Internet-Draft | Updates to X.509 Policy Validation | July 2023 |
Benjamin | Expires 6 January 2024 | [Page] |
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 which scaled exponentially in the worst case, leaving implementations vulnerable to denial-of-service attacks.¶
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[RFC5280] defines a suite of extensions for specifying certificate policies, along with a mechanism for mapping policies between subject and issuer policy domains in cross-certificates. This mechanism, when evaluated according to the algorithm in [RFC5280], Section 6.1 produces a policy tree, describing policies asserted by each certificate, and mappings between them. This tree can grow exponentially in the depth of the certification path. This can lead to a denial-of-service attack in X.509-based applications.¶
The algorithm for processing certificate policies and policy mappings is replaced with one which builds an equivalent, but much more efficient structure. This new algorithm does not change the validity status of any certification path, nor which certificate policies are valid for it.¶
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.¶
The valid_policy_tree
, defined in Section 6.1.2 of [RFC5280], is a tree of
certificate policies. The nodes at any given depth in the tree correspond to
policies asserted by a certificate in the certificate path. A node's
parent policy is the policy in the issuer certificate which was mapped to this
policy, and a node's children are the policies it was mapped to in the subject
certificate.¶
For example, suppose a certificate chain contains:¶
This would result in the tree shown in Figure 1.¶
The complete algorithm for building this structure is described in steps (d), (e), and (f) of Section 6.1.3 of [RFC5280], steps (h), (i), (j) of Section 6.1.4 of [RFC5280], and steps (a), (b), and (g) of Section 6.1.5 of [RFC5280].¶
The algorithm described in [RFC5280] grows exponentially in the worst case. In step (d.1) of 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:¶
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 2.¶
[RFC5280] describes a tree structure, but this is an unnecessarily inefficient representation of this information. A single certificate policy may produce multiple nodes, but each node is identical, with identical children.¶
This document replaces the tree structure with a directed acyclic graph. Where [RFC5280] adds multiple duplicate nodes, this document adds a single node with multiple parents. See Section 4 for the procedure for building this structure. Figure 3 shows the updated representation of the above example.¶
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 from [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.¶
Implementations of X.509 SHOULD implement a policy graph structure instead of a policy tree.¶
Section 6.1.6 of [RFC5280] describes the entire valid_policy_tree
structure as
an output of the verification process. Section 12.2 of [X.509] instead only
outputs the authorities-constrained policies, the user-constrained policies,
and their associated qualifiers.¶
An implementation which outputs the entire tree may be unable switch the format
to a more efficient one, as described in Section 3.2. X.509 implementations
SHOULD NOT output the entire valid_policy_tree
structure and instead SHOULD
limit output to just the set of authorities-constrained and/or user-constrained
policies, as described in [X.509]. X.509 implementations are additionally
RECOMMENDED to omit policy qualifiers from the output, as this simplifies the
process. Note Section 4.2.1.4 of [RFC5280] conversely recommends that
certificate authorities omit policy qualifiers from policy information terms.
This document reiterates this and RECOMMENDS that certificate authorities omit
the policyQualifiers field in PolicyInformation structures.¶
X.509 implementations that are unable 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.¶
X.509 validators typically already allow limiting the depth of a certificate chain. This can limit the attack, however a large depth limit may still admit attacks. By modifying the example in Section 3.1 to increase the number of policies asserted in each certificate, an attacker could still achieve O(N^(depth/2)) scaling or higher.¶
If existing stable interfaces force the validator to build a full policy tree (see Section 3.3), the validator SHOULD limit the number of nodes in the policy tree, and reject the certification path if this limit is reached.¶
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 certificate paths which rely on policy mapping.¶
To faciliate 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.¶
An X.509 validator can mitigate this attack by disabling policy validation entirely. This may be viable for applications which do not require policy validation. In this case, critical policy-related extensions, notably the policy constraints (Section 4.2.1.11 of [RFC5280]), MUST be treated as unrecognized extensions as in Section 4.2 of [RFC5280] and be rejected.¶
X.509 validators SHOULD verify signatures in certificate 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:¶
This section provides updates to [RFC5280].¶
This update replaces a paragraph of Section 6.1 of [RFC5280] 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.¶
This update replaces entry (a) of Section 6.1.2 of [RFC5280] with the following text:¶
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, 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:¶
valid_policy
is a single policy OID representing a valid policy for the path of length x.¶
qualifier_set
is a set of policy qualifiers 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).¶
expected_policy_set
contains one or more policy OIDs 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:¶
valid_policy
is P-OID.¶
expected_policy_set
of a node whose valid_policy
is anyPolicy is always {anyPolicy}.¶
valid_policy
is anyPolicy, except for the one at
depth zero, always has exactly one parent: a node at depth x-1 whose
valid_policy
is also anyPolicy.¶
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 OID.¶
Figure 4 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.¶
This update replaces steps (d), (e), and (f) of Section 6.1.3 of [RFC5280] with the following text:¶
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:¶
For each policy P not equal to anyPolicy in the 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:¶
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_tree
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 not.
This rule will generate a child node of depth i for the Gold policy.
The result is shown as Figure 5.¶
If there was no match in step (i) and the 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, 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 6.¶
If the certificate policies extension includes the policy anyPolicy with the qualifier set AP-Q and either (a)
inhibit_anyPolicy
is greater than 0 or (b) i<n and the certificate is self-issued, then:¶
For each policy OID P-OID (including anyPolicy) which 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 P-OID and qualifier set AP-Q.¶
For example, consider a valid_policy_graph
with a node 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, Bronze}.
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 7.¶
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 8 below. The two nodes at depth i-1 that are 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.¶
valid_policy_graph
to NULL.¶
explicit_policy
is greater than 0 or the valid_policy_graph
is not equal to NULL;¶
This update replaces step (b) of Section 6.1.4 of [RFC5280] with the following text:¶
If a policy mappings extension is present, then for each issuerDomainPolicy ID-P in the policy mappings extension:¶
If the policy_mapping variable is greater than 0, if 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.¶
If no node of depth i in the valid_policy_tree
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:¶
valid_policy
to ID-P;¶
qualifier_set
to the qualifier set of the
policy anyPolicy in the certificate policies
extension of certificate i; and¶
expected_policy_set
to the set of
subjectDomainPolicy values that are specified as
equivalent to ID-P by the policy mappings extension.¶
If the policy_mapping
variable is equal to 0:¶
This update replaces step (g) of Section 6.1.5 of [RFC5280] with the following text:¶
Calculate the user_constrained_policy_set
as follows.
The user_constrained_policy_set
is a set of policy OIDs, along with associated policy qualifiers.¶
valid_policy_graph
is NULL, set valid_policy_node_set
to the empty set.¶
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.¶
valid_policy_graph
is not NULL and contains a node of depth n with the valid_policy
anyPolicy, add it to valid_policy_node_set
.¶
Compute authority_constrained_policy_set
, a set of policy OIDs and associated qualifiers as follows. For each node in valid_policy_node_set
:¶
user_constrained_policy_set
to authority_constrained_policy_set
.¶
If the user-initial-policy-set is not anyPolicy:¶
user_constrained_policy_set
which do not appear in user-initial-policy-set.¶
authority_constrained_policy_set
with qualifiers AP-Q, for each OID P-OID in user-initial-policy-set which does not appear in user_constrained_policy_set
, add P-OID with qualifiers AP-Q to user_constrained_policy_set
.¶
Additionally, this update replaces the final paragraph 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
variable is greater than
zero or (2) the user_constrained_policy_set
is not empty, then path processing
has succeeded.¶
This update replaces Section 6.1.6 of [RFC5280] with the following text:¶
If path processing succeeds, the procedure terminates, returning a
success indication together with final value of the user_constrained_policy_set
,
the working_public_key
, the working_public_key_algorithm
, and the
working_public_key_parameters
.¶
This document addresses a denial-of-service vulnerability in [RFC5280]'s policy tree algorithm.¶
This document has no IANA actions.¶
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.¶