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CVE-2026-33936 — python-ecdsa: Denial of Service via improper DER length validation in crafted private keys

Affected: tlsfuzzer python-ecdsa

The `ecdsa` PyPI package is a pure Python implementation of ECC (Elliptic Curve Cryptography) with support for ECDSA (Elliptic Curve Digital Signature Algorithm), EdDSA (Edwards-curve Digital Signature Algorithm) and ECDH (Elliptic Curve Diffie-Hellman). Prior to version 0.19.2, an issue in the low-level DER parsing functions can cause unexpected exceptions to be raised from the public API functions. `ecdsa.der.remove_octet_string()` accepts truncated DER where the encoded length exceeds the available buffer. For example, an OCTET STRING that declares a length of 4096 bytes but provides only 3 bytes is parsed successfully instead of being rejected. Because of that, a crafted DER input can cause `SigningKey.from_der()` to raise an internal exception (`IndexError: index out of bounds on dimension 1`) rather than cleanly rejecting malformed DER (e.g., raising `UnexpectedDER` or `ValueError`). Applications that parse untrusted DER private keys may crash if they do not handle unexpected exceptions, resulting in a denial of service. Version 0.19.2 patches the issue.

GHSA-9f5j-8jwj-x28g — python-ecdsa: Denial of Service via improper DER length validation in crafted private keys

Affected: PyPI: ecdsa

## Summary

An issue in the low-level DER parsing functions can cause unexpected exceptions to be raised from the public API functions.

1. `ecdsa.der.remove_octet_string()` accepts truncated DER where the encoded length exceeds the available buffer. For example, an OCTET STRING that declares a length of 4096 bytes but provides only 3 bytes is parsed successfully instead of being rejected.

2. Because of that, a crafted DER input can cause `SigningKey.from_der()` to raise an internal exception (`IndexError: index out of bounds on dimension 1`) rather than cleanly rejecting malformed DER (e.g., raising `UnexpectedDER` or `ValueError`). Applications that parse untrusted DER private keys may crash if they do not handle unexpected exceptions, resulting in a denial of service.

## Impact

Potential denial-of-service when parsing untrusted DER private keys due to unexpected internal exceptions, and malformed DER acceptance due to missing bounds checks in DER helper functions.

## Reproduction

Attach and run the following PoCs:

### poc_truncated_der_octet.py

```python from ecdsa.der import remove_octet_string, UnexpectedDER

# OCTET STRING (0x04) # Declared length: 0x82 0x10 0x00 -> 4096 bytes # Actual body: only 3 bytes -> truncated DER bad = b"\x04\x82\x10\x00" + b"ABC"

try: body, rest = remove_octet_string(bad) print("[BUG] remove_octet_string accepted truncated DER.") print("Declared length=4096, actual body_len=", len(body), "rest_len=", len(rest)) print(…

PYSEC-2026-2467 — python-ecdsa: Denial of Service via improper DER length validation in crafted private keys