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SSRF bypass via incomplete IP validation in Photo::fromUrl — loopback and link-local IPs not blocked

Moderate
ildyria published GHSA-vq6w-prpf-h287 Mar 21, 2026

Package

composer lychee-org/lychee (Composer)

Affected versions

<= 7.5.0

Patched versions

> 7.5.0

Description

Summary

The patch introduced for GHSA-cpgw-wgf3-xc6v (SSRF via Photo::fromUrl) contains an incomplete IP validation check that fails to block loopback addresses (127.0.0.0/8) and link-local addresses (169.254.0.0/16, including the AWS EC2 instance metadata endpoint 169.254.169.254).

An authenticated user can still reach internal services using direct IP addresses, bypassing all four protection configuration settings even when they are set to their secure defaults.

Vulnerable Code

app/Rules/PhotoUrlRule.php lines 85–101:

// Uses FILTER_FLAG_NO_PRIV_RANGE — only blocks RFC-1918 (10.x, 172.16.x, 192.168.x)
// Does NOT block 127.0.0.0/8 (loopback) or 169.254.0.0/16 (link-local)!
if (
    $this->config_manager->getValueAsBool('import_via_url_forbidden_local_ip') &&
    filter_var($host, FILTER_VALIDATE_IP) !== false &&
    filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE) === false
) {
    $fail($attribute . ' must not be a private IP address.');
    return;
}

// Only checks the string 'localhost' — does not block direct IP 127.0.0.1!
if (
    $this->config_manager->getValueAsBool('import_via_url_forbidden_localhost') &&
    $host === 'localhost'
) {
    $fail($attribute . ' must not be localhost.');
    return;
}

Root Cause

PHP's FILTER_FLAG_NO_PRIV_RANGE blocks only RFC-1918 private ranges (10.x, 172.16.x, 192.168.x). It does NOT block reserved ranges covered by FILTER_FLAG_NO_RES_RANGE: 127.0.0.0/8 (loopback), 169.254.0.0/16 (link-local / AWS metadata), 0.0.0.0/8.

The localhost string check ($host === 'localhost') only matches the hostname string, not the loopback IP 127.0.0.1.

Proof of Concept

With all 4 protection configs at their secure defaults, the following requests succeed:

Bypass 1 — Loopback via direct IP (bypasses forbidden_localhost string-only check):

POST /api/v2/Photo::fromUrl
Authorization: Bearer <token>
{"urls": ["http://127.0.0.1/"], "album_id": null}

Bypass 2 — AWS EC2 metadata endpoint (bypasses forbidden_local_ip range check):

POST /api/v2/Photo::fromUrl
Authorization: Bearer <token>
{"urls": ["http://169.254.169.254/latest/meta-data/iam/security-credentials/"], "album_id": null}

169.254.169.254 passes FILTER_FLAG_NO_PRIV_RANGE because link-local is a reserved range, not RFC-1918 private. It is only blocked by FILTER_FLAG_NO_RES_RANGE.

Impact

  • Authenticated attacker can reach HTTP services on 127.0.0.1 even with forbidden_localhost=1 and forbidden_local_ip=1
  • On cloud deployments (AWS/GCP/Azure), 169.254.169.254 is reachable, potentially exposing IAM credentials and instance metadata
  • Users who patched GHSA-cpgw-wgf3-xc6v believe they are protected but remain vulnerable

Fix

// Use BOTH flags to cover private AND reserved ranges
if (
    $this->config_manager->getValueAsBool('import_via_url_forbidden_local_ip') &&
    filter_var($host, FILTER_VALIDATE_IP) !== false &&
    filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) === false
) {
    $fail($attribute . ' must not be a private or reserved IP address.');
    return;
}

// Cover loopback IPs, not just the hostname string
if (
    $this->config_manager->getValueAsBool('import_via_url_forbidden_localhost') &&
    in_array($host, ['localhost', '127.0.0.1', '::1'], true)
) {
    $fail($attribute . ' must not be localhost.');
    return;
}

Reporter

Assaf (@offensiveee)

Fix

4138667

Severity

Moderate

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 Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
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:L/UI:N/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N

CVE ID

CVE-2026-33537

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.

Credits