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scim_proton and kanidm_proto have an authenticated process abort via SCIM filter stack exhaustion

High severity GitHub Reviewed Published Apr 30, 2026 in kanidm/kanidm • Updated May 19, 2026

Package

cargo kanidm_proto (Rust)

Affected versions

<= 1.9.2

Patched versions

1.9.3
cargo scim_proto (Rust)
<= 1.9.2
1.9.3

Description

Summary

A single unauthenticated GET to any /scim/v1/... endpoint with a ?filter= query string of a few thousand nested parentheses (≈ 4–12 KB) drives the recursive-descent PEG parser past the worker thread's stack guard page. Rust responds to stack overflow with std::process::abort() — the entire kanidmd process exits. The parse runs inside axum's Query<ScimEntryGetQuery> extractor, before any handler body and therefore before any ACL check.

Details

The SCIM filter grammar recurses on ( and not ( with no depth bound.

proto/src/scim_v1/mod.rs:263-433 — peg::parser! { grammar scimfilter() ... }:

// line 281
"not" separator()+ "(" e:parse() ")" { ScimFilter::Not(Box::new(e)) }
// line 293
"(" e:parse() ")" { e }

Both rules re-enter parse() without a depth counter.

proto/src/scim_v1/mod.rs:442-447 — impl FromStr for ScimFilter calls scimfilter::parse(input) directly on the raw string with no length or depth pre-check.

proto/src/scim_v1/mod.rs:80-81 — ScimEntryGetQuery.filter is #[serde_as(as = "Option<DisplayFromStr>")], so deserialising the query struct invokes ScimFilter::from_str on attacker bytes.

Unauthenticated reachability — nine handlers in server/core/src/https/v1_scim.rs (route table at lines 865-1029) take Query<ScimEntryGetQuery> as an argument: /scim/v1/Entry, /scim/v1/Entry/{id}, /scim/v1/Person/{id}, /scim/v1/Application, /scim/v1/Application/{id}, /scim/v1/Class, /scim/v1/Attribute, /scim/v1/Message, /scim/v1/Message/{id}. The SCIM router is merged unconditionally for every server role (server/core/src/https/mod.rs:312).

Axum extracts handler arguments before the handler body runs. The preceding VerifiedClientInformation extractor (server/core/src/https/extractors/mod.rs:16-91) always returns Ok (line 89) regardless of credentials; authorization is deferred to the handler body, which is never reached.

The existing semantic depth limit (DEFAULT_LIMIT_FILTER_DEPTH_MAX = 12, server/lib/src/constants/mod.rs:212) is enforced in Filter::from_scim_ro (server/lib/src/filter.rs:786) after the PEG parse has already produced an AST, so it cannot prevent the parser itself from blowing the stack.

The production daemon (server/daemon/src/main.rs:735-744) uses new_multi_thread() with default 2 MiB worker stacks; hyper's max_buf_size (~400 KiB) is not lowered (server/core/src/https/mod.rs:708-727), so a 12 KB URI is accepted.

An identical unbounded grammar exists in libs/scim_proto/src/filter.rs:112-276 (not network-reachable, but should be fixed in the same patch).

PoC

curl -sk "https://idm.example.com/scim/v1/Application?filter=$(python3 -c 'print("("*3000+"a+pr"+")"*3000)')"
# → curl: (52) Empty reply from server
# → server journal: "fatal runtime error: stack overflow, aborting", SIGABRT

Release-build threshold measured at ~2 000 nesting levels / ~4 KB:

$ cargo test --release -p kanidm_proto --test scim_filter_depth -- --nocapture
parens depth=1500 len=3004
  -> survived
parens depth=2000 len=4004

thread 'audit_scim_filter_nested_parens' has overflowed its stack
fatal runtime error: stack overflow, aborting
  (signal: 6, SIGABRT: process abort signal)

End-to-end against an in-process server via kanidmd_testkit (no authentication performed):

Testkit server setup complete - http://localhost:18080/
audit_scim_dos: sending unauthenticated GET, url len = 12056

thread '...' has overflowed its stack
fatal runtime error: stack overflow, aborting
  (signal: 6, SIGABRT: process abort signal)

Impact

Process-wide availability loss; no confidentiality or integrity impact.

  • Unauthenticated, default install, no feature flag required.
  • Process abort, not task panic. Stack overflow triggers libstd's guard-page handler, which calls std::process::abort(). tokio's per-task catch_unwind isolation does not apply to aborts. All in-flight HTTP requests, OAuth2/OIDC sessions, LDAP binds, and the web UI are terminated.
  • Repeatable. One ~12 KB GET per crash; a while true; do curl ...; done loop holds the service down indefinitely across supervisor restarts.
  • The 6 011-byte variant (depth=3000) fits under the nginx default large_client_header_buffers limit of 8 KB, so a typical reverse proxy does not mitigate.

Affected: v1.7.0 through master @ edf50b9da.

References

@Firstyear Firstyear published to kanidm/kanidm Apr 30, 2026
Published to the GitHub Advisory Database May 6, 2026
Reviewed May 6, 2026
Last updated May 19, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(13th percentile)

Weaknesses

Uncaught Exception

An exception is thrown from a function, but it is not caught. Learn more on MITRE.

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

Uncontrolled Recursion

The product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack. Learn more on MITRE.

CVE ID

CVE-2026-46689

GHSA ID

GHSA-r5fr-9gmv-jggh

Source code

Credits

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