DeepSec: custom HTTP handlers bypass connection-time private-IP filtering

[chaliy]

Source: DeepSec Rust-focused direct pass 20260507013924-5f4812745ba339b2.

Severity: CRITICAL
Confidence: medium
File: crates/bashkit/src/network/client.rs:402
Slug: ssrf

Finding

HttpClient performs a best-effort DNS/private-IP precheck, then calls a custom HttpHandler directly when one is configured. The reqwest PrivateIpFilteringResolver is only installed on the default reqwest client, so network-capable custom handlers do not get the connect-time protection that prevents DNS rebinding between validation and send.

The precheck also fails open on DNS lookup errors. A script can request an allowlisted attacker-controlled hostname that resolves public during the precheck and private when the handler connects, causing SSRF if the handler proxies or performs real HTTP.

Suggested Fix

Do not treat pre-resolution as the security boundary for network-capable handlers.

Options:

• provide handlers with a prevalidated resolved endpoint
• require/verify the same private-IP filtering in a handler adapter
• fail closed on DNS check failures
• document custom handlers as fully responsible for SSRF enforcement