From 494b5486ba3b01ee9a4fbcc8eceb96d76e6c0bc7 Mon Sep 17 00:00:00 2001
From: bptato <60043228+bptato@users.noreply.github.com>
Date: Mon, 22 Apr 2024 09:44:33 +0200
Subject: [PATCH] Fix std/base64.decode out of bounds read (#23526)

inputLen may end up as 0 in the loop if the input string only includes
trailing characters. e.g. without the patch, decode(" ") would panic.

(cherry picked from commit 30cf570af997a0c705f7b3f194eea7337cb44185)
---
 lib/pure/base64.nim      | 2 +-
 tests/stdlib/tbase64.nim | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/pure/base64.nim b/lib/pure/base64.nim
index 6af5345f29..591d22cc00 100644
--- a/lib/pure/base64.nim
+++ b/lib/pure/base64.nim
@@ -244,7 +244,7 @@ proc decode*(s: string): string =
     inputLen = s.len
     inputEnds = 0
   # strip trailing characters
-  while s[inputLen - 1] in {'\n', '\r', ' ', '='}:
+  while inputLen > 0 and s[inputLen - 1] in {'\n', '\r', ' ', '='}:
     dec inputLen
   # hot loop: read 4 characters at at time
   inputEnds = inputLen - 4
diff --git a/tests/stdlib/tbase64.nim b/tests/stdlib/tbase64.nim
index 98388bb6c5..c3bfb818e2 100644
--- a/tests/stdlib/tbase64.nim
+++ b/tests/stdlib/tbase64.nim
@@ -18,6 +18,8 @@ template main() =
   doAssert encode("") == ""
   doAssert decode("") == ""
 
+  doAssert decode(" ") == ""
+
   const testInputExpandsTo76 = "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++"
   const testInputExpands = "++++++++++++++++++++++++++++++"
   const longText = """Man is distinguished, not only by his reason, but by this