channel: fix off-by-one write in X11 cookie generation #1696
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The loop hex-encodes 16 random bytes into a 32-byte cookie but used snprintf directly into the output buffer s, writing 3 bytes per iteration (2 hex + NUL) and overflowing by 1 byte on the last write. Hex-encode into a small temp and memcpy 2 bytes per iteration so exactly cookie_len bytes are written with no trailing NUL.
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There is a comment on line 1425 that says the null terminator is already being taken into account with the +1 on the size of |
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The
So, on the final loop, we add an extra |
Michael-Panic
approved these changes
Nov 15, 2025
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The loop hex-encodes 16 random bytes into a 32-byte cookie but used snprintf directly into the output buffer s, writing 3 bytes per iteration (2 hex + NUL) and overflowing by 1 byte on the last write. Hex-encode into a small temp and memcpy 2 bytes per iteration so exactly cookie_len bytes are written with no trailing NUL.