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Server-supplied `svn:externals` URL reaches `svn checkout` in option position (argument injection, no malicious formula)

Low
MikeMcQuaid published GHSA-9g4r-vmj2-j2gj Jul 6, 2026

Package

brew (Homebrew)

Affected versions

Tested on c0ed315adb (5.1.15-619)

Patched versions

6.0.7

Description

Summary

SubversionDownloadStrategy reads a Subversion repository's svn:externals property — a value controlled by the server, not the formula author — and feeds the resulting external URL into svn checkout in option position with no -- end-of-options guard. A svn:externals URL beginning with a dash is therefore parsed by svn as an option. This is the genuinely untrusted-input instance of the download-strategy argument-injection class: a benign formula pointing at a malicious or compromised svn repository is sufficient — no malicious formula content is required. (Reachable when the formula uses svn's multi-revision spec, revisions:, which is what triggers the externals walk in clone_repo; the default/single-revision: svn path does not iterate externals.)

Details

Sink chain in Library/Homebrew/download_strategy/subversion_download_strategy.rb:

  • :60 svn propget svn:externals <@url> reads the server-controlled externals property.
  • :62 name, url = line.split(/\s+/) parses each external (the external URL is server-supplied).
  • :116 fetch_repo(..., external_url, ...) →
  • :91 command! "svn", args: ["checkout", url, target, *args] — url is the first positional after checkout, with no -- before it.

A dash-leading svn:externals URL (for example --config-dir=<dir>) is parsed by svn as an option. The same :91 sink also takes the formula-author @url; that variant is formula-author-controlled and is reported separately as hardening parity. The svn:externals vector here is the server-supplied, advisory-grade one.

The reporter recommends passing -- before the URL/target in the svn checkout invocation (:91) and rejecting dash-leading svn:externals URLs parsed at :62. Final severity and remediation are the PSRT's to decide.

PoC

Loopback/synthetic. Driving the real HEAD strategy, a server-supplied svn:externals value of --config-dir=<dir> reaches svn checkout in option position and svn acts on it (the named config directory is created/populated) — captured via the real argv plus the on-disk side effect. Negative controls: a benign (non-dash) externals URL is positional with no effect; a ---guarded checkout makes the token positional (rejected as a non-URL). (Full recorded stdout available on request / in the private review bundle.)

Impact

A malicious or compromised Subversion server (or a MITM of a plain svn:///http:// repo) injects svn client options during a Homebrew download via the svn:externals property — without any malicious formula. Who is impacted: a user installing a formula that uses svn's multi-revision spec (revisions:) against an svn source whose svn:externals are attacker-influenced. Bounded to what an injected svn option can do (for example config-dir redirection / behaviour change); no direct command execution was reached. CWE-88 (Argument Injection).

AI/LLM use disclosure

This finding was researched and validated with AI/LLM assistance; the reporter verified its correctness against the source and the recorded run, and takes full responsibility for it.

  • How: an AI agent system (Anthropic Claude, via Claude Code) performed native source-code review to surface the candidate, then orchestrated a live proof-of-concept on a disposable, isolated Linux VM (loopback servers, synthetic inputs only). AI output was treated as a fallible first draft; the sink, reachability, and PoC result were human-verified before reporting.
  • When: 2026-06-26 – 2026-06-27; live PoC recorded at bc237f676c, sink re-verified unchanged (empty git diff) at master HEAD c0ed315adb.
  • Where: in the research (discovery + PoC validation) and in drafting this write-up.
  • Prompts: the operative instruction was — "find every 0-day from the latest HEAD; every finding must have a PoC/wire-proof with recorded reachability stdout; conform with the security model and vuln-filing conventions; use disposable VMs within policy to fan out." For this finding: serve a loopback svn repo whose svn:externals URL is a dash-leading svn option, brew fetch, and assert the option reaches svn checkout in option position via the real argv plus its on-disk side effect. Full prompt/agent logs are available to the PSRT on request.

Severity

Low

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 Present
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability None
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:P/PR:N/UI:A/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

CVE ID

No known CVE

Weaknesses

Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

The product constructs a string for a command to be executed by a separate component in another control sphere, but it does not properly delimit the intended arguments, options, or switches within that command string. Learn more on MITRE.

Credits