Fix global-buffer-overflow in Base64Decode on bytes >= 0x80

[groeneai]

Bug

sdk/core/azure-core/src/base64.cpp mishandles invalid Base64 input in two ways, both before the existing validity check runs:

1. Out-of-bounds read. Input bytes are read through const char* and used to index Base64DecodeArray, an int8_t[256] reverse-lookup table. char is signed on most platforms, so any byte >= 0x80 sign-extends to a negative index and reads out of bounds of the global table. AddressSanitizer reports global-buffer-overflow, READ of size 1.
2. Undefined left shift. The table maps invalid bytes to a -1 sentinel, but the decoder shifts the looked-up value (i0 <<= 18, etc.) before checking for the sentinel. Left-shifting a negative value is undefined behavior prior to C++20 (the standard this library targets); -fsanitize=undefined reports left shift of negative value.

Both defects are present in two places:

• the 4-byte loop helper int32_t Base64Decode(const char*)
• the trailing-group tail of std::vector<uint8_t> Base64Decode(const std::string&)

Fix

• Read each input byte as unsigned char before indexing, so the index is always in [0, 255].
• Reject the -1 sentinel before any shift, in both decode paths.

Malformed input now throws std::runtime_error("Unexpected character in Base64 encoded string") cleanly on every code path, instead of reading out of bounds or shifting a negative value. Valid Base64 decodes unchanged.

Test

Extended Base64.InvalidDecode to exercise an invalid byte (high-bit 0x80/0xC0/0xFF and low-value 0x01/0x7F) in every position of both the single-group tail path and the multi-group loop path.

Validated with AddressSanitizer + UndefinedBehaviorSanitizer (clang, -fno-sanitize-recover=all): pre-fix the inputs read out of bounds and abort on the shift under C++17; post-fix they throw cleanly under C++17/20/23 and all existing valid/invalid decode cases still pass.

[github-actions]

Thank you for your contribution @groeneai! We will review the pull request and get back to you soon.

[Copilot]

Pull request overview

Fixes a signed-char indexing bug in azure-core Base64 decoding that could trigger an out-of-bounds read when decoding bytes with the high bit set (>= 0x80). The change ensures bytes are treated as unsigned char before indexing the 256-entry reverse lookup table, and extends unit tests to cover these malformed inputs.

Changes:

• Cast Base64 input bytes to unsigned char before indexing Base64DecodeArray in both the 4-byte decode helper and the trailing-group decode path.
• Extend Base64.InvalidDecode to include high-bit bytes in each position for the tail path and in the loop path, verifying clean std::runtime_error behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdk/core/azure-core/src/base64.cpp	Prevents negative indexing into Base64DecodeArray by reading input bytes as unsigned before lookup.
sdk/core/azure-core/test/ut/base64_test.cpp	Adds invalid-decode coverage for bytes >= 0x80 across both decode paths to guard against regressions and ASan-detected OOB reads.

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