From 45f1b92b72d5993d4568f31e1db903f9efd77291 Mon Sep 17 00:00:00 2001
From: Miroslav Shubernetskiy <miroslav@miki725.com>
Date: Tue, 24 Feb 2026 03:37:46 -0500
Subject: [PATCH] fix: double check inputIndex in base64.decode (#25531)

fixes https://github.com/nim-lang/Nim/issues/25530

this double checks the index to make sure whitespace related index
increments cannot cause index defect error

(cherry picked from commit 86b9245dd69b1ea7ba9a4f73fd3651808052ca6c)
---
 lib/pure/base64.nim | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/pure/base64.nim b/lib/pure/base64.nim
index fd61119fa2..e18e9d3b4a 100644
--- a/lib/pure/base64.nim
+++ b/lib/pure/base64.nim
@@ -253,6 +253,9 @@ proc decode*(s: string): string =
   while inputIndex <= inputEnds:
     while s[inputIndex] in {'\n', '\r', ' '}:
       inc inputIndex
+    # double check inputIndex as it can be incremented due to whitespace
+    if inputIndex > inputEnds:
+      break
     inputChar(a)
     inputChar(b)
     inputChar(c)