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Pin validated DNS resolution in fetch proxy #3

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Pin validated DNS resolution in fetch proxy #3

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24 changes: 16 additions & 8 deletions index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -39,21 +39,29 @@ app.get('/fetch', async (req, res) => {
return res.status(400).send('URL not allowed');
}
try {
// DNS resolution to prevent DNS rebinding
try {
// Resolve once and pin the request to the validated address to avoid DNS rebinding TOCTOU.
const addresses = await dns.lookup(parsedUrl.hostname, { all: true });
for (const { address } of addresses) {
if (isPrivateIp(address)) {
return res.status(400).send('URL not allowed');
}
}
} catch (e) {
return res.status(400).send('Invalid hostname');
}

const resp = await axios.get(url);

const [firstAddress] = addresses;
if (!firstAddress || !firstAddress.address) {
return res.status(400).send('Invalid hostname');
}

const lookup = (_hostname, _options, callback) => {
callback(null, firstAddress.address, firstAddress.family);
};

const resp = await axios.get(url, {
lookup,
maxRedirects: 0,
});
res.send(resp.data);
} catch (e) {
res.status(500).send(e.message);
return res.status(400).send(e.message);
}
});