PYSEC-2026-1835

### Impact An attacker who uses this vulnerability can craft a PDF which leads to an infinite loop if `__parse_content_stream` is executed. This infinite loop blocks the current process and can utilize a single core of the CPU by 100%. It does not affect memory usage. That is, for example, the case if the user extracted text from such a PDF.

Example Code and a PDF that causes the issue:

```python from pypdf import PdfReader

# https://objects.githubusercontent.com/github-production-repository-file-5c1aeb/3119517/11367871?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20230627%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20230627T201018Z&X-Amz-Expires=300&X-Amz-Signature=d71c8fd9181c4875f0c04d563b6d32f1d4da6e7b2e6be2f14479ce4ecdc9c8b2&X-Amz-SignedHeaders=host&actor_id=1658117&key_id=0&repo_id=3119517&response-content-disposition=attachment%3Bfilename%3DMiFO_LFO_FEIS_NOA_Published.3.pdf&response-content-type=application%2Fpdf reader = PdfReader("MiFO_LFO_FEIS_NOA_Published.3.pdf") page = reader.pages[0] page.extract_text() ```

The issue was introduced with https://github.com/py-pdf/pypdf/pull/969

### Patches

The issue was fixed with https://github.com/py-pdf/pypdf/pull/1828

### Workarounds

It is recommended to upgrade to `pypdf>=3.9.0`. PyPDF2 users should migrate to pypdf.

If you cannot update your version of pypdf, you should modify `pypdf/generic/_data_structures.py`:

``` OLD: while peek not in (b"\r", b"\n"): NEW: while peek not in (b"\r", b"\n", b""): ```