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fix(@angular/ssr): decode x-forwarded-prefix before validation #32904

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fix(@angular/ssr): decode x-forwarded-prefix before validation #32904

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f100f2b
fix(@angular/ssr): decode x-forwarded-prefix before validation
alan-agius4 Mar 31, 2026
642988a
fixup! fix(@angular/ssr): decode x-forwarded-prefix before validation
alan-agius4 Mar 31, 2026
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20 changes: 16 additions & 4 deletions packages/angular/ssr/src/utils/validation.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -281,9 +281,21 @@ function validateHeaders(request: Request): void {
}

const xForwardedPrefix = getFirstHeaderValue(headers.get('x-forwarded-prefix'));
if (xForwardedPrefix && INVALID_PREFIX_REGEX.test(xForwardedPrefix)) {
throw new Error(
'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
);
if (xForwardedPrefix) {
let xForwardedPrefixDecoded: string;
try {
xForwardedPrefixDecoded = decodeURIComponent(xForwardedPrefix);
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@dgp1130 dgp1130 Mar 31, 2026

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Consider: Is there any risk of double encoding potentially causing an issue? Some case like: %252e%252e?

I feel like we're ok since you're not returning the xForwardedPrefixedDecoded, and only using it for validation (it wouldn't error but also wouldn't compromise). But just wanted to call out that situation in case there's something I'm not seeing.

} catch (e) {
throw new Error(
'Header "x-forwarded-prefix" contains an invalid value and cannot be decoded.',
{ cause: e },
);
}

if (INVALID_PREFIX_REGEX.test(xForwardedPrefixDecoded)) {
throw new Error(
'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
);
}
}
}
25 changes: 23 additions & 2 deletions packages/angular/ssr/test/utils/validation_spec.ts
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Expand Up @@ -154,8 +154,17 @@ describe('Validation Utils', () => {
);
});

it('should throw error if x-forwarded-prefix starts with a backslash or multiple slashes', () => {
const inputs = ['//evil', '\\\\evil', '/\\evil', '\\/evil', '\\evil'];
it('should throw error if x-forwarded-prefix starts with a backslash or multiple slashes including encoded', () => {
const inputs = [
'//evil',
'\\\\evil',
'/\\evil',
'\\/evil',
'\\evil',
'%5Cevil',
'%2F%2Fevil',
'%2F..%2Fevil',
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];

for (const prefix of inputs) {
const request = new Request('https://example.com', {
Expand Down Expand Up @@ -220,6 +229,18 @@ describe('Validation Utils', () => {
.not.toThrow();
}
});

it('should throw error if x-forwarded-prefix contains malformed encoding', () => {
const request = new Request('https://example.com', {
headers: {
'x-forwarded-prefix': '/%invalid',
},
});

expect(() => validateRequest(request, allowedHosts, false)).toThrowError(
'Header "x-forwarded-prefix" contains an invalid value and cannot be decoded.',
);
});
});

describe('cloneRequestAndPatchHeaders', () => {
Expand Down
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