Re: [libc musl] - Algorithmic complexity DoS in iconv GB18030 decoder

[Rich Felker <dalias () libc org>]

On Thu, Apr 02, 2026 at 10:27:38PM +0200, Jens Jarl Nestén Hansen-Nord wrote:
> ==========================================
> libc musl Security Advisory: April 2, 2026
> ==========================================
> Description:
> The GB18030 4-byte decoder in musl libc's iconv() implementation
> contains a gap-skipping loop that performs a full linear scan of the
> gb18030126 lookup table (23,940 entries) on each iteration of an
> outer loop whose iteration count is input-dependent. For 4-byte
> sequences whose linear index falls just below the dense CJK Unified
> Ideographs range, the outer loop executes approximately 20,905
> times, resulting in approximately 500 million comparisons per input
> character.
> Classification:
> Inefficient Algorithmic Complexity (CWE-407)
> Impact:
> This allows a remote attacker to cause denial of service via CPU
> exhaustion by sending a crafted GB18030 payload to any network
> service that uses musl's iconv() for character encoding conversion.
> Measured on musl 1.2.6 and 1.2.5: a single 4-byte input character
> (bytes 0x82 0x35 0x8F 0x33) takes approximately 260ms to decode,
> compared to approximately 13 microseconds for a benign character — a
> 19,000x slowdown. A payload of 40kB will take ~43 minutes to decode.
>
> Versions affected: 
> musl 0.8.0 to 1.2.6
>
> Status:
> The issue has been confirmed and fixed by maintainer, Rich Felker. 
> A CVE has been requested and is pending assignment.
>
> Reported by:
> Jens Jarl Nestén Hansen-Nord
>
> Upstream fix:
> Iconv-gb18030-fix.diff
>
> diff --git a/src/locale/iconv.c b/src/locale/iconv.c
> index 52178950..e559aa4c 100644
> --- a/src/locale/iconv.c
> +++ b/src/locale/iconv.c
> @@ -74,6 +74,10 @@ static const unsigned short gb18030[126][190] = {
>  #include "gb18030.h"
>  };
>  
> +static const unsigned short gb18030utf[][2] = {
> +#include "gb18030utf.h"
> +};
> +
>  static const unsigned short big5[89][157] = {
>  #include "big5.h"
>  };
> @@ -224,6 +228,8 @@ static unsigned uni_to_jis(unsigned c)
>     }
>  }
>  
> +#define countof(a) (sizeof (a) / sizeof *(a))
> +
>  size_t iconv(iconv_t cd, char **restrict in, size_t *restrict inb, char **restrict out, size_t *restrict outb)
>  {
>     size_t x=0;
> @@ -430,16 +436,14 @@ size_t iconv(iconv_t cd, char **restrict in, size_t *restrict inb, char **restri
>                 d = *((unsigned char *)*in + 3);
>                 if (d-'0'>9) goto ilseq;
>                 c += d-'0';
> -               c += 128;
> -               for (d=0; d<=c; ) {
> -                   k = 0;
> -                   for (int i=0; i<126; i++)
> -                       for (int j=0; j<190; j++)
> -                           if (gb18030[i][j]-d <= c-d)
> -                               k++;
> -                   d = c+1;
> -                   c += k;
> +               for (int i=0; i<countof(gb18030utf); i++) {
> +                   if (c<gb18030utf[i][1]) {
> +                       c += gb18030utf[i][0];
> +                       break;
> +                   }
> +                   c -= gb18030utf[i][1];
>                 }
> +               c += 0x10000;
>                 break;
>             }
>             d -= 0x40;

The above patch was a proposal for testing. It should mitigate the
extreme slowness for characters encoded in GB18030's UTF, but it does
not work correctly and has not been confirmed not to have other
problems. I will follow up with a correct patch.

Rich