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Check base64 without a backtracking regex #1144

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Merged
helpcodeai merged 2 commits into from
Oct 19, 2026
Merged

Check base64 without a backtracking regex #1144

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d18c119
Check base64 without a backtracking regex
helpcodeai Oct 19, 2026
6ce2da1
Merge branch 'main' into fix/base64-check-no-regex-stack
helpcodeai Oct 19, 2026
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12 changes: 10 additions & 2 deletions packages/backend/src/connectors/response-images.util.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -170,7 +170,8 @@ export function readImages(responseMapping: unknown): ImagesConfig | null {
/* Bytes */
/* ------------------------------------------------------------------ */

const BASE64 = /^[A-Za-z0-9+/]+={0,2}$/;
/** Any character outside the base64 alphabet. A single-character class never backtracks, so it is safe on multi-megabyte strings. */
const NOT_BASE64 = /[^A-Za-z0-9+/]/;
const DATA_URL = /^data:([\w.+-]+\/[\w.+-]+)?(?:;[\w-]+=[\w.-]+)*;base64,/i;

/** Standard, padded base64 of a base64 or base64url string; null when it is neither. */
Expand All @@ -181,7 +182,14 @@ function normalizeBase64(text: string): string | null {
const pad = s.length % 4;
if (pad === 1) return null;
if (pad > 0 && !s.endsWith('=')) s += '='.repeat(4 - pad);
return BASE64.test(s) && s.length % 4 === 0 ? s : null;
if (s.length % 4 !== 0) return null;
// Up to two '=' of padding at the end, base64 alphabet before them. Checked
// without an anchored `+` regex: on a 7 MB image V8 overflows its stack
// backtracking through one.
let end = s.length;
while (end > 0 && s.charCodeAt(end - 1) === 61 /* = */) end--;
if (end === 0 || s.length - end > 2) return null;
return NOT_BASE64.test(s.slice(0, end)) ? null : s;
}

function decodedLength(b64: string): number {
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