Q} | | {AP-Q} | depth i
| +-----------------+ +-----------------+
| | {Silver} | | {Gold} |
| +-----------------+ +-----------------+
|
| Figure 6: Processing Unmatched Policies When the
| Certificate Policies Extension Specifies anyPolicy
|
| (3) If there is a node in the valid_policy_graph of depth
| i-1 or
| less without any child nodes, delete that node.
| Repeat this
| step until there are no nodes of depth i-1
| or less without
| children.
|
| For example, consider the valid_policy_graph shown in
| Figure 7 below. The two nodes at depth i-1 that are marked
| marked with an 'X' have no children, and they are
| deleted. Applying
| this rule to the resulting graph will
| cause the nodes at
| depth i-2 that is marked with a 'Y'
| to be deleted. In the
| resulting graph, there are no
| nodes of depth i-1 or less
| without children, and this
| step is complete.
|
| +-----------+
| | | depth i-3
| +-----------+
| / | \
| / | \
| v v v
| +-----------+ +-----------+ +-----------+
| | | | | | Y | depth i-2
| +-----------+ +-----------+ +-----------+
| | \ | |
| | \ | |
| v v v v
| +-----------+ +-----------+ +-----------+
| | X | | | | X | depth i-1
| +-----------+ +-----------+ +-----------+
| / | \
| / | \
| v v v
| +-----------+ +-----------+ +-----------+
| | | | | | | depth i
| +-----------+ +-----------+ +-----------+
|
| Figure 7: Pruning the valid_policy_graph
|
| (e) If the certificate policies extension is not present, set the
| valid_policy_graph to NULL.
|
| (f) Verify that either explicit_policy is greater than 0 or the
| valid_policy_graph is not equal to NULL.
The text following step (f) in Section 6.1.3 of [RFC5280], beginning
with "If any of steps (a), (b), (c), or (f) fails", is left
unmodified.
5.4. Updates to Section 6.1.4
The following replaces step (b) in Section 6.1.4 of [RFC5280]:
| (b) If a policy mappings extension is present, then for each
| issuerDomainPolicy ID-P in the policy mappings extension:
|
| (1) If the policy_mapping variable is greater than 0 and
| there is
| a node in the valid_policy_graph of depth i
| where ID-P is the
| valid_policy, set expected_policy_set
| to the set of
| subjectDomainPolicy values that are
| specified as equivalent
| to ID-P by the policy mappings
| extension.
|
| (2) If the policy_mapping variable is greater than 0 and no node
| node of depth i in the valid_policy_graph has a
| valid_policy of
| ID-P but there is a node of depth i with
| a valid_policy of
| anyPolicy, then generate a child node
| of the node of depth
| i-1 that has a valid_policy of
| anyPolicy as follows:
|
| (i) set the valid_policy to ID-P;
|
| (ii) set the qualifier_set to the qualifier set of the policy
| policy anyPolicy in the certificate policies
| extension of
| certificate i; and
|
| (iii) set the expected_policy_set to the set of
| subjectDomainPolicy values that are specified as equivalent
| equivalent to ID-P by the policy mappings
| extension.
|
| (3) If the policy_mapping variable is equal to 0:
|
| (i) delete the node, if any, of depth i in the
| valid_policy_graph where ID-P is the valid_policy.
|
| (ii) If there is a node in the valid_policy_graph of
| depth i-1 or
| less without any child nodes, delete
| that node. Repeat this
| step until there are no
| nodes of depth i-1 or less without
| children.
5.5. Updates to Section 6.1.5
The following replaces step (g) in Section 6.1.5 of [RFC5280]:
| (g) Calculate the user_constrained_policy_set as follows. The
| user_constrained_policy_set is a set of policy OIDs, along
| with associated policy qualifiers.
|
| (1) If the valid_policy_graph is NULL, set valid_policy_node_set
| valid_policy_node_set to the empty set.
|
| (2) If the valid_policy_graph is not NULL, set
| valid_policy_node_set to the set of policy nodes whose
| valid_policy is not anyPolicy and whose parent list is a
| single node with valid_policy of anyPolicy.
|
| (3) If the valid_policy_graph is not NULL and contains a
| node of
| depth n with the valid_policy anyPolicy, add it to
| to valid_policy_node_set.
|
| (4) Compute authority_constrained_policy_set, a set of policy
| policy OIDs and associated qualifiers as follows. For
| each node in
| valid_policy_node_set:
|
| (i) Add the node's valid_policy to
| authority_constrained_policy_set.
|
| (ii) Collect all qualifiers in the node, its ancestors, and
| and descendants and associate them with valid_policy.
| valid_policy. Applications that do not use policy
| qualifiers MAY skip this
| step to simplify
| processing.
|
| (5) Set user_constrained_policy_set to
| authority_constrained_policy_set.
|
| (6) If the user-initial-policy-set is not anyPolicy:
|
| (i) Remove any elements of user_constrained_policy_set
| that do
| not appear in user-initial-policy-set.
|
| (ii) If anyPolicy appears in authority_constrained_policy_set
| authority_constrained_policy_set with qualifiers
| AP-Q, for each OID P-OID in user-initial- user-initial-policy-
| policy-set set that does not appear in
| user_constrained_policy_set, add P-OID with
| qualifiers AP-Q
| to user_constrained_policy_set.
In addition, the final paragraph in Section 6.1.5 of [RFC5280] is
updated as follows:
OLD:
| If either (1) the value of explicit_policy variable is greater
| than zero or (2) the valid_policy_tree is not NULL, then path
| processing has succeeded.
NEW:
| If either (1) the value of explicit_policy is greater than zero,
| or (2) the user_constrained_policy_set is not empty, then path
| processing has succeeded.
5.6. Updates to Section 6.1.6
The following replaces Section 6.1.6 of [RFC5280]:
| If path processing succeeds, the procedure terminates, returning a
| success indication together with the final value of the
| user_constrained_policy_set, the working_public_key, the
| working_public_key_algorithm, and the
| working_public_key_parameters.
|
| Note that the original procedure described in [RFC5280] included a
| valid_policy_tree structure as part of the output. This structure
| grows exponentially in the size of the input, so computing it
| risks denial-of-service vulnerabilities in X.509-based
| applications, such as [CVE-2023-0464] and [CVE-2023-23524].
| Accordingly, this output is deprecated. Computing this structure
| is NOT RECOMMENDED.
|
| An implementation that requires valid_policy_tree for
| compatibility with legacy systems may compute it from
| valid_policy_graph by recursively duplicating every multi-parent
| node. This may be done on-demand when the calling application
| first requests this output. However, this computation may consume
| exponential time and memory, so such implementations SHOULD
| mitigate denial-of-service attacks in other ways, such as by
| limiting the depth or size of the tree.
6. Other Mitigations
X.509 implementations that are unable to switch to the policy graph
structure SHOULD mitigate the denial-of-service attack in other ways.
This section describes alternate mitigation and partial mitigation
strategies.
6.1. Verify Signatures First
X.509 validators SHOULD verify signatures in certification paths
before or in conjunction with policy verification. This limits the
attack to entities in control of CA certificates. For some
applications, this may be sufficient to mitigate the attack.
However, other applications may still be impacted, for example:
* Any application that evaluates an untrusted PKI, such as a hosting
provider that evaluates a customer-supplied PKI
* Any application that evaluates an otherwise trusted PKI that
includes untrusted entities with technically constrained
intermediate certificates. If the intermediates do not constrain
policy mapping or path length, those entities may be able to
perform this attack.
6.2. Limit Certificate Depth
The policy tree grows exponentially in the depth of a certification
path, so limiting the depth and certificate size can mitigate the
attack.
However, this option may not be viable for all applications. Too low
of a limit may reject existing paths that the application wishes to
accept. Too high of a limit may still admit a denial-of-service
attack for the application. By modifying the example in Section 3.2
to increase the number of policies asserted in each certificate, an
attacker could still achieve O(N^(depth/2)) scaling.
6.3. Limit Policy Tree Size
The attack can be mitigated by limiting the number of nodes in the
policy tree and rejecting the certification path if this limit is
reached. This limit should be set high enough to still admit
existing valid certification paths for the application but low enough
to no longer admit a denial-of-service attack.
6.4. Inhibit Policy Mapping
If policy mapping is disabled via the initial-policy-mapping-inhibit
setting (see Section 6.1.1 of [RFC5280]), the attack is mitigated.
This also significantly simplifies the X.509 implementation, which
reduces the risk of other security bugs. However, this will break
compatibility with any existing certification paths that rely on
policy mapping.
To facilitate this mitigation, certificate authorities SHOULD NOT
issue certificates with the policy mappings extension
(Section 4.2.1.5 of [RFC5280]). Applications maintaining policies
for accepted trust anchors are RECOMMENDED to forbid this extension
in participating certificate authorities.
6.5. Disable Policy Checking
An X.509 validator can mitigate this attack by disabling policy
validation entirely. This may be viable for applications that do not
require policy validation. In this case, critical policy-related
extensions, notably the policy constraints extension
(Section 4.2.1.11 of [RFC5280]), MUST be treated as unrecognized
extensions as described in Section 4.2 of [RFC5280] and be rejected.
7. Security Considerations
Section 3 discusses how the policy tree algorithm in [RFC5280] can
lead to denial-of-service vulnerabilities in X.509-based
applications, such as [CVE-2023-0464] and [CVE-2023-23524].
Section 5 replaces this algorithm to avoid this issue. As discussed
in Section 4.1, the new structure scales linearly with the input.
This means input limits in X.509 validators will more naturally bound
processing time, thus avoiding these vulnerabilities.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[CVE-2023-0464]
CVE, "Excessive Resource Usage Verifying X.509 Policy
Constraints", CVE-2023-0464, March 2023,
<https://www.cve.org/CVERecord?id=CVE-2023-0464>.
[CVE-2023-23524]
CVE, "Processing a maliciously crafted certificate may
lead to a denial-of-service", CVE-2023-23524, February
2023, <https://www.cve.org/CVERecord?id=CVE-2023-23524>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[X.509] ITU-T, "Information technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks", ITU-T Recommendation X.509,
October 2019, <https://www.itu.int/rec/T-REC-X.509>.
Acknowledgements
The author thanks Bob Beck, Adam Langley, Matt Mueller, and Ryan
Sleevi for many valuable discussions that led to discovering this
issue, understanding it, and developing the mitigation. The author
also thanks Martin Thomson, Job Snijders, and John Scudder for their
review and feedback on this document.
Author's Address
David Benjamin
Google LLC
Email: davidben@google.com