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Fix two issues in the new APNG handling (read_chunks bounds + progressive reader diagnostic) #911

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Fix two issues in the new APNG handling (read_chunks bounds + progressive reader diagnostic) #911

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2af2568
fix: Correct contradictory fcTL length bounds in read_chunks table
17krishna8 Aug 26, 2026
20939ec
fix: Report a rejected fcTL accurately in the progressive reader
17krishna8 Aug 26, 2026
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14 changes: 10 additions & 4 deletions pngpread.c
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Original file line number Diff line number Diff line change
Expand Up @@ -269,10 +269,16 @@ png_push_read_chunk(png_struct *png_ptr, png_info *info_ptr)
png_read_reset(png_ptr);
png_ptr->mode &= ~PNG_HAVE_fcTL;

png_handle_fcTL(png_ptr, info_ptr, png_ptr->push_length);

if (!(png_ptr->mode & PNG_HAVE_fcTL))
png_error(png_ptr, "Missing required fcTL chunk in APNG stream");
png_handle_fcTL(png_ptr, info_ptr, png_ptr->push_length);

/* We only get here for an fcTL chunk, so if PNG_HAVE_fcTL was not
* set above the chunk was present but rejected (e.g. an oversized
* or misplaced frame description); say that rather than claiming
* that the chunk is missing.
*/
if (!(png_ptr->mode & PNG_HAVE_fcTL))
png_error(png_ptr,
"Invalid or misplaced fcTL chunk in APNG stream");

png_read_reinit(png_ptr, info_ptr);
png_progressive_read_reset(png_ptr);
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9 changes: 8 additions & 1 deletion pngrutil.c
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Expand Up @@ -3232,7 +3232,14 @@ read_chunks[PNG_INDEX_unknown] =
/* Allocates 'length+1'; checked in the handler */
# define CDtIME 7U, 7U, 0, hIHDR, 0
# define CDacTL 8U, 8U, hIDAT, hIHDR, 0
# define CDfcTL 25U, 26U, 0, hIHDR, 1
# define CDfcTL 26U, 26U, 0, hIHDR, 1
/* The minimum and maximum fcTL lengths are equal because an fcTL is
* exactly 26 bytes long: a 4-byte big-endian sequence number followed by
* 22 bytes of frame data. The earlier value of 25U for max_length was
* inconsistent with min_length=26U and would have rejected every valid
* fcTL if this table entry were ever consulted (the handler is currently
* compiled out; see the '#define png_handle_fcTL NULL' above).
*/
# define CDfdAT Limit, 4U, hIDAT, hIHDR, 1
/* Supported chunks from PNG extensions 1.5.0, NYI so limit */
# define CDoFFs 9U, 9U, hIDAT, hIHDR, 0
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