CVE-2026-31959: Quill Mac Binary Signing SSRF Vulnerability

CVE-2026-31959 Overview

CVE-2026-31959 is a Server-Side Request Forgery (SSRF) vulnerability affecting Quill, a cross-platform tool for macOS binary signing and notarization. The vulnerability exists in the mechanism used to fetch Apple notarization submission logs, where Quill fails to properly validate URLs provided in API responses before making requests.

Critical Impact

Exploitation could lead to exfiltration of sensitive data such as cloud provider credentials or internal service responses from environments with TLS-intercepting proxies or compromised certificate authorities.

Affected Products

• Quill versions prior to 0.7.1
• Quill CLI (when used to retrieve notarization submission logs)
• Quill library (when used to retrieve notarization submission logs)

Discovery Timeline

• 2026-03-11 - CVE CVE-2026-31959 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-31959

Vulnerability Analysis

This vulnerability is classified as CWE-918 (Server-Side Request Forgery). When Quill retrieves notarization submission logs from Apple's notarization service, it fetches a URL provided in the API response without performing adequate validation. The application fails to verify that the URL scheme is HTTPS or that the target host does not resolve to local or multicast IP addresses.

Exploitation requires the ability to modify API responses from Apple's notarization service, which is not possible under standard network conditions due to HTTPS with proper TLS certificate validation. However, environments with TLS-intercepting proxies (common in corporate networks), compromised certificate authorities, or other trust boundary violations are at increased risk.

Root Cause

The root cause of this vulnerability is insufficient URL validation when processing API responses from Apple's notarization service. Quill trusts the URL provided in the response without checking:

1. Whether the URL scheme is restricted to HTTPS
2. Whether the target host resolves to a private, local, or multicast IP address
3. Whether the destination is an internal network resource

This lack of validation allows an attacker with the ability to tamper with API responses to redirect requests to arbitrary destinations.

Attack Vector

The attack requires adjacent network access and involves manipulating the API response that Quill receives when querying Apple's notarization service. An attacker who can intercept and modify TLS traffic (through a compromised proxy, certificate authority, or similar trust boundary violation) can inject a malicious URL into the response.

When Quill processes this tampered response, it will make an HTTP or HTTPS request to the attacker-specified URL. This could be leveraged to:

• Access internal network services that are not exposed to the internet
• Retrieve cloud provider metadata endpoints (such as AWS IMDSv1 at 169.254.169.254)
• Exfiltrate data from the local network to an attacker-controlled server
• Probe internal infrastructure to discover additional attack surfaces

The vulnerability affects both the Quill CLI and the Quill library when either is used to retrieve notarization submission logs.

Detection Methods for CVE-2026-31959

Indicators of Compromise

• Unexpected outbound HTTP requests to internal IP addresses or metadata endpoints from systems running Quill
• Network traffic from Quill processes to destinations other than Apple's notarization service endpoints
• DNS queries for internal hostnames originating from Quill CLI or library processes

Detection Strategies

• Monitor network traffic from systems running Quill for connections to non-Apple destinations during notarization operations
• Implement egress filtering to detect and alert on requests to cloud provider metadata endpoints (e.g., 169.254.169.254)
• Review proxy logs for unusual URL patterns in requests originating from Quill processes
• Deploy network detection rules to identify SSRF patterns targeting internal resources

Monitoring Recommendations

• Enable verbose logging for Quill operations to capture URL requests made during notarization log retrieval
• Implement network segmentation to limit the impact of potential SSRF exploitation
• Monitor for anomalous data exfiltration patterns from build servers or CI/CD systems running Quill

How to Mitigate CVE-2026-31959

Immediate Actions Required

• Upgrade Quill to version 0.7.1 or later immediately
• Audit systems running Quill in environments with TLS-intercepting proxies for signs of exploitation
• Review network logs for any suspicious outbound connections from Quill processes
• Consider restricting network access for build systems running Quill to only necessary endpoints

Patch Information

This vulnerability is fixed in Quill version 0.7.1. Users should upgrade to this version or later to remediate the SSRF vulnerability. The fix includes proper validation of URLs received from Apple's notarization service, ensuring that only HTTPS URLs pointing to legitimate external hosts are processed.

For more details, refer to the GitHub Security Advisory.

Workarounds

• Restrict network egress from systems running Quill to only allow connections to Apple's notarization service endpoints
• Implement network-level controls to block requests to internal IP ranges and cloud metadata endpoints from build systems
• Avoid using Quill in environments with TLS-intercepting proxies until the upgrade is complete
• Consider using network isolation for CI/CD systems that perform notarization operations

# Example: Network egress restriction using iptables (Linux)
# Allow only connections to Apple's notarization services
# Note: Verify current Apple IP ranges before implementation

# Block metadata endpoint access
iptables -A OUTPUT -d 169.254.169.254 -j DROP

# Block private network ranges for SSRF protection
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP