GHSA-4f6g-68pf-7vhv CVE-2026-22691 PYSEC-2026-1828
pypdf has possible long runtimes for malformed startxref
Medium
PyPI
- Severity
- 0.0 / 10
- Medium · CVSS 4.0
- Exploit chance (EPSS)
- 0.43%
- Chance of exploitation in the next 30 days. Higher than 37.66% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Fix
6.6.0- Upgrade pypdf to this version or later
Description
Impact
An attacker who exploits this vulnerability can craft a PDF which leads to possibly long runtimes for invalid startxref entries. When rebuilding the cross-reference table, PDF files with lots of whitespace characters become problematic. Only the non-strict reading mode is affected.
Patches
This has been fixed in pypdf==6.6.0.
Workarounds
from pypdf import PdfReader, PdfWriter
# Instead of
reader = PdfReader("file.pdf")
# use the strict mode:
reader = PdfReader("file.pdf", strict=True)
# Instead of
writer = PdfWriter(clone_from="file.pdf")
# use an explicit strict reader:
writer = PdfWriter(clone_from=PdfReader("file.pdf", strict=True))
Resources
This issue has been fixed in #3594.
Affected versions
PyPI
| Package | Vulnerable | Fixed in |
|---|---|---|
| pypdf | < 6.6.0 |
6.6.0 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
pypdf PyPI → 6.6.0
- pip
pip install --upgrade "pypdf>=6.6.0"- poetry
poetry add "pypdf>=6.6.0"- uv
uv add "pypdf>=6.6.0"
Severity breakdown
CVSS 4.0 0.0
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- None
- User interaction
- None
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- None
- Availability (vulnerable system)
- Low
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
- Exploit maturity
- Unreported
CVSS 3.1 5.3
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- Low
Weakness
CWE-1333: Inefficient Regular Expression Complexity
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
How to prevent it
- Architecture and Design: Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
- System Configuration: Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
- Implementation: Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
- Implementation: Limit the length of the input that the regular expression will process.
CWE-400: Uncontrolled Resource Consumption
The product does not properly control the allocation and maintenance of a limited resource.
How to prevent it
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Implementation: Ensure that all failures in resource allocation place the system into a safe posture.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-5849-834g-hvw5 pypdf has possible infinite loop when hitting EOF in standard xref table during trailer search | High | Also affects pypdf |
| GHSA-cggf-277g-3q54 pypdf has possible large memory usage when parsing page tree | High | Also affects pypdf |
| GHSA-mfr5-77xh-9j7h pypdf has possible infinite loop when cloning dictionaries | High | Also affects pypdf |
| GHSA-42gh-23hr-vphw pypdf has possible long runtimes when retrieving page labels | High | Also affects pypdf |
| GHSA-4vqv-7265-gm2w pypdf has possible large memory usage for exceptions in strict mode | High | Also affects pypdf |
| GHSA-62m5-6wh8-28x7 pypdf has possible long runtimes/large memory usage for encrypted PDF files | High | Also affects pypdf |
| GHSA-x3rh-gjgm-fmqc pypdf has possible large memory usage when parsing cross-reference streams | High | Also affects pypdf |
| GHSA-jr96-gvgh-j3r4 pypdf: Possible long runtimes for repeated malformed cross-reference entries (Follow-up) | High | Also affects pypdf |
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