CVE-2025-12781
Description
A flaw was found in the base64 module in the Python standard library. The b64decode, standard_b64decode and urlsafe_b64decode functions will always accept the '+' and '/' characters even when an alternative base64 alphabet is specified via the altchars parameter that excludes them. This input validation bypass allows malformed or unexpected data to pass through decoding filters, potentially causing logical errors or data integrity issues in applications relying on strict character sets.
Statement
This issue can only be exploited by Python applications that use the base64 functions to validate if a string is safe for a URL or filename without additional input validation. Additionally, this flaw affects only the base64 functions when an alternative alphabet is specified via the altchars parameter, limiting its exposure. This issue can cause logical errors and integrity issues but it does not allow direct access to system data or critical files, as the exploitation depends on how the application processes the decoded data. Due to these reasons, this vulnerability has been rated with a moderate severity.
Mitigation
To mitigate this issue, implement manual input validation before calling the base64 functions with an alternative alphabet, ensuring to reject any string containing the standard base64 characters, such as '+' and '/', that are not part of the expected alphabet.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | 5.3 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | None | None | N/A |
| Integrity Impact | Low | Low | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory)
An attacker could provide unexpected values and cause a program crash or arbitrary control of resource allocation, leading to excessive consumption of resources such as memory and CPU.
Confidentiality
Technical Impact: Read Memory; Read Files or Directories
An attacker could read confidential data if they are able to control resource references.
Integrity,Confidentiality,Availability
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
An attacker could use malicious input to modify data or possibly alter control flow in unexpected ways, including arbitrary command execution.
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